44 Common Misconceptions About Astronomy

(Inspired by the legendary Nails song “88 lines about 44 women”)

Many people love astronomy – it is the sexy science – and learn about it from TV, popular books, etc.  In spite of this, there is a lot of misunderstanding going on.  If you see something on this list and think “Hey, that’s what I thought!”, you are not alone.  There are many more than 44 misconceptions – Neil Comens in his book Heavenly Errors identifies over 1600 (!), some of which can be found below.  I’ve made this list with only 44 items so it’s a proper homage.  They aren’t listed in any particular order…so enjoy!

Important!  Please note that the misconception is in the title line of each item, followed by the facts.

All Stars are oursAsteroids everywhereAstrology & AstronomyA Young Universe
Bigger is betterBlack holes destroy everythingBright Moon, still shiningColors of stars
Crystal clear skiesDark side of the MoonDrifting continentsEmpty space
Eternal starsFar Side = Dark SideFiery cometsFree in orbit
Galactic gulfsGreenhouse tall tale Hair of the cometHell on Earth
Heavenly motionsIs that all there is?Lunar phase namesMercury rising
Moon phasesMoonlight everywhereNeighboring starsNo gravity on the Moon
Ocean tidesOne size fits all Planets are basically the samePlanets close enough to touch
Run for your lives!  It’s an eclipse!Seeing Mars through a telescopeShooting deathstarsSunpower
There’s no Earth like my EarthThe lesser lightThe North StarThe polymorphous Zodiac
Under a yellow sunWater, water everywhere Weighty mattersWhither seasons?


Whither seasons?  Seasons are caused by Earth changing its distance from the Sun as it orbits the Sun.

It is true that Earth changes its distance from the Sun over the course of a year, but the change is about 3%.  It’s pretty clear that the temperature changes on Earth from season to season (in many places) much more than 3%.  Also, most people are vaguely aware that the Southern Hemisphere has different seasons from the Northern Hemisphere, “they” have winter when “we” have summer.  If that’s true, how can it be connected to the distance between the Earth and Sun?  If distance to the Sun played a major role in the seasons, then all Earth should have the same seasons at the same time.

So what causes the seasons?  Ah… that would be the tilt of Earth.  As we spin, we have a tilt (from the Sun’s point of view) of about 23°.  This means that sometimes North Earth is tilted toward the Sun and sometimes tilted away (and vice versa for South Earth).  If you’re wondering why that matters, consider this:  When the Sun is higher in our sky, the light it shines is more concentrated – think how sunlight around sunset doesn’t warm you much compared to sunlight at noontime.  Also, when the Sun is higher in the sky, it stays up longer because it appears to follow a longer path (Earth is really doing the moving here) and takes longer to go from one end of that path (sunrise) to the other (sunset).

Shooting deathstars:  If stars are so enormous in size, then why don’t shooting stars destroy Earth?

Because shooting stars aren’t really stars.  They’re bits of debris that usually vaporize in Earth’s atmosphere.  Bigger bits might make it to the ground and become meteorites.  Real big bits (10 km across or more) do things like take out the dinosaurs and cause mass-extinctions.  There have been five of those in the last half-billion years.

Sunpower:  The Sun is on fire, or burning.

The Sun couldn’t last as long as it has (it’s 4.5 billion years old) from any chemical reaction that could be giving it energy.  The Sun is actually fusing hydrogen atoms into helium, giving it energy that will last for about another 4 billion years from now (when the hydrogen at the Sun’s core will run out).  This is a nuclear fusion reaction and gives off much more energy than any chemical reaction.  Ironically, the same people who think “nuclear is bad, solar is good” seem unaware that the Sun is the largest nuclear reactor around.

Dark side of the Moon:  The Moon has a “dark” side that is in eternal darkness.

The Moon is lit up about half the time and in darkness about half the time, just like any other moon, planet or piece of debris in our Solar System.  No body in the Solar System has a large part of itself in permanent darkness.  When we see the phases of the Moon change, we are watching the changing day on the Moon.  If the face of the moon changes its lighting over the course of a month, then how can half of the Moon be in eternal darkness?  Now there are small parts of the Moon near its south and north poles where sunlight never reaches into some craters, but that’s not the same thing, is it?

Now if you’re wondering if a planet could have a permanently dark side, then read the next one…

Far Side = Dark Side:  The Moon’s dark side is the side we can never see.

We only see one face of the Moon.  This is because the Moon spins in a way that it takes the same amount of time to spin around once as the Moon takes to move (revolve) around the Earth.  In a way, the Moon’s “day” is the same as its “year” – so we only see the side of the Moon that faces us.  No matter where on Earth you are, the same face of the Moon stares down at us.  This is called the near side.  If you want to see the far side of the Moon, you have to go there:  the first humans to see it with their eyes were the astronauts of Apollo 8.  The far side gets lit by the Sun the same as the near side, just at different times.  During New Moon, the far side is completely lit up and the near side is in darkness.

Now if the Moon keeps the same face pointed toward Earth (this is also true of many moons of Jupiter and Saturn), then shouldn’t it be possible for a planet to always keep one face toward the Sun thus making the far side of this planet the “dark side”?  Yes, it’s possible.  The only planet in the Solar System that could do this is Mercury, and astronomers used to think this happened there.  But it’s doesn’t, because of the nature of Mercury’s orbit around the Sun.

Yet over the last decade, we’ve discovered dozens of rocky worlds orbiting stars other than the Sun.  Many of these do in fact have “daysides” where the star is always out and it is always daytime and “nightsides” where the star’s light never reaches.  The closest habitable world to Earth, Proxima Centauri B, is in just this situation.  It’s habitable because it’s at the right distance from its star to have liquid water on its surface…if there’s any water.  It’s a rocky world more massive than Earth, so it’s a Superearth.  So such planets do exist.

Asteroids everywhere:  Flying through an asteroid field is fast and dangerous.

Space is mostly empty.  Things in it are far apart because if they weren’t, over the eons things would collide.  Planets (and moons) are here today after surviving for billions of years.  You don’t survive that long if you are too close to your neighbors.  The asteroids are spaced out at distances far larger than the distance between the Earth and Moon (400,000 km).  If you were heading right toward an asteroid, you would likely have days if not weeks to fix the problem.  Again, space is mostly empty.  If you found yourself one day on a ship heading toward the Sun near the speed of light, you would have almost eight minutes to change your course.  No Nintendo reflexes required.  Do asteroids collide?  Sure, but it’s rare as they are very far apart on average.

Planets close enough to touch:  Movies show three planets hanging in the sky of an alien world.

This is a problem because large things in space are moving, and fast.  Planets too close together would eventually collide and this would lower the real estate values.  Every once in a while, someone Photoshops an image of Mars to make it appear huge in the sky.  In truth, Mars is one of the most disappointing things to see through your neighborhood telescope because it is so far away (see Seeing Mars through a telescope).  It takes about six months for a space probe to reach Mars from Earth even at the enormous speeds of space probes.

A recent exoplanet discovery of three habitable worlds orbiting one star (the TRAPPIST 1 System) shows that maybe this isn’t as unlikely as we thought (see below):



Neighboring stars:  Getting from one star system to another only takes a few days.

As seductive as the idea of faster-than-light travel is, there’s no evidence that it’s possible.  When I say that, I don’t mean that there’s some widget we need to invent and then it’s all good.  I mean not possible.  If we could find a way to take shortcuts through space (wormholes, hyperspace, etc.) that would be cool too, but the jury’s out on whether that’s possible.  This means the stars may be beyond our practical reach, unless a human culture emerges that plans far into the future.

Stars are millions of times further away than even the most distant planet in our Solar System.  The space between stars is so huge compared to their sizes that when entire galaxies with hundreds of billions of stars collide, only a few stars actually physically collide.  A few, out of hundreds of billions.  This enormous void takes tens of thousands of years for our fastest space probes to reach only the nearest star.

About one-third of all stars come in sets (binaries or multiples).  Even for multiple stars, the spaces between them are large or else they would be at risk of colliding.  As an example, the star Sirius (in Canis Major) is actually two stars that orbit each other.  They are as far apart from each other as the Sun and Uranus are.  Other multiple stars have still greater distances between them, such as Rigel (in Orion).

Ocean tides:  Only the Moon causes tides.  Nothing else is involved.

Not quite.  If the moon were destroyed tomorrow (like in the skit from the cult classic show Mr. Show with Bob and David), there would still be tides in the oceans of Earth.  They wouldn’t be as strong, but they would exist.  How’s that?  Because tides are also caused by the Sun.  The Sun may be far away compared to the Moon, but it’s a lot more massive.

Tides are complicated because of this.  Sometimes the Moon and Sun work together to make tides higher or lower than normal, and sometimes they work against each other – making tides less extreme.  Tides have confused people for thousands of years, so if they confuse you, you’re not alone.  Even today, how tides work is one of the hardest concepts to explain without using math.  So please forgive my passing on giving a full explanation here and now.

No gravity on the Moon:   Because gravity is only on Earth.

Most people would not agree with the second part of the above, yet they sometimes believe the first part.  If you were on a very small body like a tiny comet or asteroid, the gravity would be so low that you could jump off into orbit.  Yet even these small bodies have gravity.  The Moon is much bigger than any piece of debris, and so it has significant gravity.  It’s just less than Earth’s gravity.  Astronauts on the Moon couldn’t fly off into space without a giant rocket firing underneath them.  Mars has even more gravity than the Moon, and Venus still more.  Gravity is a truly universal force and everything has some, even you and I.

The North Star:  It’s the brightest star in the sky.

Many people are taught this in school, but not by astronomers.  A clear night can quickly settle this.  At that time, go outside.  Look up in the sky toward the north (if you’re not sure what direction this is, map your house on Google, or watch the sunset – North is to the right of it).  If you see stars in the northern sky, one of them is likely the North Star – it’s true name is Polaris (in Ursa Minor).  Now, turn around and look to the southern sky.  Do you see anything brighter than the stuff in the northern sky?  Odds are you will.

In the days pre-GPS, Polaris was important for navigation because of its location in the sky, not its brightness.  It is very close to the North Celestial Pole (NCP) , or the north pole of the sky.  All the sky turns around that point (or its equivalent in the southern sky – the South Celestial Pole).  Since the NCP is pretty important in finding your way, Polaris is important by association.

Fiery comets:  They burn when near the Sun.

Comet do change in appearance spectacularly as they approach the Sun.  But they’re just melting, not burning.  Once the comet moves far from the Sun, it stops melting and becomes an undistinguished lump of weird ices (including the type of ice we all know and love).  That’s assuming that the comet doesn’t hit the Sun – about two comets a month die that way.

Seeing Mars through a telescope:  It should look big as life.

It’s sad that most people never look through a telescope.  This is a shame, for whether you live in Chicago or X’ian, somebody near you has a telescope and would like you to look through it.  If you live in Sacramento and want to look through a telescope, check out our Observatory schedule.  Many things look good through a telescope (although it’s best in dark skies), things such as the Orion Nebula, Saturn, the Ring Nebula, the globular cluster M13 and so on.  One object that almost always disappoints, however, is Mars.  This is because Mars is very small and usually far away.  Saturn looks larger than Mars through a telescope because although it’s even further from us than Mars, it’s really, really big and reflective.

Even on those occasions when Mars is kind of close to us, it’s hard to see things on Mars.  Consider that one hundred years ago, some observers thought canal networks built by advanced peaceful Martians crisscrossed Mars.  That’s how bad the view of Mars is under the best of circumstances.  It took sending robot probes to Mars to give us our first good views of Mars.

Moon phases:  They’re caused by the shadow of Earth.

When part of the Moon is in darkness, that’s just because that part is out of view of the Sun and so that part is having its night.  The Earth is only half-lit by the Sun at any time, why should the Moon be different?  The Moon does take much longer to spin around once than Earth, and that is what we see when watch the phases over the course of a month –  which is how long the Moon’s “day” is (and its “year”, see Far Side = Dark Side).

The shadow of Earth does cross the Moon during a lunar eclipse, but this can only happen during a full moon – when the Earth can get in the way between the Sun and Moon.  It also only happens at most, a couple times a year.

Astrology and Astronomy:  They’re basically the same thing.

No.  To a math phobic person, astrology might seem like a science because it uses extensive math to calculate things like planet positions and hour angles. Numerology does as well, yet it’s certainly not a science.  Astronomy, on the other hand is a science – not because of its use of math, but because it creates testable ideas about the Universe.  Astronomers then go out and test those ideas.  Some turn out to be based in reality, but many more do not.  In science, ideas that don’t correspond to what we see as reality are junked.  Some think this is small minded but if there’s anything to the idea, at some point in the future as our knowledge expands it will return in one form or another to make its presence known.  Some ideas however, have stayed in the junk pile for centuries.  Astrology is one such idea.

When people test an idea, have it fail, and then continue to make excuses that allow the idea to go on, that’s not science.  When people avoid testing an idea because they don’t want to risk it being wrong, that’s also not science.  That’s faith.  While there’s nothing wrong with faith per se, it isn’t science and shouldn’t be claimed as science.

Astrology has been tested extensively in the 20th century and has failed again and again and again.  Astrologers continue to excuse these huge failures but then as mentioned above, that shows that they don’t do science.  It makes more sense to think of Astrology as a very old religion that requires math skills if you practice it – as opposed to just believing in it which requires no math. More information about astrology from an astronomer’s point of view.

Colors of stars:  All stars look white.

This idea is more common in the Eastern U.S. than the Western.  This is partly because out west (or in any dry climate) most people see colors in stars.  But whether you live in Boston or Santa Barbara, if you view a star through a telescope you will see color.  Some stars like Betelgeuse (in Orion) have a clearly orange color.  Other stars, like Vega (in Lyra) or Sirius (in Canis Major) have a harsh blue-white color to them.  So what’s going on?

It has to do with human eyes.  Our eyes have two types of cells that absorb light, one sees color and the other can see in very faint light.  In a dark room or under a dark sky, your color cells don’t have enough light to work with and they are “off”.  The cells that can see faint light are working, but they can’t see color.  This is why colors disappear in the dark.

For very bright stars or planets, the color seeing cells in our eyes do have enough light to work with and this lets us perceive color.  Mars looks red compared to Jupiter for example, but it is a subtle difference.

Black holes destroy everything:  Any nearby star or planet is history.

It’s true that black holes have gravity, because everything does.  And it’s true that black holes have much more gravity than most stars.  But they are not gravitational vacuum cleaners that suck in everything around them.  Like any star, things can orbit them.  If the orbiting thing is far enough away (about as far as Jupiter is from the Sun), it can have a stable orbit that lasts for a very long time.  The only danger near black holes occurs if there is a disk of crap around the black hole.  This disk has stuff that is slowly falling into the black hole.  An orbiting planet plowing through the stuff in a disk would slow down and later become part of the disk (after being shredded like pasta) and fall into the black hole with all the other stuff.  Other than that, it’s a perfectly safe neighborhood.

If the black hole has no such disk, then it could have planets or another star stably orbiting it until the end of the Universe.  Black holes are like most dangerous things in the Universe – don’t live right on top of it and you’re safe.

Drifting continents:  California’s gonna fall off into the Pacific.

Earth is a dynamic planet (planets are astronomy) that has a number of plates in its crust.  These plates move around in a process called Plate Tectonics.  Many people think that when plates move, holes in the crust open up and swallow people.  It doesn’t work that way.  Plates move very slowly – at about the rate your fingernails grow – and although they hit each other like bumper cars and get pushed up and down, the changes are very slow.  A plate “sinking” into the Earth like the mythical Atlantis can’t happen within human lifetimes.  California is not going to sink into the ocean.  In addition, the plate boundary going through California is a sliding boundary, not one where the plates are being pushed together or pulled apart.

Over the billions of years we know plate tectonics has been operating on Earth, continental plates have always been taller than ocean plates.  They stay taller, too.  The basic continental plates have been around a long time, although they move and change their shape over the eons:  continental plates don’t “fall” into oceans.

Eternal stars:  Stars never die.

Stars are born and if you look at the Orion Nebula (the fuzzy thing in the sword of Orion, below Orion’s Belt) you can see an area where stars are forming today.  Stars are stars because they fuse hydrogen in their cores into helium (see Sunpower).  This gives stars the energy they need to fight against the force of gravity, which keeps stars from becoming one-shot fusion bombs.

The day finally comes when they run out of hydrogen in their cores (which is where it’s needed).  Stars then start fusing helium to keep going.  At this point they are large and cooler than they were – Red Giants and such – and already dying.  Stars like the Sun will end when they run out of helium in their cores  and become dead stars, white dwarves in the case of the Sun.

More massive stars will fuse elements beyond helium – carbon, oxygen, neon, magnesium and silicon.  Once big stars fuse iron, it’s game over as fusing iron gives no energy and actually takes energy away from the star.  It’s somewhat like hanging on the edge of a cliff by grabbing a branch and then discovering that the branch is actually a lit rocket that’s pointing down.  Massive stars collapse within seconds after iron fusion starts then they explode as supernovas.  The dead star then becomes either a neutron star or black hole (see Black holes destroy everything).

Although supernovas are rare in our part of the galaxy (there hasn’t been a good one in over 400 years!), many are seen every year in other galaxies.  Like the mythical swan song, the explosion of light that is a supernova is the death flash of a massive star.

We see much evidence of white dwarves, neutron stars and black holes, so we know there are dead stars.  Like humans – stars are born, live, then die.

Empty space:  There’s nothing out there between the stars.

This is the exact opposite of Asteroids everywhere, Planets close enough to touch, and Neighboring stars above, in thinking that space is totally empty.  By our standards on Earth that’s certainly true, but space is insanely big.  For centuries, astronomers thought that if there was any gas or dust in space it was few and far between.  Because of this, they misunderstood how much stuff is between us and the center of our Galaxy – among other things.

With the 20th century and radio astronomy, the amount of stuff between the stars became hard to miss.  It’s everywhere – which is good as new stars form out of it.  The problem comes when we think: “Gee, it’s just a few atoms for every cubic centimeter, so it’s too spread out to matter.”  Space has many, many cubic centimeters and over great distances the amount of stuff adds up to equal the mass of thousands of stars!

There’s an entire branch of astronomy that studies this Interstellar Medium (ISM), because it plays a vital role in the formation and life cycle of stars (see Eternal stars).  We now have much more respect for clouds of crap than we used to.

When you see a nebula, the density of it is very low, but the volume of the space it occupies is enormous – thousands to millions of cubic light years!

A young Universe:  The Universe is only a few thousand years old.

As mentioned above (see Astrology and Astronomy), people that persist in making excuses to maintain ideas that have failed testing are not doing science.  There are those who claim evidence to support a young Universe (and therefore a young Earth).  The problem with all this claimed evidence is that it doesn’t stand up to even basic investigation.  These people often quote scientists to support their claims.  The problem is, either the work quoted was done long ago (when we thought different), or information is quoted out of context (so are some scientists), or it’s a (usually not honest) misunderstanding of something in physics or astronomy.  Since almost all people pushing this idea have a flamingly obvious agenda, this claim is not taken seriously by scientists.  Is this close-minded?  No more so than dismissing claims by some people – still made today – that Earth is actually flat.  Some ideas don’t reflect reality no matter how much some may wish.

So what about the evidence that the Universe is ancient?  Just sampling some of the massive scientific work, let’s start with the most abstract and move toward more concrete things.  Based on the expansion of the Universe, you can make a crude guess as to when it all started based on the speed at which it now expands.  This assumes that the expansion is constant, which it probably isn’t, but I did say crude guess.  From this, the Universe is between 10 billion and 20 billion years old.

Next, we can see the echo of the Big Bang in the Cosmic Background Radiation which is a kind of glowing leftover from that unimaginably violent event.  Recent examinations of this glow tell us that the Universe is about 13.7 billion years old, assuming that the models used in these studies are correct.  The point is, it doesn’t disagree with the earlier value.  If we found stars that are older than the Universe, then there’s a problem with the model.

We have a good understanding of how stars work, how long they live and when they will die (see Eternal stars).  We can build star models from this to estimate star ages.  All around us in the Galaxy are many stars of ages from the newly born to the dying (or even dead).  The measurable features that we look for in stars of certain ages agree with the predictions of our star models.  From this, we know that the Sun is about 4.6 billion years old and the oldest stars in the Galaxy are about 13 billion years old.  So far, so good…as the ages don’t contradict each other and they are both younger than the oldest age of the Universe.

Meteorites can be dated radiometrically just like Earth rocks, and the oldest ones found so far (including rocks brought back from the Moon) are about 4.5 billion years old.  So the Solar System isn’t older than the Sun, which agrees again.  Although we can’t find rocks on Earth older than about 3.8 billion years  because of Earth’s active geology, no Earth, Mars or Moon rock has been found that is older than the Sun.

The point of all this is that the Universe, Sun, Moon and the planet beneath your feet are billions of years old.  So it’s an ancient Universe for such a young species as we to live in.  Perhaps there are species out there who have been around since the formation of the Sun.  I’m sure they have some stories!

Planets are basically the same:  Jupiter’s just a giant version of Earth.

Few people think that each planet is alike, but some folks think that the basic stuff planets are made out of is the same.  But giant planets are different in quality as well as quantity.  Small planets and moons can be made of rock and/or ice depending on where they formed.  Worlds distant from the Sun are more ice than rock because at very cold temperatures, ice is common and very strong.  Worlds close to the Sun can’t be made of ice, so they are mostly rock and/or metal.

Giant planets, however, are more like stars than rocky worlds.  They are mostly made of hydrogen and helium and have no solid surfaces – in fact they may not have anything solid in them at all.  It’s thought that giant planets can only form far from their progenitor stars, yet so far most planets found around other stars are gas giants very close to their stars.  It’s a puzzle, but maybe planets move a lot more during their lives than we thought they did.

Recently, two planets have been discovered around the star Gliese 581 that may be rocky worlds bigger than Earth.  Such worlds are called super-Earths.  Some believe the next big “paradigm shift” will come when we discover new worlds that resemble Earth.  Perhaps.

The polymorphous Zodiac:  The planets travel through only 12 constellations

Constellations are arbitrary patterns humans read into the stars we see in the sky.  Most of the constellations used in astronomy are from the Middle East by way of Greece and Rome.  Other cultures of course, have different constellations.  The Sun, Moon and planets travel through a specific band in the sky called the Zodiac.  The planets and the Sun travel pretty close to a line in the middle of the Zodiac called the ecliptic, although the Moon’s path wanders further.  The classical zodiac has twelve constellations.  This isn’t a mystery, as the year has twelve months for the same reason – it’s how many times the Moon goes around the Earth in a year (give or take).  With twelve constellations, the Sun is in a different one each month.  It doesn’t work well, because the number of months in a year doesn’t divide evenly and this fact drove ancient skywatchers nuts – and leaving leftover days in their calendars.

There are 88 constellations and actually thirteen of them are on the ecliptic.  The 13th one is Ophiuchus.  If we allow for the tilt of the Moon’s orbit, there are about twenty zodiacal constellations.  Some constellations that can host the Moon (and some planets) but aren’t thought of as part of the Zodiac include Auriga, Orion and Crater.

So how many constellations are on the Zodiac?  Depends on what sky object you care about.  The Sun has thirteen along its path, the Moon has about twenty.  If this seems quite arbitrary, it is.  Remember that all constellations are random patterns invented by humans, which is why they’re not imbued with a deeper significance.  Those who believe astrology is real think otherwise but see Astrology and Astronomy for more about that.

All stars are ours:  Stars shine by reflected light from the Sun.

Planets do shine from reflected sunlight.  Stars, however, make their own light through nuclear fusion (see Sunpower and Eternal Stars).  The Sun is a star too, so it does the same thing.  When you look up at the stars, remember that each one you see is a giant nuclear fusion reactor.  Some, maybe most of them have planets – planets that shine with borrowed light.

Lunar phase names:  A first quarter moon is 25% lit up.

Some names stick even when they make no sense.  Lunar phases have two points named quarters:  First Quarter and Last (or Third) Quarter.  It’s a source of huge confusion that when the Moon is in a “quarter” phase it’s half lit by the Sun.  The problem is that the word quarter doesn’t refer to the illumination of the Moon, it refers to which of four main stages the lunar cycle is in (hence quarters).  Each month has about four weeks because they are stand-ins for the phases of the moon.  There are four quarters in the total lunar cycle, so each week represents one quarter of the cycle.  In order, they are:

  • New Moon
  • First Quarter
  • Full Moon
  • Last (or Third) Quarter
  • New Moon (start of a new cycle)

The phase of the Moon where it’s more than half lit (from our point of view) is called gibbous.

Hair of the comet:  The tails of comets always flow behind them

Comets are beautiful mostly because of their tails.  Bright comets show two tails, one of dust peeling off the melting dirty snowball that is the heart of any comet. and one of plasma being vented by the comet’s heart.  The plasma starts out as gas and is turned into lighted plasma (like in a fluorescent light) by the Sun and the conditions in space around the comet.  The dust tail usually looks yellowish and the plasma tail usually looks bluish.  The tails only exist when the comet is close enough to the Sun to melt the comet’s heart and release the dust and gas/plasma.

A strange thing is that comet tails, being creatures of the Sun, are controlled by the Sun.  They always point away from the Sun.  Always.  This gets weird, because when the comet starts moving away from the Sun and back to its winter home the tails end up in front of the comet.  A comet literally chases its own tails on its way back into the outer dark.

Galactic gulfs:  Galaxies are too far away to be easily seen.

Galaxies are very far away, even the closest large one is over two million light years away.  It’s easy to think that they are very hard to see.  The main problem with seeing galaxies isn’t their distance, it’s that too many people have no access to a dark sky.  They are faint, but some can be seen by the eye alone, if you have a dark sky.  Even a small telescope can reveal basic structure in galaxies like the Whirlpool Galaxy.

The nearest large galaxy, the Andromeda Galaxy is clearly visible as a fuzzy patch to the eye – as long as you are looking in a dark sky.  The true tragedy of astronomy in the 20th century is how much has been lost to light pollution across Earth.  Dark skies are becoming as rare as your favorite cute endangered mammal.  Ironically, light pollution is the most easily fixed of all pollution.  By using light fixtures that cost no more yet direct the light down instead of into space, tens of billions of dollars can be saved in energy costs alone.  The best part is we get our skies back.  The International Dark Sky Association raises awareness of this “lesser evil” pollution.

Free in orbit:  Satellites aren’t affected by Earth’s gravity.

Things that orbit the Earth (or Moon, or anything) are very much affected by the gravity of what they are orbiting.  Gravity is actually what makes them orbit.  When you are in orbit, you are basically aiming for the ground and constantly missing (how the Hitchhiker’s Guide to the Galaxy defines flying).  In orbit you have enormous horizontal speed.  The ground below curves beneath you faster than you can fall to hit it, so you never catch up.  If there were no gravity at all in space, once a probe got there, it would move away from its planet of origin at a constant speed in a straight line.  It wouldn’t come back and it wouldn’t be in orbit.

The reason there seems to be no gravity in the orbiting International Space Station is that you, it, your stuff, and everything else around you are falling at the same rate.  You feel weightless because you are falling (and you feel it, constantly).  If you jumped off a cliff, you would also feel weightless until you hit the ground.  The difference is that things in orbit never hit the ground unless something gets in their way.

29).  Under a yellow sun:  The Sun is a yellow star.

Many people think the Sun is yellow in color.  This is an idea found everywhere from computer space games to comic books (Superman gets his powers partly from our “yellow sun”).  The problem is defining the Sun’s color.  If you take a prism (or diffraction grating), you can split sunlight into all the colors of the rainbow – they’re all there.

So why is the Sun thought to be yellow?  Sane people don’t look directly at the Sun when it is high in the sky for good reason.  But people often see the Sun when it is near the horizon which is when it can look red, orange, or yellow depending on the conditions.  For the record, it’s never safe to look directly at the Sun without protection regardless of where it is in the sky.  In New Mexico for example, the setting Sun is yellow, not red.  Since most folks get that the red color of the Sun is from it being low on the horizon, it’s not a long jump to the idea that its “normal” color is yellow.

So what color is the Sun?  On a sunny winter day, go outside and look at any snow that’s around.  Snow is an excellent reflector of light and faithfully reflects visible colors.  Most people will agree that snow looks white under these conditions, so the Sun is a white star.

There’s one more catch, though.  If you look at all the colors of light from the Sun, they’re not equally strong – this is true of all stars.  From carefully comparing the different color brightnesses, we see that the Sun gives off more green light than any other visible color.  So the Sun could be called a green star.

So which is it?  To our eyes, the Sun is white.  To the instruments of an outside (or alien) observer, the Sun is green.  So objectively, the Sun is green but we see it as white.  That says more about our eyes than the Sun, but that’s another story (see Colors of Stars).

Bright Moon, still shining:  The Moon is really reflective.

There’s no doubt that the full Moon is bright.  This is because it reflects light from the Sun.  If the Moon were as reflective as a mirror, then a full Moon would be like having another Sun in the sky at night.  Clearly, it’s not that bright, so the Moon is not as reflective as a mirror.  How shiny is the Moon?

Not very, it turns out.  The Moon reflects about 12% of the light from the Sun.  That’s it.  It’s about as shiny as a battleship-gray rock, which is what it is.  Even the Earth reflects more light (39%).  Moons in the outer Solar System are mostly made of ice and some of them are very reflective, though not all of them because ice in space darkens over time.  The winner is Saturn’s moon Enceladus, which reflects 99% of the sunlight that falls on it back into space.

There’s no Earth like my Earth:  All planets are unique, especially Earth.

Earth is different from any other known planet, but that doesn’t mean there isn’t any planet out there that’s similar to it.  Mars is the world closest to Earth (from an ‘is-there-life?’ point of view) we’ve found so far, but few would be surprised if we found Mars-like worlds circling distant stars.  Around other stars, we haven’t found planets that are twins of Earth yet, but there’s no reason to think they’re not out there.

In truth, Earth has many similarities to Mars and Venus as they do to each other.  Earth also has unusual qualities too – like its freakishly large moon.  Science throughout its history has taught from the time of Copernicus that anytime you claim you live in a special place in time or space, you’re likely wrong.  The Copernican Principle states that “We don’t live in a particularly special time or place in the Universe”.  In five hundred years, this principle has worked well.

For a fair comparison, we can look at stars – far more stars have been cataloged than planets.  The Sun is a particular type of star (G2 V, if you care).  Our Galaxy has hundreds of billions of stars and about 7% of them are similar to the Sun.  That’s billions of stars.  Of that lot, some are more like the Sun and fewer yet are basically identical to the Sun.  This means that there millions of stars in the Galaxy that are identical to the Sun.  Why should it be any different for Earth?

Is that all there is?  Visible light is the only light.

The idea of invisible light sounds contradictory, but think about it:  you can feel heat – which is a form of infrared light.  You can also feel the aftereffects of ultraviolet light via a sunburn.  These are forms of light you can’t see, but you can feel (either immediately or later).  The amount of light in the Universe that we can’t see dwarfs the tiny range of colors that we do see.

The types of light that humans can’t see also include radio (which is light, not sound), x-rays, gamma rays, and microwaves (a type of radio light).  These types of light can be detected, but not with human eyes.  Many wonder if any aliens out there can see this other light with whatever organ they have that’s equivalent to our eyes.  Who knows?  X-rays and gamma rays are dangerous or outright lethal (sorry to any Incredible Hulk fans), so it’s unlikely an alien species would be able to “see” them, but radio, infrared and ultraviolet are quite possible.

Run for your lives!  It’s an eclipse!  Looking at any eclipse is dangerous.

There are two types of eclipse:  lunar and solar.  All lunar eclipses are always safe to look at.  No one ever went blind staring at the Moon.  On the other hand, most solar eclipses have an element of danger in them.  Partial solar eclipses (where the Sun isn’t ever totally covered) are the most dangerous, and you should never observe them without proper protection.  Total solar eclipses are safe to look at – when the Sun is completely eclipsed.  This only lasts a few minutes, but during that time viewing the totally eclipsed Sun is safe – and glorious to see.  Total solar eclipses are one of those rare things in life that everyone should make an effort to try and see.

Partial solar eclipses get really dangerous when the Sun is more than 95% covered by the Moon.  The reason is that you get tempted to look at the Sun by thinking: “Well, most of it is covered up”.  Bad idea!  If ANY part of the Sun is stared at, it can cause permanent eye damage or blindness.  Even 1% of the disk of the uncovered Sun can do this, because the Sun has about a constant brightness across itself – if the whole Sun can blind you, then any part of it no matter how small can do it as well.  So protect yourself with true solar filters, available from many dealers.

If you want to see the effects of a 90% (or more) partial solar eclipse, then just look around.  Things will be dimmer – about what it looks like when wearing sunglasses.  Pay close attention to the shadows of nearby objects – they will get very weird.

Weighty matters:  Heavier things fall faster.

This idea is as old as Aristotle.  If you take two different weights and drop them off a tower, you might think that the heavier one will reach the ground first.  Aristotle did.  But then, no one actually tested this idea until Galileo.  When Galileo tried it out, he discovered that the two weights (as long as they were both heavy so air resistance isn’t a player) hit the ground at the same time.  It’s kinda amazing that no one in Aristotle’s time bothered with this simple experiment and it’s a great example of why science is so powerful.

If you try this with a steel hammer and a feather, they don’t hit the ground at the same time.  This is because air resistance affects the feather much more than the steel hammer.  So what if you tried this on the Moon, where there is basically no air?  Been there, done that.  Check out the result. [link to http://nssdc.gsfc.nasa.gov/planetary/lunar/apollo_15_feather_drop.html expired]

To date, this is the most expensive science demonstration ever done!

Crystal clear skies:  A clear sky is always a good sky for astronomers.

If the sky is cloudy or in some way not transparent, this is clearly bad (sorry).  But if the sky is totally clear, it may not be good for observing because of the turbulence in the atmosphere.  Air is always moving different speeds at different altitudes and any light from stars, planets, etc. must travel through that shimmering mass of air.  This is why stars twinkle and partly why they always appear to be disks in a telescope – though they should appear as points of light.  If the air is particularly unsettled, then things seen through a telescope will be blurry (especially planets) and stars will appear to twinkle.  Brighter stars (like Sirius) will even appear to scintillate or pulsate.  This is still true even if the sky is clear.

Astronomers describe the sky using two scales:  the first is transparency and is concerned only with the sky’s clarity – no clouds, particles, haze, etc.  The second is seeing, which involves the steadiness of the air.  A great night has high values of both seeing and transparency.

Hell on Earth:  It’s possible for humans to turn Earth into Venus.

The Earth is getting warmer with time.  Much of this is due to human activity because the amount of carbon dioxide (CO2) in our atmosphere is tiny.  That’s why it’s a problem – because the normal amount of CO2 is so small, it’s possible for our puny contributions to have an effect.

Some environmentalists have carried their argument to this:  If we burn all of our carbon energy sources, enough CO2 will be released to make a Runaway Greenhouse Effect like the one that keeps Venus hot enough to melt lead.  Can’t happen – simply because there isn’t enough CO2 to generate a runaway greenhouse.  Venus has an atmosphere 100 times thicker than ours and it’s almost all CO2.  Add to that the fact that the Earth naturally removes CO2 through the formation of carbonate rocks and it’s clear that CO2 can’t build up to the levels needed for a runaway greenhouse.

Anyone reading the last paragraph might wonder why there’s concern about global warming if there’s a natural way for the Earth to remove CO2 from the atmosphere.  The problem is that it works over hundreds of thousands of years and Earth is warming over centuries.  So, there’s no easy answer to Earth’s coming climate change.

Ironically, as the Sun ages it will give off more energy.  All the planets will warm over time until one day when the Earth gets hot enough to go runaway greenhouse.  This is billions of years in the future (which is about when the Social Security funding problem will be solved) and will happen whether humans are around then or not.  The point is:  it won’t be caused by us.

Heavenly motions:  Planets orbit in circles around the Sun.

The ancient Greeks thought that things on Earth only moved in straight lines (apparently, they never noticed that thrown discuses in the ancient Olympics follow curved paths.).  They also thought that circular motions were found only in the perfect heavens.  This assumption survived for thousands of years through models of the Solar System developed by the Romans, early Muslims, Europeans, etc.  Even Copernicus held onto this idea though it raised problems with his Sun-centered model of the Solar System.

The first person (we know of) who thought differently was Johannes Kepler.  After many years of making models using the infamous Tycho Brahe’s data, he figured out that planet orbits weren’t quite circles, but ellipses.  Some planets have orbits that are only slightly elliptical, like Venus.  Other planets’ orbits are more elliptical, such as Mars.

This business about orbits being ellipses applies to moons orbiting planets and stars orbiting each other.  Kepler figured out something big and universal.

Mercury rising:  Mercury is always hot.

Mercury is the closest planet to the Sun, so it should be hot.  It is, but it spins,  so it has a day side and a night side like any planet.  This means Mercury cools off at night, like Earth or the Moon.

What’s interesting about Mercury is how cold it can get.  Since a Mercury “day” is about 59 Earth days, night lasts awhile.  The coldest it can get on Mercury is about 20 degrees above the temperature where the air you’re breathing right now would form liquid puddles at your feet.  That’s far colder than it ever gets on Earth (or even Mars).

Not every planet cools off at night – Venus stays hot all the time everywhere.  This is why Venus is actually the hottest planet even though it is further from the Sun than Mercury.  The reason Venus stays so hot is due to it’s thick atmosphere of mostly CO2.  This gives it a Runaway Greenhouse Effect (see Hell on Earth).

The lesser light:  The Moon can only be seen during the night.

Most people don’t look up in the sky unless there’s a reason to.  This is a major reason for most UFO reports – most people reporting them have minimal or no sky familiarity.  During the day when the Sun is out it makes less sense to take time out of your busy day to watch the sky.  Because of this, most people are unaware that the Moon is often visible in the sky during the day.

Two good times to go moon watching during the day are:

  1. A few hours before sunset when the Moon is past the phase of First Quarter but isn’t yet Full.  The Moon will be in the east in the late afternoon while the Sun is in the west.
  2.  Early in the morning a few days after the Moon is Full, but before Last Quarter.  At this time, the Moon is in the west.  The closer to full the Moon is, the more of it is lit up and the easier it is to see during the day.

The Sun and Moon are not the only things visible during the day.  It is possible to see Venus in the sky during the day – if you know exactly where to look.  The easiest way to do this is to see Venus before dawn and track it across the sky as the day goes on.  From time to time you may lose it, but if you know the area of the sky it’s in, you can find it again.

Bigger is better:  The most important thing telescopes do is magnify stuff.

At star parties and observatories all across the land, The Question is asked:  How much magnification does your telescope have?  I always respond by asking “What magnification would you like it to be?”  Changing the magnification takes no more time than it takes to change the eyepiece.  The problem with high magnification is that you get a bigger image, but it’s fainter.  A large image is also blurrier.  With bad seeing (see Crystal Clear Skies), a big image of Saturn quickly becomes a blurry washed out mess.

The most important thing telescopes do is gather light which is why they’re often called light buckets.  With more light, you can see fainter things or more detail in brighter things.  The larger the light gathering surface (lens or mirror), the better.  This is why we try to make telescopes with the largest light gathering surfaces possible, which is leading to telescopes with mirror diameters of thirty meters or more.

Moonlight everywhere:  All planets have moons.

Our planet certainly does.  Even small bodies such as Pluto and some asteroids have moons.  It seems straightforward to think that anything in space larger than a zeppelin has a moon.  The problem is that this isn’t true.  Two planets have no moons (Mercury and Venus).  Even if you count Earth’s moon and Mars’ moons, the inner Solar System is pretty sparse on moons.

Once we reach the outer Solar System, things change dramatically.  Both Jupiter and Saturn have over sixty moons, and the outer ‘snow giants’ of Uranus and Neptune have a respectable set of known moons – over a dozen each. It seems that large planets have plenty of moons, along with rings.  Rocky worlds like Earth seem to have few or no moons.

Greenhouse tall tale:  The Greenhouse Effect is like a blanket.

This idea is still found in some textbooks – and it’s completely wrong.  Blankets work by keep the warm air around you from mixing with cooler air outside the blanket.  A thick blanket traps more air next to you than a thin blanket.  This has nothing to do with the Greenhouse effect.

So what is the Greenhouse Effect?  Instead of a blanket, think of a foot warmer (or hot water bottle if you’re Old School British).  During the day the greenhouse gases warm up just like everything else.  Nighttime is when greenhouse gases show a difference.  They absorb some of the heat given off by the cooling Earth.  They then release this heat – but – they can release it up toward space or down toward the ground.  This makes the greenhouse gases in the air a second source of heat besides the cooling Earth.  It’s like having a foot warmer next to you instead of a blanket.

The effect isn’t much (unless you live on Venus:  see Hell on earth), but it does keep the Earth from cooling below freezing everywhere every night.  If night lasted a very long time, these gases would lose all their heat to space and stop heating the Earth, but nights are usually short on Earth.

Water, water everywhere:  Water and other things life needs are only found on Earth.

One of the reasons astrobiologists are optimists about life out there is because the things (Earth-like) life needs are found in many places throughout the Galaxy – and probably in other galaxies.   Water is made from two of the five most common elements in the Universe, so it’s not surprising it’s pretty common.  Water in the form of ice is all over the outer Solar System and it makes up a big part of the moons of the gas giants.  It has been found in distant clouds of interstellar gas throughout the Galaxy, and has been located in the atmosphere of at least one exoplanet (a planet that orbits any star besides the Sun).

Organic stuff has carbon in it, which is why it’s called organic.  Carbon is made by old and dying stars and is also plentiful (it’s also one of the five most common elements) .  Even amino acids (the building blocks of proteins) have been found in meteorites and in giant clouds of crap between the stars.

Life also needs energy, and that’s common too.  Near stars, sunlight is abundant and even far from stars, life can gain energy from all sorts of chemical reactions.  Strange forms of life on Earth called extremophiles can survive in conditions that would kill humans quickly, so we have insights into life living in difficult environments right here on Earth!

The things that (Earth-like) life needs:  Water, organic stuff and energy are quite plentiful throughout the Galaxy.  Perhaps being used by life right now on a world orbiting a star 20 light-years from us in the constellation of Libra (Gliese 581).

One size fits all:  All stars are the same size.

Stars are as different as people except that almost everything about a star comes down to how much mass it has.  More massive stars are bigger, hotter, and give off more light.  They also die quicker when they run out of hydrogen in their core (see Sunpower and Eternal Stars).

Dead stars, like white dwarfs and neutron stars, are tiny compared to living stars.  Black holes are a special case, because they have no size at all (more or less).  Dying stars are huge compared to normal stars.  Examples of dying stars that you can easily see in the sky are Betelgeuse (in Orion), Aldebaran (in Taurus) and Antares (in Scorpius).

“Normal” stars –  which are called Main Sequence stars – also vary in size.  While they are all bigger than dead stars and smaller than dying stars, they vary widely within that range.  Generally, the hotter a main sequence star, the larger and more massive they are.  The hottest main sequence stars are the biggest (like Rigel [in Orion] and Spica [in Virgo]), and the coolest main sequence stars are the smallest (such as Proxima Centauri [in Centaurus] and Barnard’s star [in Ophiuchus]).

The Sun is somewhere in the middle of the Main Sequence – middle temperature and middle size.  This is deceptive, though, because only 9% of all stars are hotter (and so bigger) than the Sun.  But those rare stars are enormous compared to our Sun.  If we replaced the Sun with Antares (in Scorpius), Earth would be inside it!