- How much astronomy do you know? I mean, really know.
Completely, self-assuredly, bet-your-bottom-dollar, 100 percent absolutely
certain you know.
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- Hmmmwanna bet? On these pages are five astronomy misconceptions
that are so common they're almost canonical. Is one of these lurking in
your brain? I bet at least one is.
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- Let's find out how much you know that you think you know,
but really don't know.
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- 1. There is no gravity in space.
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- We've all seen videos of astronauts floating weightlessly
above the Earth, and of course you've heard the expression "zero-g."
But that's a misnomer. Gravity gets weaker with distance (in fact, with
the square of the distance), but it never falls all the way to zero. In
point of fact, gravity goes on essentially forever.
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- You cannot "escape the bounds of gravity" anymore
than you can escape the grasp of the IRS.
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- Astronauts look like they are experiencing no gravity
because they are orbiting the Earth. What they are really feeling is freefall,
since they are in reality "falling" around the Earth. In effect,
they are falling toward the Earth, but moving sideways enough to continuously
miss it. The net result is they follow the curvature of the Earth, always
falling but never hitting.
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- At the typical shuttle orbital height of 250 miles (400
kilometers) off the Earth's surface, the force of gravity is roughly 90
percent what it is here on the surface. Gravity is still very much in control
of the shuttle's (and astronauts') motion. Inevitably, when they land,
they return to its full effects.
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- Some things even astronauts cannot escape. They even
have to pay their taxes too.
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- 2. The Moon looks bigger on the horizon because the
air acts like a lens, magnifying it.
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- When on the horizon, the Moon appears huge and flat from
space, too.
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- Almost everyone has seen the Moon, red and swollen, looming
hugely as it rises over the horizon. A few hours later, when it's high
in the sky, it has shrunk considerably, looking more "normal."
Most people are also aware the Sun exhibits this behavior, and even constellations
do, too.
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- It's true that the Earth's air is thicker near the horizon.
When you look up, you are looking through the thinnest part of the atmosphere,
and the closer you look toward the horizon, the more air you look through.
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- However, the air actually compresses the Moon's image,
instead of magnifying it. Have you noticed that the Moon looks noticeably
squashed when it's right on the horizon? That's because the varying thickness
of the air near the horizon distorts the Moon's shape, making it smaller
top-to-bottom.
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- It turns out this effect of the Moon looking larger near
the horizon, called the Moon Illusion, really is an illusion. You can see
this for yourself, by comparing the rising Moon's size with some household
object (say, the tip of a pencil eraser held at arm's length), and then
wait a few hours and do it again. You'll find the size hasn't changed appreciably.
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- This illusion is convincing, but it's not real.
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- What's going on here is that your brain is interpreting
the sky as being farther away near the horizon, and closer near the zenith
(directly overhead). This isn't surprising; look at the sky on a cloudy
day and the clouds overhead may be a few kilometers above you, but near
the horizon they might be hundreds of kilometers away. The Moon, when it's
on the horizon, is interpreted by your brain as being farther away. Since
it's the same apparent size as when it's high up, your brain figures it
must be physically bigger. Otherwise, the distance would make it look smaller.
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- This effect is the well-known Ponzo Illusion. Recent
tests have shown pretty conclusively that this is indeed the cause of the
Moon Illusion.
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- By the way, when it's on the horizon, the Moon is actually
a few thousand miles (kilometers) farther away than when it's overhead.
So in reality, it's actually a bit smaller when it's on the horizon! Our
brains may be smart, but they are very easily fooled.
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- 3. Seasons are caused by the Earth's distance from
the Sun.
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- On a cold winter's evening, you can huddle near a fire
for warmth. If you get too close it can burn you, and if you are too far
away it can hardly warm you at all. Clearly, the amount of warmth you get
from something hot depends on its distance.
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- And hey, the Earth's orbit is an ellipse! So sometimes
it's closer to the Sun, sometimes farther away. This must be why we have
seasons, right?
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- Wrong. If you do the math, you'll find that the Earth
should only be a few degrees warmer when it is at perihelion (closest to
the Sun) than when it's at aphelion (farthest from the Sun). Yet the difference
between summer and winter in most locations is a lot more than just a few
degrees.
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- Even worse, when it's summer in the Northern Hemisphere,
it's winter in the south, and vice-versa. So clearly it can't be the distance
to the Sun that makes the difference.
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- The real reason for the seasons is the tilt of the Earth.
Ever notice that a globe of the Earth is always tilted? That's because
the Earth's spin axis (the line connecting the north and south poles) is
tilted to the plane of the Earth's orbit around the Sun. The amount of
the tilt is about 23.5 degrees.
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- In the summer, the Earth's axis is pointed toward the
Sun (well, not exactly at the Sun, but in that direction). When that happens,
the Sun gets higher in the sky. Its light is more concentrated, and it
heats the ground more efficiently. Also, days are longer, giving it more
time to heat things up. Summers are hot.
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- In the winter, when the Earth's axis is directed away
from the Sun, the Sun is lower in the sky. The light hits the ground slanted,
spreading it out. That makes it heat things a lot less efficiently. Days
are also shorter, giving it less time to heat things up. Winters are cold.
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- That's why the opposite hemispheres have opposite seasons,
too. When the Northern Hemisphere of the Earth is tipped toward the Sun,
the southern one is tipped away, and vice-versa.
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- Sometimes, good science just depends on your slant on
things.
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- 4. Meteors are heated by friction as they pass through
the atmosphere.
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- This one makes sense, which is why it's so pernicious.
But it's still wrong.
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- Meteoroids are tiny bits of dust, rock, ice or metal
that have the unfortunate luck of having their orbits intersect the Earth's.
When they pass through our atmosphere, they are heated so ferociously that
they glow (and at this point are called meteors), and are visible for hundreds
of miles.
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- However, it is not friction that heats them. Think of
it this way: a space shuttle's tiles are extremely delicate; they crumble
easily in your hand. If they were heated by friction as the shuttle de-orbits
and enters the atmosphere at Mach 25, the tiles would disintegrate. That's
not a very good design characteristic.
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- In reality, it isn't friction, but ram pressure that
heats the meteoroid. When a gas is compressed it gets hot, like when a
bicycle pump is vigorously used to inflate a tire. A meteoroid, moving
at 33,500 mph (15 kilometers a second) or more compresses the air in front
of it violently. The air itself gets very hot, which is what heats the
meteoroid. That's the fact, not friction.
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- 5. Meteors are still very hot when they hit the ground.
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- You'd expect that something heated up so much that it
glows would still be hot a couple of minutes later. Actually, the situation
is a bit more complicated.
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- The super-hot air in front of the meteoroid is not actually
in contact with the particle. (A particle can still be referred to as a
meteoroid as it races through the atmosphere, while "meteor"
is meant to describe the whole glowing phenomenon.)
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- The meteoroid's quick motion sets up a shock wave in
the air, like from a supersonic airplane. The shocked air sits in front
of the meteoroid, a few centimeters away (depending on the meteoroid's
size) in what's called a standoff shock. Between the shocked air and the
surface of the meteoroid is a relatively slow-moving pocket of air.
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- The surface of the meteoroid melts from the heat of the
compressed gas in front of it, and the air flowing over it blows off the
melted portion in a process called ablation. The meteoroid's high velocity
provides the energy for all this heat and light, which rob it of speed.
When it falls below the speed of sound, the shock wave vanishes, the heating
and ablation stop, and the meteoroid then falls rather slowly, perhaps
at a couple of hundred mph (or a few hundred kilometers per hour).
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- It's still pretty high up in the atmosphere at this point,
and takes several minutes to fall to the ground. Remember, this tiny bit
of rock spent a long time in space, and the core is pretty cold. Also,
the hottest parts were melted and blown off. Even more, the air up there
is cold, which chills the rock as well.
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- All of these things together mean that not only is the
rock not hot when it hits the ground, it can actually be very cold. Some
meteorites (what a meteoroid is called after it impacts) have actually
been found covered in frost!
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- Philip Plait is the author of "Bad Astronomy"
(Wiley & Sons, 2002). For more about these and other astronomy misconceptions,
you can buy his book or visit his Bad
Astronomy website.
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- Copyright © 2002
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