Top 5 Cosmic Myths
By Philip Plait <>

How much astronomy do you know? I mean, really know. Completely, self-assuredly, bet-your-bottom-dollar, 100 percent absolutely certain you know.
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.
Let's find out how much you know that you think you know, but really don't know.
1. There is no gravity in space.
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.
You cannot "escape the bounds of gravity" anymore than you can escape the grasp of the IRS.
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.
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.
Some things even astronauts cannot escape. They even have to pay their taxes too.
2. The Moon looks bigger on the horizon because the air acts like a lens, magnifying it.
When on the horizon, the Moon appears huge and flat from space, too.
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.
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.
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.
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.
This illusion is convincing, but it's not real.
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.
This effect is the well-known Ponzo Illusion. Recent tests have shown pretty conclusively that this is indeed the cause of the Moon Illusion.
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.
3. Seasons are caused by the Earth's distance from the Sun.
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.
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?
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.
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.
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.
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.
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.
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.
Sometimes, good science just depends on your slant on things.
4. Meteors are heated by friction as they pass through the atmosphere.
This one makes sense, which is why it's so pernicious. But it's still wrong.
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.
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.
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.
5. Meteors are still very hot when they hit the ground.
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.
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.)
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.
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).
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.
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!
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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