The following questions were answered by astronomer Dr. Cathy Imhoff of the Space Telescope Science Institute.

How old is the earth?
The earth is about 4.5 billion years old. That is 45,000,000,000 years. The sun, the moon, the planets, and nearly everything in our solar system was formed at the same time, so they are all 4.5 billion years old.

How does the earth spin?
If you look around at planets, stars, and galaxies in the universe you will see that EVERYTHING spins! This is because planets like Earth (and stars and galaxies) formed from a large cloud of gas and dust. As gravity pulled the cloud together, any little bit of spin gets multiplied. This is known to scientists as the Conservation of Angular Momentum. You can try this out yourself. Find a chair that spins easily and sit on it. Start out by spinning with your arms outstretched, then pull them in to your body. Did you notice that the chair started to spin faster when you pulled your arms in? The same thing happens as the planet forms — as its gas, dust, and rocks fall together the planet spins up faster and faster. Later other things can affect the spin. Earth and the moon pull on each other, affecting both their spins and the orbit of the moon. It's quite a dance!

How fast is the earth spinning and why can't we feel it spin?
The circumference of (distance around) the earth is 2 x PI x radius, which turns out to be about 24,000 miles. Since the earth turns once per day (or 24 hours), that is 1,000 miles per hour! We don't feel that because everything else is moving with us and because the motion is very, very smooth. It's kind of like sitting in a car and reading a book or playing a game. If the car is going very smoothly, with no stops or bumps, and you don't look out the window, you wouldn't know you were moving. The only reason we know the earth is turning is because the sun and stars move across the sky.

What is the exact tilt of the earth? How much does it change throughout the year?
The tilt of the earth is 23.45 degrees. This is measured as the tilt of the earth's equator compared to its orbit around the sun. The amount of the tilt doesn't change with time, but the direction that the earth's poles are pointed does. This is called precession. It is just like the wobbling of a spinning top. The direction that the North Pole (and South Pole) is pointed moves in a circle in the sky very slowly, completing each circle once every 26,000 years.

Why does only Earth have so much oxygen?
That's a very good question — no other planet in the solar system has so much oxygen. The answer is — life! Earth did not originally have a lot of free oxygen in the atmosphere. The oxygen was mostly combined with other elements to make carbon dioxide, water, or in rocks. But as early life evolved, especially plants, well they give off oxygen. So the reason we have so much oxygen in our atmosphere — and we are here and can breathe it! — is due to plant life on the earth!

How can rockets get through the earth's atmosphere, but comets can't?
Objects hitting our atmosphere usually burn up because they are hitting it so fast. Usually they are moving at hundreds or thousands of miles per second. Rockets and the shuttle move more slowly (although pretty fast compared to cars on the highway). Here's a comparison. If you jump into a swimming pool from the high board and hit the water really hard, it stings and you make a big splash. But if you step into the water slowly, it feels fine and you make only a little ripple. The same thing happens to meteors — they hit the earth's atmosphere real hard and go splat!

If a comet hit the earth, what would happen?
It could cause a big explosion. The nucleus of a comet is thought to be a mile or two in size. The nucleus is made up of ice and rock, but we aren't sure how solid it is. Something that big hitting the earth from space would be bigger than any bomb. Since the earth is mostly ocean, there would be a good chance that it would hit water. That might not be so bad. Fortunately the chances of this happening are very small.

Did the earth always weigh the same as it does now?
No, it is actually a tiny bit heavier. Over time it has swept up a lot of space dust, many thousands of tons of it. The moon has done the same thing. Before the Apollo missions, there was some concern that the astronauts might sink deep into moon dust! But it was only about an inch deep. Of course on the earth the dust has fallen into oceans, been swept away by winds, and so on.

Is the earth alive? Will it someday die and turn into an uninhabitable place?
Well, there is certainly life on the earth. But life exists only on a very narrow "skin" on the surface of the earth. We don't really think of the earth as itself being alive. Some day, about five billion years from now, the sun will run out of energy and the earth will become uninhabitable. But it will still be here. Hopefully we will have gone to another star by then.

Could Earth ever come so close to the sun that it would burn up?
Fortunately, the answer is no. Earth is in a nice, stable orbit, so there is no danger of that. However, when the sun is very, very old (about 4.5 billion years from now), it will turn into a red giant star. It will become much bigger than it is now and will make Earth very hot. So Earth won't get closer to the sun, but in a sense the sun will get closer to Earth! But that is a long time from now and I hope we will be traveling around the universe in spaceships by then!

Is Earth moving away from the sun?
No, I don't think Earth is moving away from the sun. It has been in orbit at about the same distance for a long, long time or life would not have survived here.

What happens to the temperature when you live close to the equator?
As you know, it's hotter at the equator. This is because the sun shines down from almost overhead. At the North and South Poles, even when the sun is above the horizon so you can see it, its light is coming at an angle. This means that the sunlight is spread out over more area and it can't warm the ground as much. So the poles are the coldest places on the earth.

Which would get more sunlight, Miami or the North Pole?
The answer depends on how you count "more sunlight." If you mean the most sunlight over one year, then this would be Miami because the sun is shining more directly to the ground in Florida. If you mean which place has the longest day, that would be the North Pole, because in the summer the sun is up 24 hours a day!

Do all places on earth get the same amount of sunlight?
No, the amount of sunlight that you receive depends on where you are. It mainly depends on how far from the equator and the North/South Poles you are. The closer you are to one of the poles, the more ground the sunlight has to cover. So each little patch of ground receives less sunlight.

Why doesn't Earth have the extreme temperature differences of Mercury? Is it the atmosphere on Earth?
That is part of the answer. The earth's atmosphere and oceans help keep the earth's temperature moderate. Another BIG factor is that Mercury rotates very slowly. It rotates with respect to the sun only once every 59 earth days. So one side gets roasted for a long time by the sun, with nothing to protect it, before it gets any shade. Then on the night side, there is no sun to warm it for a long time and no blanketlike atmosphere to help.

How far has anyone from Earth seen into space?
When we look at the most distant galaxies, known as quasars, we are looking very, very far. Some of them are ten to twelve billion light-years away. That also means that we are seeing them as they were ten to twelve billion years ago!

How far away from Earth is Mars?
Mars'orbit is about 1.5 times Earth's orbit. So if both planets were on the same side of the sun, at their closest point they would be about 46 million miles apart. But if Mars is on one side of the sun and Earth is on the other, Mars is 233 million miles away!

Do Mercury and Venus ever get between Earth and the sun? If so, what is it called?
Yes, this happens — it is called a transit. Since the planets are so much smaller than the sun, it looks like a small black dot moving across the face of the bright sun. A transit of Mercury happens about 13 times in 100 years. Transits by Venus are more rare. There were two back in 1874 and 1882. The next ones you and I may see — they are coming up on June 8, 2004, and June 6, 2012.

How is gravity measured?
We measure it by dropping something! Of course to measure gravity correctly, we have to be careful. For instance, the air helps to slow something that is falling. So to do the measurement properly, we would have to have a long tube with no air in it, then very carefully measure how long the tube is and how long it takes for something to fall.

What is earth's gravity?
We measure gravity by how much faster a falling object falls due to the pull of gravity. The earth's gravity is 32 feet per second per second. Suppose you jumped out of a plane. In the first second, you would be falling at a speed of 32 feet per second (22 miles per hour). After another second, you would be falling at 64 feet per second (44 miles per hour). In another second, you would be falling 96 feet per second (66 miles per hour). And so forth.

How do you make a non-gravity area on Earth?
We don't have any anti-gravity machines yet. That is only in science fiction. But we can make it feel like there is no gravity. The astronauts go up in a special plane that makes a big, high loop. If the plane goes just right, the passengers onboard float around the cabin as if there were no gravity. They can do this only for a few minutes.

If you dug a hole through the earth and dropped something down it, would it stay in the earth or fall out the other side?
First, I assume you know that we can't really dig a hole all the way through the earth! Also for our test we will have to stop the spinning of the earth so that our object doesn't hit the side of the tunnel! But if we could do that.... Let's suppose we stood over the hole and dropped a ball from shoulder height. The ball would fall faster and faster as it falls because gravity keeps pulling on it. When it gets to the center of the earth, it is falling very fast. There is nothing to stop it so it keeps falling, except now the pull of gravity is in the other direction. So it starts slowing down. If there were no air in our tunnel to slow the ball down, the ball would pass all the way through the earth and come out the other side. It would stop at about shoulder height, then start falling back down the tunnel. It could keep doing this forever. If this seems strange, think about the moon. It has been "falling" in orbit around the earth for billions of years!