Most planets in the solar system have moons. Because our planet, Earth, has only one moon, we call it the Moon, so sometimes when people use the word "moon," it can be confusing whether they are talking about our Moon or any moon in the solar system. This month, when I'm thinking about the Moon, I mean the one and only that belongs to Earth. And I think we're lucky to have it, because it regularly gives us a beautiful view of a world out in space.

Although it's one of the largest moons in the solar system, the Moon is smaller than all the planets except Pluto*. Still, it's so large that if it orbited the Sun instead of Earth, we would call it a small planet and not a moon. [Go here to compare the sizes of the planets and moons in the solar system.]

But the Moon does orbit Earth, just as the space shuttle and thousands of other satellites do. However, at 239,000 miles -- almost a quarter of a million miles -- the Moon is much farther away than most satellites we launch.

The Moon's size and distance contribute to a wonderful coincidence for those of us who live here on Earth. The Moon is about 400 times smaller than the Sun, but it also just happens to be about 400 times closer. The result is that from Earth, they appear to be the same size. And when its orbit around Earth takes the Moon directly between Earth and the Sun, the Moon blocks our view of the Sun in what we call a solar eclipse. This is just the same as when you use your thumb to block your view of something that is both much larger and much farther away.

To imagine this, think of Earth as about the size of a basketball. Then the Moon would be 30 feet away and about the size of an apple. The Sun would be a ball as big as a 10-story building more than 2 miles away. And if you were with the basketball, the nearby apple and the distant building would look to be about the same size.

To cover the distance between Earth and the Moon took Apollo astronauts 3 days. Now that's much longer than the 8 minutes it takes to reach Earth orbit, but still not uncomfortably long. Unlike Earth, the Moon does not have air or water, and without them, living there would be impossible. So the astronauts had to bring along their own air and water for the trip to the Moon and back, as well as their stay of a few days there. I was a child during those Apollo missions, and even then I was fascinated by space, just as I hope you are. It was thrilling for me to watch each one of those journeys that carried not just 3 men, but my imagination and dreams, as well as those of millions and millions of others. In fact, the amazing success in sending people to the Moon was one of the reasons I grew up wanting to work for NASA.

By studying the darker and lighter regions that you can see on the Moon, scientists discovered that they are made of different materials on the Moon's surface, just as Earth is covered with different kinds of materials in different places.

Be sure to take the time to go outside on a warm evening this month to gaze at the Moon, which has inspired so many scientists, writers, artists, and people like you and me who love to wonder about the Universe.

While you think more about the Moon, you can enjoy delicious Moon cookies made from ingredients easily available wherever you are on Earth. Go [here] to The Space Place web site at spaceplace.nasa.gov, and click on Friends Share to find the recipe created by one of our Space Place Library Partners, the Daniel Boone Regional Library in Columbia, Missouri.

Thanks again for calling Dr. Marc here at The Space Place.

I think I'm going to go have a Moon Cookie!

* On August 17, 2006, the International Astronomical Union redefined "planet," calling Pluto a "dwarf planet."