Moon Half Full Or Half Empty?

Mar 9, 2015

Credit photo by Mike Procell

This is “Looking UP! in southern Colorado,” from the Colorado Springs Astronomical Society. I’m Hal Bidlack, and there are lots of reasons to look up in the night’s sky!

If I asked you to guess where you can take a walk and know that your footprints will last for a very long time, what would you guess?

If you guessed the surface of the moon, you’d be right! The moon is the only place in the universe, other than the Earth, where humans have walked. Because there is no wind or rain on the Moon, the footprints left by the Apollo astronauts will last for millions of years!

There are four moons orbiting other planets in our solar system that our larger than Earths moon

    Because we are so used to the Moon in the sky we tend to take it for granted. But the Moon is a remarkable place, and it is fun to explore by telescope!

The moon may have been formed when an object the size of Mars smashed into the early Earth billions of years ago. There are four moons orbiting other planets in our solar system that our larger than Earth’s moon, but our moon is still pretty big, at about 1/4th the size of the Earth. And it is relatively close, so its gravity affects us every day, as tides rise and fall.

And in spite of what you might have heard, there is no “dark side” to the moon. It is true that only one side of the moon faces the Earth, because it rotates on its axis at about the same rate as it orbits the Earth. So while it keeps the same side toward us, whenever we have a “new moon” it means that the Sun is illuminating what we think of as the far side, and when we have a full moon, the Sun is lighting up the side facing the Earth.

If you could put a speedometer on the Earth’s rotation here in Southern Colorado, you would see that we are spinning at a speed of about 700 miles per hour. At the equator, it is closer to 1000 miles per hour. The Earth is about 24,000 miles around, spinning at 1000 miles per hour. That gives us a 24 hour day.

The moon, however, is rotating at a speed of only ten miles per hour! Therefore a day on the moon lasts as long as 28 Earth days it takes the moon to rotate around the Earth. And, due to the moon’s smaller mass, the gravity there is only 1/6th that of Earths. So, if you want to lose weight, get to the moon! If you weighed 150 pounds on Earth, you’d only weigh 25 pounds on the Moon!

If you’d like to take a closer look at the moon, or any of the other wonderful and amazing things in the sky, please visit or for a link to information on our monthly meetings and our free public star parties! 

This is Hal Bidlack for the Colorado Springs Astronomical Society, telling you to keep looking up, Southern Colorado!