Looking Up: Waiting For A Full and Sincere Apogee

September 5, 2016
Moon mirageCredit M. Procell
Moon mirage

This week on LOOKING UP we learn about the true distance of the moon from the earth.

Take a look at the moon tonight. Does it seem a little smaller to you? Well, it should. Because the Moon is very near its apogee point for September. What does Apogee mean? It means it’s the farthest distance the Moon gets from Earth in its orbit.

We usually say the moon is about 240,000 miles from Earth, and that’s true if you average out the distances involved. But the Moon does not orbit the earth in a perfect circle – rather, the Earth and the Moon orbit a common center of mass, and they do so in elliptical orbits. Therefore the distance, as measured from the center of the Earth to the center of the Moon, varies a bit over the course of a month. The farthest the Moon will be from Earth in September is 251,689 miles, and that happens tomorrow night. The closest the moon will be in September is on the 18th, when the moon gets within 224,872 miles. That’s a difference of nearly 27,000 miles, so if the moon looks a little smaller to you tonight and a little bigger and a couple of weeks, now you know why.

And did you know the Moon wobbles just a little bit? While it always shows, for the most part, one side to the Earth. It is “tidally locked” which is common in our solar system. For example, both of Mars’s moons are tidally locked and only show one side to that red planet and Jupiter has quite a few moons that only show one side to it.

But as it turns out, our Moon isn’t quite as tightly locked  as you might guess. There is a slight North – South rocking, and a bit of an East – West wobble, known as lunar libration. And that rocking and wobbling means that over the course of a month it’s possible to see as much as 59% of the Moon’s surface.

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 CSASTRO.org 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!