Let's talk about the Moon

In all the years I have been writing this column, I have never devoted an article specifically to the Moon. Of course, the Sun and the Moon are the most common objects most people are familiar with as they look at the day and night sky, so it is about time to focus on the Moon. I have recently become interested in the Moon again after many years of relative neglect on my part. To many amateurs, the Moon is sort of the vanilla of astronomical objects. But ... this may not be true for many of you.

The Moon is about 1/3 the size of the Earth and has an average distance from us of about 240,000 miles, give or take. I say 'give or take' because the Moon's orbit around us is slightly elliptical rather than circular. This means that sometimes we see the Moon a bit larger in apparent size and a bit smaller in apparent size, depending on where the Moon is when we see it. Because the Moon is about 1/3 the size of the Earth, its gravity is about 1/3 the size of the Earth's. Many of you may remember images broadcast from the Moon showing astronauts waltzing about on the Moon's surface in a sort of slow glide. This glide was the result of the low gravity compared to Earth's.

The Moon is a geologically inactive rock ball, made of the same rocky materials as Earth. However, the Moon has not always been geologically inactive. In this article, I have included two images of the Moon I made about a month ago and these images show two distinct eras of the Moon's history. In one of the pictures, one can see that the Moon has been very heavily cratered. Cratering still occurs in a minor today, but much less intensely than in the remote past. Most of the big rocks that have caused the cratering have been swept out of the Moon's orbit long ago. The Moon has taken a lot of hits from big objects that Earth might have taken if the Moon were not there. Many of the Moon's craters have central peaks. This is because when large rocks struck the Moon, they liquefied the Moon's surface at the collision points and then the underlying molten rock rebounded and solidified as central peaks.

Later in the Moon's history, there was a period of intense volcanic activity. When you look at the full Moon, you will see many dark areas. These are lava fields left over from that period of volcanic activity. If one looks carefully at the second photo I have included, you will see that the surface of the Moon is darker than the cratered area, smoother, and many of the bottoms of many craters have been filled in when the lava flow. We know that the extensive outpouring of lava from the interior of the Moon occurred mostly after the period of meteoric bombardment because otherwise, many large craters would not be filled with the solidified lava. If one could have seen this volcanism from Earth, it would have been quite a spectacle to see active volcanoes spewing lava onto the Moon's surface when this occurred. Some of this could have been seen with the naked eye.

September will be another good month to view the Milky Way and Saturn will still be visible in the southwest when the sky is fully dark. Soon, from our perspective, Saturn will pass behind the Sun and we will see the ringed planet at dawn, along with mighty Jupiter. Autumn is coming on and some of these cooler nights will be ideal for viewing the night sky.

-- Dr. David Cater is a former faculty member of JBU. Email him at [email protected]. The opinions expressed are those of the author.

Community on 09/02/2015