Image of Copernicus Crater, courtesy NASA

Last week all eyes were on the sun. This is a good week to look at the moon.

The first thing you’ll see is billions of craters, sights of impacts of asteroids and comets from the sky which blast out huge basins. The craters were formed when comets and asteroids slammed into the surface and exploded. Comets and asteroids are not made of explosive material, but even an inert chunk of ice or rock explodes when it is traveling at hypersonic speeds and then stops suddenly. The object’s energy of motion is abruptly changed into heat, and enough heat is generated to vaporize the impacting object and a great deal of whatever it hits. An object the size of a large house traveling at 100,000 miles per hour creates a crater a mile across in whatever it hits. This is how craters formed on all the planets and moons. The earth has craters too, but erosion soon erases them here while on the airless moon craters remain fresh for hundreds of millions of years.

They exist by the billions, but they’re not spread randomly. Most are in the lighter highlands, while fewer are in the darker smoother seas.

Because everything there is essentially the same gray color, shadows reveal topography. The boundary between the lit side of the moon and the dark side is called the terminator. The terminator is a constantly moving line that runs from near the moon’s North Pole to South Pole, and it marks the points where the sun is rising or setting. Between new moon and full moon, the terminator marks the sunrise line, and between full moon and the following new moon the terminator marks the sunset line. The terminator moves across almost one-thirtieth of the face of the moon each day, and each night it is 12° from where it was the night before. Follow the terminator during the lunar month and watch it reveal, though shadows, a changing procession of craters, seas and mountains. Each feature is visible well for only a few days before the sun is high in its sky and its features become “flattened” by the lack of shadows. The full moon is the worst time to moon gaze because there are no shadows then (although that is when the white rays that radiate from major craters show up best).

Perhaps the most beautiful crater is Copernicus. It is one of the moon’s youngest major craters, since it formed “only” 900 million years ago. Its walls are as high as the Rocky Mountains. Copernicus’ rays, which are made of rock debris violently thrown outward during the impact, are conspicuous when the sun angle is high near full moon. Tycho is also conspicuous and beautiful. Notice the chains of smaller craters radiating away from it.

The moon is at 1^st quarter phase on the 21^st.

Stellar Vista Observatory provides portable telescopes and tripod mounted binocular kits on loan for free to residents and visitors in Kane County. Enhance your enjoyment of the night sky! To learn more, request a loan, or attend one of SVO’s free public star parties for 2023, visit https://stellarvistaobservatory.org/ or drop in to the Kane County Office of Tourism. Everyone is welcome!