Surely the Moon is boring because nothing ever changes on it? Not at all! As the Moon orbits the Earth every month the features change dramatically as the angle of Sun hitting the surface varies and casts strange and beautiful shadows across its landscape.
Of course nothing much actually changes on the surface of the Moon itself (not totally true – new craters are still being created by impacts) but the appearance of its features change daily. It’s visible for a lot of each month so make the most of it. It’s also a good object to start your astronomical journey with because it’s so bright and easy to observe and your first views of the Moon through a telescope are mind-blowing!
A couple of years ago I decided to follow one prominent feature – the crater Plato – through several lunar phases (this took several Lunar cycles to do because of weather). As you can see from the main image above, Plato changes dramatically as the sunlight strikes it at different angles. The phases where the crater is closest to the terminator (the divider between Lunar day and night) are the easiest to observe and image, Lunar morning and evening, because shadows are more prominent and the features clearer. (Seeing conditions also changed through the observing cycle making some phases clearer than others.)
The first image above shows the sunlight striking the eastern wall of Plato, making it bright white, with shadows on the crater floor cast by the western wall. The third image shows the opposite effect. The central image however shows the sun overhead the crater, causing no shadows at all and making it very indistinct.
Plato animation, Mark Phillips
My intention was to create an animation showing how it changed with different illumination. But there was a problem: Libration. The aspect and shape of the crater appears to change throughout the month, making an animation very difficult to put together accurately. This is what I managed:
Libration is when the Moon appears to change its orientation towards us, showing us more of each limb or pole throughout a lunar month (see animation). So although the Moon keeps only one face towards us, you can actually see about 59% of its surface, just not all at once. The 3 images above show the change in shape of Plato from an elliptical to a more circular shape because of the changing angle it presents to us – caused by libration.
There are 3 types of libration:
Lunar Libration (Wikipedia, Public Domain)
- longitude – caused by the elliptical orbit of the Moon
- latitude – caused by the inclination of it’s axis of rotation to the plane of its orbit around the Earth
- diurnal – a small daily oscillation caused by the Earth’s rotation and our position on it (we move from one edge of the Earth facing the Moon to the other edge)
We have a number of Lunar features in our ASE-24: Observing list for beginners, I also did an article on the Lunar 100, if you want to take Lunar observations further and we also have our own ASE Lunar 100 Project.
There are so many interesting geological features on the Moon that change their appearance throughout the Lunar month (interestingly the definition of the Lunar month varies quite a bit). Have a look at them with whatever optical instruments you have. It’s very rewarding.
Article and images: Mark Phillips
Originally published 24 November 2019