The Moon at 9.7 days

We refer to the age of the moon as the time since the last new moon, when it is between the Earth and the Sun and hence not visible (unless there’s an eclipse, when its silhouette is visible). In this image the age of the moon was 9.77 days.

The image is thus very similar to that on my post of July 2018, taken with the same equipment when the moon was 10.3 days old – about 12 hours later in the lunar month. But looking closer there are a few differences in the images.

In the earlier image the morning Sun is illuminating a little more of the Oceanus Procellarum, to the west of the large crater Copernicus (since the Apollo missions, east and west directions on the moon are given as they would be on earth, with west to the left as seen with the naked eye; this differs from the direction ‘on the sky’, where the moon’s eastern edge would be the last to rise in the east).

Another difference is that, due to the libration of the moon, the circular* Mare Crisium on the moon’s western limb is rotated more into view in the earlier image, and the Mare Marginis and Mare Smythii can just be glimpsed on the western horizon, whereas in the current image they are virtually invisible. (Libration refers to the apparent ‘rocking’ back and forth of the moon as it speeds up and slows down slightly during its travel along its elliptical orbit.)

*Mare Crisium is in fact oval but appears circular as it is foreshortened by its position near the moon’s limb.

What should be the most obvious difference between the two images is, in fact, barely discernible: the 2018 image was monochrome whereas this image is in colour. On second glance some shades of colour can be made out, for example in the Sea of Tranquillity which suggests a dark blue-grey, but to quote Jim Lovell on the Apollo 8 mission, “The moon is essentially grey”.

Equipment and processing:

Video frames taken on 21 April 2021 through Baader R, G, B and UV/IR-cut L filters, using a Takahashi TS-120 refractor with a focal length of 900mm and a ZWO ASI174 monochrome camera. Each video was processed with AutoStakkert! 3 and Registax 6, stacking the best 200 frames from each filter. Images aligned in PixInsight and finished in Photoshop.