Moon Wobble

We had several nights of clear weather around the full moon back in November, so I took pictures of the moon at each available opportunity. The moon cycles through its phases every 29.5 days, starting with new moon, filling in as a waxing crescent until it hits full, and then empties as a waning crescent until new moon.

I managed to capture images on November 12th, 13th, 15th, 17th, and 18th, starting at 80% waxing and ending at 94% waning. Seeing the progression of the terminator line across the moon’s surface is always interesting as craters near the terminator have deep shadows. I noticed when I tried to align the images that they never actually lined up. Strange!

Upon further research, I learned that the moon actually wobbles over the course of its cycle! The moon is tidally locked to Earth, meaning we always see the same side of it, but rather than being able to only see the 50% of the moon that is facing us, we can observe about 59% of the moon over the course of its cycle. To demonstrate this, I made sure all of the images were the same size (the moon’s apparent size changes, but we’ll get to that later). Next, I rotated all of the images to put Tycho crater in the same place - you can flip through the images by clicking on one below.

The wobble can be a bit subtle, so I’ve put the images from the 15th and 17th next to each other. By moving the slider, you can compare the two images. Pay particular attention to the top and bottom. The left image shows a crater on top that does not show in the right image at all and vice versa. Super cool!

The wobble is even more dramatic if we compare the first image of this set (November 12) to the last (November 18).

I mentioned apparent size and November’s moon happened to be a “supermoon”. This is a media term describing when the full moon coincides with being at the closest point in its elliptical orbit. Because of this, the moon will appear larger and brighter, although you won’t necessarily notice. Brightness is a difficult comparison to make with varied conditions and exposure settings, but we could observe the size difference since I am using the same camera, telescope, and Barlow lens to capture images. Or at least we could if we hadn’t had 4 consecutive supermoons since I got the 2.25x Barlow lens. I’ll add a comparison for January or February when the moon is further away. The next “micromoon” (when the full moon coincides with the farthest distance) is February, so that will be an interesting comparison! Stay tuned!

Useful Resources

• https://www.rmg.co.uk/stories/topics/how-far-away-moon: this has pretty much everything you would want to know about the moon.
• https://www.spaceweatherlive.com/en/moon-phases-calendar/2024/12.html: Shows moon phases over the course of the month including moon distance.
• https://science.nasa.gov/moon/moon-phases/: This describes the wobble and has an animation to demonstrate over a long period.