04/26/2026
Last night we witnessed a lunar occultation of the bright star Regulus in the constellation Leo.
These occultations are interesting as they give us an opportunity to observe the moon's proper motion in the sky versus its apparent motion.
As the moon rises in the East on any particular evening, over the course of the night it appears to be moving to the West. But that's only because of the rotation of the Earth moving in the opposite direction from how the moon appears to be traveling. That is the moon's apparent motion.
The moon's proper motion however, is actually moving the opposite direction, to the East. That's because the moon relative to the Earth orbits in an Easterly direction.
We can measure the moon's proper motion however against background stars, as they also appear to move to the West as the Earth rotates.
But as the moon orbits around the Earth, it will slowly pass in front of those background stars enabling us to visually measure how far it moves. As it turns out, it moves about one moon width per hour.
When a bright star passes behind the moon, it gives us better opportunity to observe and measure this proper motion.
Last night Regulus disappeared behind the dark edge of the moon, on the left around 8:20, and about an hour later, appear out from behind the other side, on the right.
This demonstrates how the moon is orbitng in the oposite direction than it appears to move on a nightly absis.
Did you see it? Let us know.
Photos by CAS member Jun Lao.
