As the lunation draws to a close, the time between moonrise and sunrise gets smaller and smaller. This morning’s moon rose at 4:39 a.m. local time (EST), and the sun was only two hours and twenty minutes behind, rising today at 6:59 a.m. To those of you who are keeping track of definitions, the moon has only 50 minutes before the onset of astronomical twilight (defined as 90 minutes before sunrise).
Now, I was outside shooting the moon at 6:10 a.m., some 91 minutes after moonrise. In those 91 minutes, the waning crescent moon had only managed to get 16° above the horizon—not very far at all.
Now, as I understand celestial motion, the “normal” rate of elevation of an object on the celestial equator would be 15° per hour. So the moon “should have been” 22.5° above the horizon. Instead, it was a full 6 degrees behind schedule! Why would the moon take so much longer to rise today?
Well, for one thing, the moon is only on the celestial equator for a very short time each month. Remember that graph I made in an earlier post? If not, here it is again:
As you can see, the moon’s position above the horizon varies throughout the month and throughout the year; its position with respect to the celestial equator (its declination) will vary similarly, although perhaps not to the same extent as in the graph above, which is simplified to ignore the various irregularities in the speed of the moon throughout the month (as you’ll recall, it moves slightly faster when closest to Earth, and slightly slower when farther away).
This morning the moon’s declination was 24° S, well below the celestial equator. And the further you go from the equator, the more slowly you move across the sky.
Another thing that keeps the moon from getting to 22.5° elevation in the time one might expect is the retrograde motion of the moon itself in the sky; it’s slipping eastward by an average of 50 minutes per day as a result of its own orbital motion. (If it didn’t, the moon would always rise and set at the same time.)
And finally, there’s the egocentric view of things: the moon is rising more and more slowly just to annoy those people—like me—who would like to take its picture at this lovely slender crescent phase:
