Shallow Sky Object of the Month: Moon Dance

Original article appears in GuideStar May, 2012.

The moon setting just before mid-night on May 25, 2012. From TheSkyX

Object: The Moon
Class: An event, actually a non-event
Optics needed: You can see the moon rise and set without optical aid.

Why this is interesting:

We tend to think about the moon rising and setting each day. Most of us who like to get out and observe under dark skies are aware of moon rise and set times. I was just checking the weather reports for the week and noticed that on this coming Thurday (April 24), there’s no moonset. How can that be?

The moon appears to move to the west in the sky because of the eastward rotation of the earth. This is true with the stars, planets, the sun and all celestial objects.

The moon setting after midnight on May 27, 2012. From TheSkyX

But the moon, of all the major objects in the sky is one that, on its own, moves a significant distance eastward in the sky each day. We see evidence of this because of the change in phases of the moon. This movement changes the relative position of the earth / moon / sun system.

How far does the moon move across the sky every night? Things are never simple, but we can simplify this enough to come up with a good approximation. The lunar cycle (new moon to new moon) is 29.5 days. So, the moon makes a complete trip around the earth, relative to the sun, in 29.5 days. A complete trip is 360 degrees, so 360 degrees / 29.5 days = 12.2 degrees / day. The sky moves 360 degrees (relative to the sun) in 24 hours or 15 degrees per hour. So it’s easy to determine how long it is between successive moonsets 12.2/15 = .81, and .81 * 60 minutes of time = 48.8 minutes.

If the moonset time on day 1 is less than 48.8 minutes before midnight, there will be no moonset time on the next day. The moonset will occur early on day 3.

And, in fact, on this Wednesday, the 25th of April the set time is 11:52 p.m. On Thursday, the 26th, there is no moon set. Finally, on Friday at 12:37 a.m. the moon sets. If you ask about the difference in set time due to the movement of the moon it’s 8 minutes + 37 minutes = 45 minutes, not far from our back-of-the-envelope calculation. Actual results depend on other things like the angle of the moonrise and moonset, whether the moon is at apogee or perigee, and probably some things that I haven’t thought about.

The geometry of this is such that the ‘no moonset’ day occurs around first quarter and a ‘no moonrise’ day occurs around last quarter. In May…

May 9 — no moonrise this date. On May 8, the moonrise is at 23:10, and the next moonrise is on May 10 at 00:02 (local time).

May 26 — no moonset this date On May 25 moonset is at 23:58 and the next moonset is on May 27 at 00:36

By the way… I’ve had to consult several resources to confirm these times. Some sources list moonset on May 26 at 23:58, which is not correct. The correct answer is that there is no moonset on May 26!

So… check this out. Observe the moonset on May 25 and the next moonset on May 27. These will be 24 hours + 38 minutes apart by the clock, but two days apart by the calendar!

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