Q&A: Moonrise, Moonset, and Phase

Question: I’ve been observing the moon since the past full moon and trying to understand the relationship between moonrise and moonset. It made sense during the full moon and the waning gibbous moons; the moon would be up around 12 hrs. However, as we approached the last quarter moon and then during the waning crescent moons, the time the moon would be up grew shorter, around 8-9 hrs. Isn’t the moon supposed to be up around 12 hrs all the time? Actually, I’d expect to see the moonrise for the Last Quarter around midnight (that was right) but then the moonset around noon when in real life the moonset was between 9 and 10am. Could you please explain it? — AK via Gmail

Answer: What complicates things is that you’re not on the equator (presumably), and the Moon’s orbit is tilted with respect to Earth’s equator, and Earth’s equator is tilted with respect to the plane of its orbit around the Sun. Take a look at the complex geometry in my July 11, 2022 post.

Now if everything (Earth equator, Earth orbit, Moon orbit) were in the same plane, things would be a LOT simpler (especially for an observer on the equator). What we can say is that the Moon rises and sets (on average) about 50 minutes later every day. During that same 24 hour period, Earth has moved 1/365th of the way around its orbit, and that changes the angle of Earth’s equator relative to the Moon’s orbit.

How long the Moon is “up” on any given day also depends strongly on your latitude. Consider that, from above the arctic circle the Moon can be “up” for 24 hours (just like the Midnight Sun). As you move to lower latitudes, the Moon will be “up” for increasingly shorter times.

The diagram at top shows an extremely simplified geometry for phases of the Moon. The observer is on the equator, and Earth’s equator and the Moon’s orbital plane are both in the plane of Earth’s orbit. Now consider the observer at the 6 pm (sunset) position. If the phase were New, the Moon would be setting (as the rotating Earth carries you into darkness. First Quarter would be directly overhead, and a Full Moon would be rising. To see these patterns you have to put yourself into the diagram and visualize.

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Allow me to share an anecdote: Back in the 80s some friends and I hiked the Bass Canyon Trail down into the Grand Canyon. We reached the river just around sunset. From the bottom of the canyon you can’t really see the horizon, but over the canyon walls we could see the glow of the Moon rising in the east. I remarked to my pals, “Great! We’ll have a Full Moon tonight and that’ll help as we set up camp.” One of them asked “How can you know that before you see it? Oh yeah, you must have checked your astronomy charts earlier.” (He knew I was into astronomy).

I replied “No, when the Moon rises as the Sun sets, it’s gotta be a Full Moon!” I grabbed a stick and walked over to the damp sand on the south bank of the Colorado River, and drew a crude approximation of the graphic included in this post. “See?” I asked. They finally got it. I saw the pedagogical value of the diagram, so later, after our hike, I created a more sophisticated version of what I’d scratched into the sand. I added it to my Physics Sourcebook, page T-14. And now you can learn from it here.

Next Week in Sky Lights ⇒ April Un-Fools Day

Black Clouds
April Un-Fools Day