Why are there phases of the moon?

i was at a dinner party with 8 other educated people who have continued to explore their world beyond their work lives and this topic came up. I’ve noticed that most people aren’t aware the moon rises 45 minutes later each day, until it is actually rising in the day, then it synchronizes to earth rotation again and it rises as the sun sets at the full moon.

I couldn’t describe it all because I don’t have it straight in my head either. I looked it up in Wikipedia and there’s a bunch of stuff about perigees and ecliptics. So, no shame, like I said, this was 9 smart people and we couldn’t describe it. Test yourself. Why are their moon phases? Why is the moon sometimes up in the daytime? Other than using an app, how do you know where the moon will be tonight?

Think of taking a photograph of someone in the sunlight.
If They face the sun, your back is to the sun, and they are fully lit. (full moon)
If their back is to the sun, you are facing the sun, and the part of them you see is in their own shadow (new moon)

The part about sunrise and moonrise/set would be you spinning around. When your back is to the sun it is night. When you face the sun it is day.
If you both face the sun, you won’t see your subject because they are behind you.
Remember, the moon is tidal locked, the same side is always facing the earth - your subject is always facing you.
The full moon is at night.

If it is a new moon on Earth, it is a Full Earth on the moon.

I use the mnemonic DOC to remind myself if we’re headed to a new moon or full moon.
“O” is the full moon. The crescent moon with the opening to the left is a “D” to the right looks like a “C”

Now, why does the moon (or sun) look bigger when it’s closer to the horizon?


DOC, love that.
On the way to my house, there’s a hill, and at certain times, you see the big moon illusion and it suddenly gets small once you come over the hill.
The part I looked up after writing this is, the moon traces the low in the sky to higher longer arc pattern just like the sun, but it does it every month.


I’m at an advantage. At home, small bedroom, big bed, leaves me with a >2’ passage on my side of the bed and the wall.
Upside is that the wall is mainly two big windows with (besides a not too big tree) an unbroken view to the sky to the south of us. Given I usually get up a couple times a night, means I get to ponder that moon nearly three weeks out of month.

Pretty cool for sure. I know from my time in big cities, I’d get a glimpse of the moon every once in a while, so there’s little chance to get any rhyme or reason out of it.

What’s really trippy and that I’m still wrapping my head around is how much the moon moves up and down the horizon, at moon rise and set.

It does the same thing as the sun and like the sun, it has a moon rise and a moon set. What trips me out, though I think I’m starting to understand, is who we allegedly never see the opposite side of the moon.

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`Yeah, but the moon up and down along the horizon is much more dynamic and complicated. Now you’ve inspired to look it up, rather than just wondering at it.

How does the direction of the moonrise change?

October 2, 2020 by Admin

The Earth rotates counter-clockwise on its axis (picture a spinning top). Because of this motion, celestial bodies such as the Sun, Moon and stars appear to rise in the eastern sky and set in the western sky. In addition, the Earth is tilted on its axis by 23.5 degrees, relative to the plane of the orbit of the Earth around the Sun. Because of this, the Sun’s rise and set positions vary by up to 23.5 degrees north or south of due east or west throughout the course of a year (the orbital period of the Earth around the Sun).

The Moon’s pattern is close to that of the Sun’s. It orbits the Earth on a plane that is approximately 5.1 degrees offset from the orbital plane of the Earth around the Sun. This causes the position of the moonrise and Moonset to vary up to 28.6 degrees north or south (that’s 23.5+5.1 degrees).

The Moon orbits completely around the Earth in 28.5 days, about once a month. This causes the Moon to move through its 28.6 degree range of variation much quicker than the Sun appears to, creating a noticeable position change against the horizon each night. The Moon also doesn’t rise at the same time each night. Due to the speed of Earth’s rotation and the Moon’s orbit, the Moon rises about 50 minutes later each day. Interestingly, all these changes in relative position to the Sun make the Moon appear to go through its waxing and waning phases.

There is a nice animation here that shows the rotation of the Earth, and the relative timescales of the Earth and Moon orbits. …

I’m not trying to prove anything here, other than anecdote about my own life, where I’ve brought up the subject of the moon, it’s phases, it’s rise and set, and almost no one has been able to explain it in simple language. More often, I’m challenged on the basic facts, like it rises 45 minutes later each night.

My brother had to find an archeological site that had been worked on in the late 1930’s and then reburied to preserve it. He looked at their notes and they used the sunrise on the equinox to determine east, so he repeated that. He’s the only person I’ve seen look up in the sky and use his fist to measure degrees and tell you what time it is. To do that, you need to have a good idea of how many hours the sun is up that day. He’s said things about the moon too, but I never remember them.

The moon is harder I think, because if you aren’t watching it daily, you won’t know what phase it’s in. With the sun, you know when it’s spring. With the moon you have to know the difference between waxing and waning, what that means for the length of rise to set that day, and where and when it’s rising and setting. The sun is always pretty close to where it did all those things the day before, and, you can’t miss it most days, so you can’t help but be aware of it.

Pinhole camera tracing the path of the sun over 110 days.


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Kevin, who did one of the early $1K solar water heating systems and lives WAY up north in British Columbia has been working on Solargraphy, which is really fascinating.

Well that explains why it’s so radical. Very cool, thanks @mrmhead.

Now that sounds like someone worth going hiking with.