Analemmas

[cjsmith]

I think the lines on a sundial should be long thin figure-eights.

If the moon has earthalemmas, what shape are they?

I wish moon inhabitants could use a sundial during the 2-week "day" and an earthdial during the 2-week "night", but sadly for my poetic notions, that wouldn't work at all. The Moon People threw away all their earthdials generations ago, cursing the slick salesman who had sold them. He lived out his life in bitter exile.

Comments

[deyo.livejournal.com]

If the moon has earthalemmas, what shape are they?

Sort of an egg-shape, with a smaller Earth toward the point. (http://antwrp.gsfc.nasa.gov/apod/ap991108.html)

And the Earth wouldn't pass through a point on the geolemma during the day, it would move smoothly through the geolemma during the month. :)

[cjsmith.livejournal.com]

Thanks for that link! I was visualizing that very thing, that animation, while I was musing on all this stuff. But I never would have known where to go look for it again. :-)

[shadopanther.livejournal.com]

I once attempted to make a moondial out of a stick stuck in the ground and watching as the moon's shadow passed through scratch marks I had made in the soil. I'm not sure how well it worked to tell time (as I didn't work out any equations or match up scratch marks with time on my watch) -- but it was a bit of fun one backpacking trip.

[cjsmith.livejournal.com]

Moonlight across the soil, filtered through the trees - sounds beautiful, whether or not you had the time to make it accurate.

[quasigeostrophy.livejournal.com]

He lived out his life in bitter exile.

They send him to Jupiter? :-)

[cjsmith.livejournal.com]

But that would be jovial exile! He was all sad!

[selenite.livejournal.com]

Inhabitants near the limb would see Earth hanging near the horizon and could tell the time of month by how it moved relative to the skyline. Precision would probably be +/- a couple of days, though.

[cjsmith.livejournal.com]

Ooo, good point!

[hitchhiker.livejournal.com]

Now I want to write "Analemma" TTTO "Anatevka", as a sort of companion piece to this (http://zem.novylen.net/filk/earthrise.html).

The haircut's definitely growing on me, btw :)

[cjsmith.livejournal.com]

Oooo, that'd be a great companion piece! (And thanks! It's growing on me, too -- I have to keep getting it cut!)

[selenite.livejournal.com]

Ooooh. I'll be singing that at our next housefilk. Nice.

Earthalemmas

[fyellin.livejournal.com]

I don't think there is a simple earthalemma.

The analemma is basically a combination of the Earth's elliptical orbit around the Sun and its tilt. A different version of the analemma looks like the sum of two sine waves, one with period 1yr (elliptical orbit) and one with period .5 years (tilt).

The moon's orbit around the earth is way more complicated. The moon is close enough and large enough that several second order effects become significant.

Re: Earthalemmas

[cjsmith.livejournal.com]

Plus there's the complexity of adding the day-rotation to it, a thing with (seemingly) no analog in the moon's orbit around the earth.

Darn, I was so hoping for a nifty simple little shape like an oval.

Re: Earthalemmas

[fyellin.livejournal.com]

You might also need to define what exactly you mean by an earthalemma.

The real meaning of analemma can be found by looking at the picture at http://epod.usra.edu/archive/epodviewer.php3?oid=228384 . Take a picture of the Sun at the exact same time every day (remembering to ignore daylight saving time) and the Sun draws a large figure 8 in the sky in the course of a year.

What would an earthalemma mean? The moon doesn't rotate on its own axis. There is no such thing as a "day" on the moon. You can cheat and say you really mean a 24-hour period on Earth, but that's completely arbitrary. Earth's analemma has nothing to do with the Sun's rotation.

So all you're left with is the moon's wobble. I vaguely remember reading that roughly 60% of the moon's surface is visible from the Earth at some time or other. But the equations dealing with that are extremely complex.

Re: Earthalemmas

[cjsmith.livejournal.com]

You might also need to define what exactly you mean by an earthalemma.

Yes, exactly; without the "day" rotation, the analogy does not work well at all.

So all you're left with is the moon's wobble.

Right. Knowing the shape of the wobble is what I was thinking of (if I could be said to have been thinking at all), and would be cool.