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

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.