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Friday, March 13, 2015


The Celestial Equator (Earth's Equator projected on the firmament, Light Blue) Versus The Plane of the Ecliptic (the plane all the good planets use for their orbits around the Sun, Red) They are fixed relative to each other. The line of intersection is marked by the yellow ray, which is pointing at the Autumnal Equinox.
Stu corrected my estimation of the gravity on Cere, so I start checking into it and right off I run into
"Earth's rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day, is 86164.090 530 832 88 seconds of mean solar time...'
The 'misnamed' sidereal day? What? Why would you put an incorrect name in an encyclopedic definition? Jerks. Well, now I've got to look up sidereal day, which is okay, because what I am after is sidereal rotation period. I mean, I've been hearing this word sidereal for like forever, and I know it's got something to do with time because the Earth rotates relative to the sun and that's what determines our normal everyday kind of time, but the Earth also rotates around the sun, so like a planetary gear we've got one extra rotation every year.

The sun gear is red and is fixed when we start this video clip. The planet gears roll around it. The ring gear is traveling in the same direction as the planets but because this is a video, much of the time it appears to be going backwards, much like the wheels on the stagecoaches in Western movies.

The direction the Earth spins is the same as if it were rolling around the surface of the sun, which maybe why the central gear in a planetary gear system is called the sun gear, and the gears that roll around it are called planet gears. Anyway, back to sidereal.
    Basically, my assumption about the sidereal day was correct. Because the earth goes around the sun, in absolute terms it makes one extra rotation a year, so you divide 24 hours by 365 days and you get (24*60/365) about 4 minutes a day. That's not exact, but it's close enough for this diatribe. A sidereal day is 4 minutes shorter than a regular old day. You can see the same star in the sky with your telescope every night, without moving the scope, if you just look 4 minutes earlier every night. Eventually it will be daylight when you look and that's because now the Earth is on the opposite side of the sun and you will have to wait 6 months to be able to see it at the same position again.
    The problem with the so-called "sidereal day" is because it is measured relative to the Vernal equinox, indicated by the yellow line in the picture at the top, and Vernal equinox is not fixed relative to the firmament. It is also moving, albeit very slowly, one degree every 72 years, which is not enough to make a difference to backyard astronomers, but for people who make a living out of looking at stuff in the sky, well "one degree every 72 years" is not nearly accurate enough.
    So there are regular 'solar' days, sidereal days, and stellar days. The difference between a sidereal day and a stellar day is about 8 milliseconds and only matter to people who care about things like precession of the poles.

P.S. You will excuse the extraneous jaunt in gear land, but I just love this stuff. I was just going to put up a still image of a planetary gear set, but then I found a really cool animated gif, but it held the ring gear stationary, so while it was very cool, it didn't convey the correct message. So then I started looking for one that held the sun gear stationary and this video is the only one I found that did that. Keep watching and it will show you several modes.

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