Orbiting at 19 miles a second, so it's reckoned...

December 3, 2009 2:11 AM

Oh, so that's how that works. I never got the whole 'Mars in Retrograde' thing. This really helped. More than this description by Dr.Feynman of the elliptical orbit of the planets, and definitely more than this song-and-dance description of, well, the Universe.
posted by From Bklyn (18 comments total)

This is brilliant! I've run it along until 13142 AD, and it makes me feel a bit giddy just thinking about it.
posted by tawny at 2:27 AM on December 3, 2009

Here's a page with some nice gifs showing Mars in retrograde!
posted by Greg Nog at 2:50 AM on December 3, 2009

Nice to see that the planets aren't aligned on Jan 1, 2012.

And see! They think Pluto is a planet! ha!
posted by Severian at 5:29 AM on December 3, 2009

Nice app. But, tawny, I don't think the app takes into account all the changes that post-singularity super-intelligences will make to the solar system between now and 13142 AD. ;)
posted by aught at 5:45 AM on December 3, 2009

I don't think the app takes into account all the changes that post-singularity super-intelligences will make to the solar system between now and 13142 AD.

It opens a popup that tells me to stop using Internet Explorer?
posted by rokusan at 5:48 AM on December 3, 2009 [2 favorites]

From Bklyn, thanks for this -- a really nice educational tool.
posted by Killick at 6:08 AM on December 3, 2009

The full insanity of retrograde observed motion is apparent in De Dondi's Astrarium, a 1340s clock that included dials showing the position of the planets. The dials show an earth-centric view of where the planets are, requiring elaborate clockwork to simulate the bizarre dance.

de'Dondi's dial for Mercury uses a number of intermediate wheels, including: a wheel with 146 teeth, two oval wheels with 24 irregularly shaped teeth that meshed together, a wheel with 63 internal (facing inwards) teeth that meshed with a 20 tooth pinion. The 63-tooth wheel made one rotation a year, non-uniformly because of the oval driving wheels, and made the main indicator wheel rotate through 63/20 × 12 signs of the zodiac each year.

It's an absolutely marvelous bit of engineering, working so hard to model the universe with the awkwardness of having the wrong theory. There's a working replica at the watch museum in La Chaux-de-Fonds. It's pretty amazing.
posted by Nelson at 7:17 AM on December 3, 2009 [1 favorite]

What was the explanation for retrograde motions in the earth-centric theories?

I'd be hesitant to call those 'wrong' theories. They were theories, and they apparently were able to describe planetary motions accurately. Seems like it it what it was meant to do.
posted by Harry at 8:12 AM on December 3, 2009 [1 favorite]

Nelson, thanks for that reference to De Dondi's Astrarium...

WANT!!

Want to make one, and put it on display at next year's Pennsic A&S!!!

Gack! The 14th-century recreator and the engineer in me are colliding in a geekgasm!
posted by IAmBroom at 9:59 AM on December 3, 2009

What was the explanation for retrograde motions in the earth-centric theories?

Greg Nog's link has an animated gif showing this.
posted by Rhomboid at 10:23 AM on December 3, 2009

The De Dondi Astrarium is amazing (and the site it's on is also very cool).
posted by From Bklyn at 11:27 AM on December 3, 2009

Reminds me of one of the views in Microsoft Space Simulator.
posted by clvrmnky at 12:11 PM on December 3, 2009

I clicked on things and poked around and I still don't know what retrograde means. Where in this app does it make it so clear?
posted by rahnefan at 1:30 PM on December 3, 2009

Long trails...ends up looking like a spyrograph as you add planets
posted by OHenryPacey at 1:33 PM on December 3, 2009

...and zoom in and reduce planet scale
posted by OHenryPacey at 1:34 PM on December 3, 2009

I clicked on things and poked around and I still don't know what retrograde means. Where in this app does it make it so clear?

rahnefan, it's there, but not exactly clearly pointed out.

In a nutshell: if you observe the position of planets (say, by taking a photo every night at the same time, pointed at the same spot in the sky), eventually you will see a planet "reverse direction". Jupiter will be traveling (say) east to west on days 1-344*, but from days 345-359, it will go west to east. Then, starting on day 360, it will go east to west again, for many months at a time.

The short reversal period is "retrograde motion" - retro to the expected direction.

It happens because the planet (Jupiter, in this example) is not traveling around the earth, but the sun. Since they travel at different rates, the earth actually "laps" Jupiter in these races. Halfway around the track, Jupiter stops appearing to go "left to right" and starts appearing to go "right to left" for a while (until it "rounds the bend" with earth).

My explanation has a few holes & gaps, but maybe it gives a thumbnail explanation.

* All number made up.
posted by IAmBroom at 2:26 PM on December 3, 2009

I wish there was a way I could favourite this harder. Thank you for posting this.
posted by motty at 5:56 PM on December 3, 2009

surely if it make it go fast enough, the planets eventually either spiral out of orbit or crash into the sun. commoonn....
posted by Dillonlikescookies at 10:15 PM on December 3, 2009