Planetary orbital foci

Planetary orbits are not perfectly circular; in fact, they are ellipses. An ellipse is a mathematical shape approximately equivalent to what is typically called an oval. An ellipse, though, meets some very specific criteria. One is that, unlike a circle, it has two foci instead of a single center. Where a circle is defined as the set of points whose distance to the center are some constant distance (the radius), an ellipse is the set of points whose distances to the two foci add to a constant. This allows you to construct an ellipse with some pins, some string, and a pencil.

So what are the two foci of planetary orbits? Well, one is the Sun. The other one? Just some random spot in space. And because each planet has a different size orbit with a different eccentricity (a measurement of how non-circular the orbit is), each planet has a non-Sun focus in a different place. Here is a Google doc spreadsheet with information on each planet’s orbit. A visualization after the jump.

Venus and Earth have very round orbits. Mercury’s is surprisingly un-round – nearly as eccentric as (although much smaller than) Pluto’s.

Astronomy Biathalon


SPORT NAME: Astronomy Biathlon SUMMARY: Each competitor must run from the starting line, following a predefined route of approximately 1600 meters carrying a telescope of their choice. The route will end at a Sighting Area. The competitor must set up the telescope in the Sighting Area and accurately find and identify three (3) astronomical objects listed on a list provided. Then he or she will break down the telescope, and run the next route to the next Sighting Area. There will be five (5) Sighting Areas in all, followed by a final 1600 meter run to the finish line. Sixty (60) seconds will be added to a competitor’s time for each object not correctly identified. The competitor with the shortest final time will be the winner. Mixed genders.

POSSIBLE VARIATIONS: 4x1600 Astronomy Biathlon relay; Astronomy Biathlon medley (galaxies at first Sighting Area, binary star systems at second, etc); Astronomy Triathlon (routes alternate 1600m runs with 5km biking)