## Thursday, October 25, 2007

### Stripping the planetary gears

kw: musings, science fantasy

In a post in August (Geared-up Universe) I reviewed Jay Lake's novel Mainspring. Much action revolves around (giggle) the gear teeth at Earth's equator, and their meshing with a huge ring gear that forms Earth's orbit.

I couldn't resist doing the math. Earth's orbital velocity is more than 100,000 km/hr or nearly 2.6 million km/day, yet her equatorial circumference is 40,000 km (The meter was originally defined as 1/10,000th the distance from the pole to the equator at the longitude of Paris).

The disparity is a factor of more than 64:1. Thus in Jay Lake's universe, either the year has 23,500 very short days (22.4 minutes long, each), or the sun must be very dim, so Earth can reside a mere 2.3 million km away, rather than 150 million.

Now, consider dear Luna, which has a much longer day, and a much smaller orbit. Are they perhaps a better match?

The Moon's circumference is about 10,900 km. Her orbital circumference is 2.4 million km. Simple division yields just over 220; that is the number of days a ring gear would enforce. However, Luna revolves but once in traveling that distance. No ring gear is needed, just a simple arm to fix her distance, and keep her face pointed toward Earth. The day length disparity caused by the ring gear so visible in one scene of Mainspring causes a discrepancy of 220:1, then, greater than for Earth.

Well, that is what fantasy is about, making us, for a short time, believe the impossible. Readers of good fiction know what the Red Queen knew when she told Alice she was in the habit of believing seven impossible things before breakfast.

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