Sunday, September 23, 2007

Fall equinox

It's the autumnal equinox today, the day when the Sun appears to cross over into the southern sky. The stirrings of winter are beginning--little reminders of the fragile nature of our comfort zone on this planet. This last week in the evening twilight the first-quarter moon barely peeked above the trees and buildings to the south, showing roughly where the Sun would be in three months time. It seemed a little low for the winter noon, though, so I checked and indeed the Moon is at its greatest distance from the ecliptic, some 5 degrees south. I have some discussion of the tilt at this previous post. And, to confirm it, there was a lunar eclipse in late August, meaning the nodes (the points where the Moon's orbit meet the ecliptic) of the Moon's orbit were aligned in the Earth-Sun line, which meant my first-quarter moon should have been above or below the ecliptic.

And it now occurs to me the root of eclipse and ecliptic are the same, a point I never realized. This will require another post.

Tuesday, September 18, 2007

Radiation doses from above ground nuclear testing

Estimated gamma-ray radiation doses from above-ground nuclear testing in Nevada, as of 1957. Above-ground testing continued, at a higher pace, until 1962.

This doesn't include radiation from non-gamma sources, including iodine-131, as shown here.

I personally spent a lot of time in the 2-4 Roentgen range as a kid.

Tuesday, September 11, 2007

French whine about spy satellite disclosure

Via an article in the Inquirer and the original article, the French military is threatening to publish US spy satellite orbital data, which has been left out of the official US catalog of orbiting objects. Amateur satellite observers find this stupid and just figure out the orbits themselves. What the French really want is a quid pro quo agreement to not publish French spy satellite orbits. But of course, all it takes are a couple of people with clocks watching the sky to figure the orbit, so why hide the orbit elements in the first place?

The French discovered these uncataloged satellites via radar returns. The US has a fence of VHF radars across the southern US, known as NAVSPASPUR, that can detect nearly every satellite in LEO orbit over a short time period. Amateurs can pick up the radar returns from meteors and other objects from these transmitters.

Tuesday, September 04, 2007


Emily Lakdawalla at the Planetary Society Blog has spliced together Cassini images of Rhea and done a fantastic job of creating a massive image of a crescent Rhea.

It's really big, and really awesome. This deeply cratered moon is the second largest Saturnian satellite, with a bright (but not snow white) albedo.

Sunday, September 02, 2007