Friday, November 10, 2006


How Many?

Thanks to Linternaute I discovered a really interesting service: Fourmilab, using images provided by LivingEarth and NASA, and atmospheric observations provided by UW-Madison, enables one to request images of the earth (or the moon) from various vantage points. For example (the one they give on the "Custom Request" page), "Earth from Moon: current cloud cover" :
Earth from Moon: current cloud cover

Now, from what I've read it seems that what is actually provided is not really the view from the satellite, but a view constructed from a stock of images (Living Earth, NASA Visible Earth, topographic maps) from the point of view of the current position of the satellite. The cloud cover and water vapor views are "real" however, inasmuch as they use observations that are updated several times each day. The NASA images are updated monthly (at least) to get snow cover right.

The view computed for 200 km above my head (47N 7E
Living Earth Image NASA Visible Earth Image
Current view from 200km above 47N 7ECurrent view from 200km above 47N 7E

Better and better -- one can choose the view from a particular satellite. An example they give is "the current view from one of the satellites in the Iridium constellation". There is a list of satellites from which to choose. I have no idea which satellite is in an interesting position. Shall I try them, one after the other? How many are up there? Hundreds!

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