Monday, February 26, 2007

The Northern lights

Courtesy of the wonderful John C. Baez again (, here's an incredible time-lapse movie of the Northern lights (Aurora borealis), filmed on September 24th 2006 in British Columbia, Canada:

One question I have though: why are some of the stars rotating round through the course of the night, whilst other, fainter ones remain fixed?


Anonymous said...

Great link, Gordon.

Anonymous said...

I agree. One of the things I want to witness before I die. Yes, definitely, before I die. It is magical, other-worldly and beautiful.

Art Durkee said...

Speaking as a veteran sky watcher, when I go camping every August up in northern Minnesota, 20 miles from electricity, etc., etc., watching the aurora:

those are most probably satellites. The skywheel's rotation of the fixed stars is as I'm sure everyone knows because of the earth's rotation. But there's a ton of junk in orbit these days, including decommissioned satellites. That's what it looked like to me, anyway.

Gordon McCabe said...

Interesting idea Art. Would there be that many geostationary satellites within the field of view, and visible to a camera? I just wondered if the video was actually a composite of a number of different exposures.

Art Durkee said...

The video didn't seem like a composite to me, although I personally could, with the right high-end software probably make something that would fool most people. (I'm a Photoshop expert, among other things.) But I didn't catch any glitches that would make me think it's a composite.

The number geostationary satellities is in the hundreds now, so I think it's quite possible that the video, which does seem to be made using a wide-angel lens, to capture more of the sky-field, would capture a few. The GPS system uses 24 satellites; weather sats and many communications sats are also geosynchronous, and also typically use webs of 24 sats, if I recall correctly.

Something about global coverage geometry at a certain orbit level leads us towards using 24 sats; I'm sorry, it's been awhile since I looked up the governing math formulae, so I'm remembering the numbers rather than citing them.

As for visibility, satellites are high-albedo reflectors, and designed to be so in most cases. (Excepting spy sats.) Since the aurora video is essentially a time-lapse construction, there is plenty of time for each single frame's exposure to be quite long—which it would have to be, in order to capture the aurora so brightly. I'm guessing, even with a wide-angle lens, at least a second or two per frame. (I'm also a pro photographer, among other things.)

Hope that helps. Beautiful images, even if I'm dead wrong! I love it when science and aesthetics converge, and data is actually beautiful, too, and elegant.

Gordon McCabe said...

Fascinating stuff Art, and an excellent post.

Would geostationary satellites be visible all night long? I watched the video again, and tried to keep my eye on a number of the fixed points of light, and they all seem to be visible throughout the night. I guess it depends on the altitude of the satellite, and the latitude at which the're observed from (and the time of the year).

Art Durkee said...

Polar or upper-latitude geostationary sats ought to be visible from northern latitudes all night long, yes, cloud cover depending. Altitude does matter some, yes. However, sats at higher latitudes and altitudes are less likely to be occluded in the earth's shadow, either the darker main shadow, the umbra, or the more diffuse atmosphere-scatter shadow, the penumbra. So, a high-altitude sat at a high latitude, or a polar position, is likely to be visible all night because it never dips into the earth's shadow. So, from the observer's position, it should remain fixed and unchanged.

Anonymous said...

Hmm, I don't buy the geosynchronous satellite theory --- to be geosynchronous, a satellite needs to sit above some spot on the equator, I thought, so would need to be in the southern part of the sky.

From the paths of the stars, I'd say we were looking more or less northward.

My guess is that the stationary "stars" are really reflections (off internal parts of the camera) of bright lights in the town below.