Eclipse 2008 - the last for a while

kw: observations, astronomy, photography

Last evening I got out (in 5-minute spurts—it was well below freezing) with my camera to observe the last Lunar eclipse that will be visible in Eastern North America before 2010. The images in this montage are clipped from much larger frames; they are 300x300 clips from 6Mpx images. Image times are 9:02, 9:14, 9:24, 9:34, 9:46, 9:54, 10:04 and 10:19pm Eastern.

I've had my camera, a Nikon D40, about a year, and I really love it. However, it is not well-suited to astronomy. Two deficiencies bugged me: the lack of a cable release shutter (and too hard to work self-timer in the dark), and focusing troubles. As to the latter, I managed to get it into a mode so that it could autofocus on the moon, until it got too dim (the sixth image), after which I had to switch the lens to Manual focus and eyeball it. The lack of any of the focusing aids found in film SLRs made this very iffy. I basically took several pix at a time, refocusing each time, hoping to hit the sweet spot once each set. I did OK. Once shutter times got slower, however, camera shake became an added complication. At least I have a sturdy tripod.

It just occurred to me that I could have taken some of the earlier images, at least, by setting the camera to continuous shooting (2.5 per second), and held the shutter down while I slowly focused through infinity and beyond (only in a camera can you go beyond infinity!). I haven't tried continuous shooting with shutter speeds of a second or so!

This image was processed a bit to give the Moon an appearance closer to what I could see visually: lower contrast and more yellow/tan than orange/red. The tiny dot at the top is the star Regulus, which was 2° North of the Moon at the time. Saturn was also nearby, about 5° East, but my image frame wasn't wide enough to put the Moon on the autofocus spot and see Saturn also.

The ability to capture Regulus (Mag 1.4) in a 1/10-sec exposure at f/5.6 and ASA1600 gives me some idea what I'd need to do to get wide-angle sky images with this camera. First, I need a desert or dry mountaintop location, so stars of sixth magnitude are clear to my eyes. Then I need to mount my camera on a tripod with a clock drive, because each magnitude requires a 2.5x longer exposure.

To get a 1.0-magnitude star captured this well takes 1/15 sec; at f/4 (my wider-angle lenses), 1/30 will do. Then 6.0-mag stars would require 100x the exposure or 6.7 seconds. However, with proper setup, my telescope tripod will keep a camera pointed at the right part of the sky for a minute or more. A 60-sec exposure would then image 8.4-mag stars with similar intensity as that seen here, and probably show some a magnitude or so fainter. Of course, "visible" stars would be very overexposed, but that's typical of astrophotography.

The curvature of the Earth's shadow in the upper images gives one a feeling for the size of the Umbra, even 240,000 miles (385,000 km) away. The Umbra actually reaches, on average, 855,000 miles (1.38 million km) beyond Earth, and its diameter at the Moon's distance is still 5,700 miles (9,200 km), or about 2.4 times the diameter of the Moon. Though the Moon was off-center to the Umbra (as usual), and because it travels its diameter in just under an hour (orbital speed 2,100 mph or 3,350 kph), it was fully engulfed in the Umbra for just under an hour. According to the Sky and Telescope site, the central phase—totality—lasted from 10:00-10:52pm EST (UTC-5h).

Some folks wonder why the Moon turns orange. If you were on the Moon at the time, the Earth would appear dark except for a bright red rim where light scatters through the atmosphere. During the rare eclipse in which the Moon passes through the center of the Umbra, it gets much darker, as you can see to the left side of the later images. When the Earth has more cloud cover than usual, and the Moon hits the Umbra dead-on, it seems to vanish from the sky...slowly, like the Cheshire Cat's smile.