28 September 2015
HARVEST MOON ECLIPSE
The weather gods smiled on us last night and skies were perfectly clear for the Harvest Moon eclipse. As you probably heard, this was a "Super Moon" eclipse, with the closeness of the moon in its orbit around the earth making it appear 13% larger than normal. According to the story on it in September's edition of Sky and Telescope, this was the biggest eclipsed moon we'll see.
The eclipse was predicted to start at 7:40 PM CDT here at SOCO, with the moon rising only a few minutes before that. The eclipse was predicted to end at 11:27 PM CDT. I wanted to set up my Nikon D300 SLR to catch a time sequence of the eclipse, centered approximately on totality (predicted to occur at around 9:50 PM CDT). Going into Cartes du Ciel, I determined the point in the sky where totality would occur (altitude of 26° above the horizon, azimuth of 108° from North). I then tilted the camera on the tripod mount so that the long axis of the photo frame would be roughly parallel to the path of the moon along the ecliptic. I set the focal length of the Nikkor AF-S 18-200 mm telephoto lens on the camera to 35 mm. This wouldn't get the entire eclipse, but would get a good portion of it centered on totality. I could have used a shorter focal length and gotten the entire eclipse, but the size of the moon in the resulting photos would be very small— too small to show any real detail.
So with this setup, I started taking photos at 8:55 PM CDT, when the moon just entered the photo frame. At this time, the moon was 16° above the horizon, at an azimuth of 100° from North. I continued to take photos approximately every 10 minutes. I was a bit off in my time sequence a few times— once I fell in the weeds and had to get cleaned up (hey, it was dark!). I continued photographing until just before the moon moved out of the photo frame. This last photo was taken at 11:05 PM CDT, when the moon was 41° above the horizon and at an azimuth of 122° from North. For all photos, I used an exposure of approximately 1 second at f/4.5.
The set of individual photos was composited using Photoshop CS3. The resulting image is presented in Figure 1. In this image, the moon rose from the bottom of the image to the top of the image. To get a correct idea of the moon rising in the sky, remember that the ecliptic along which the moon was travelling stretched across the southeastern part of the sky.
Figure 1. Eclipse sequence. Each image of the moon is approximately 10 minutes apart, progressing from bottom to top.
As the eclipse progressed, it was remarkable how the surrounding landscape went from brightly moonlit to very dark. Totality lasted around an hour and 12 minutes, so there was quite a bit of time under dark sky conditions. During totality, the Milky Way was easily visible. In fact, I took this opportunity to image the open cluster M 25 in Sagittarius using the SOCO scope. This would have been useless to attempt under regular full moon conditions. At totality, the eclipsed moon displayed a strong orange-red color. This is shown in the enlarged photo in Figure 2.
Figure 2. The eclipsed moon at totality.
If you missed this "Super Moon" eclipse, it won't occur again until April 25, 2032. I hope I'm around to catch it!
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