Lunar Eclipse over Longs Peak: a How-To

Lunar eclipse over Longs Peak, Rocky Mountain National Park, Colorado

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Photographing the lunar eclipse over Longs Peak was a challenge on many levels. You can read about the physical challenge of reaching my shooting location atop Twin Sisters in the middle of the night here. This blog post will describe how I planned the shot to be in the right place at the right time, how I shot the component pieces, and how I assembled the final image in Photoshop.

My plan was to shoot a series of images, starting before the moon began to enter Earth’s shadow, and continuing until the eclipse ended. The first step, therefore, was to find out when the eclipse would begin and end. That information is available in many places on the web. I used www.timeanddate.com. To be specific, I needed to know when the partial eclipse would begin, when totality would begin and end, and when the partial eclipse would end, all for my location in Colorado.

Next, I needed to know where the moon would be, in terms of azimuth (compass bearing) and altitude (angular elevation above the horizon), at the beginning and end of the eclipse. For that information, I turned to Photo Ephemeris Web. I knew the partial eclipse would begin at 4:16 a.m. If I wanted to shoot one frame every five minutes, and I wanted six images of the fully illuminated moon before the partial eclipse began, I would need to start shooting at 3:45 a.m. So I set the date and time of Photo Ephemeris Web to April 4, 2015, 3:45 a.m., and saw that the moon would have an azimuth of 230 degrees and altitude of 31 degrees.

This screenshot from Photo Ephemeris Web shows the position of the moon at 3:45 a.m., when I started my sequence of moon images. The thin blue line indicates the direction of the moon as seen from the summit of Twin Sisters. Notice that it falls to the left of Mt. Meeker. The thick blue line indicates the direction of moonset. The thick yellow line indicates the direction of sunset, which of course was not relevant in my calculations for my lunar eclipse image.

Further examination of TPE showed that at moonset at 6:47 a.m., the moon would be at azimuth 263 degrees. TPE also showed me that the moon would set just a few minutes before sunrise while it was still partially eclipsed. That meant that during the later stages of the eclipse the sky behind the moon would become increasingly bright and the land would be illuminated by twilight.

Putting all this together, I concluded that the ideal shooting location would be where I was looking at something interesting to the southwest, and where I would be as high as possible, so the horizon to the west would be at my elevation (or close) and I could see as much of the eclipse as possible before moonset.

This screenshot from Photo Ephemeris Web shows the position of the moon when it sets below the skyline ridge at 6:35 a.m., a few minutes before the almanac time of moonset. The thin gray line extending from Twin Sisters towards the gray secondary marker indicates the direction of the moon. The thick blue line indicates where the moon would set above a level horizon.

It didn’t take too much map study to realize that the logical place to go was the summit of Twin Sisters, which offers a great view of Longs Peak and Mt. Meeker to the southwest. I returned to TPE and activated the gray secondary marker by clicking on it. I then dragged and dropped the secondary marker at the point where the thick blue line indicating moonset direction crossed the skyline ridge. TPE's readout showed that the ridge was only 2 degrees above a level horizon as seen from Twin Sisters, so I would be able to see the moon until a few minutes before the almanac time of moonset. I dragged the timeline marker slowly back in time until the moon's altitude was 2 degrees, then read off the time. The moon would set behind the ridge at about 6:35 a.m. at an azimuth of 261 degrees. By dropping the secondary marker where the thin blue line indicating current moon position crossed the ridge, I confirmed that the altitude of the ridge at that point was still only 2 degrees.

Now I knew that my composition would need to span a left-to-right arc of 31 degrees, from 230 degrees to 261 degrees. A 35mm lens, set vertically, has an angle of view across the short dimension of 38 degrees. Its angle of view on the long dimension is 54 degrees – plenty to include both the moon (31 degrees above a level horizon when I started shooting) and some foreground. At 3:45 a.m., when I started the sequence of images, the moon would be just to the left of Mt. Meeker. It would move downward at roughly a 45-degree angle until it set to the right of Longs Peak.

The correct exposure for the moon varies dramatically during an eclipse, from about 1/60 sec., f/11, ISO 200 when the moon is fully lit to about 8 sec., f/11, ISO 1600, when it is totally eclipsed. (If I had used an exposure much longer than 8 seconds with my 35mm lens, the moon would have blurred due to the Earth’s rotation.) I shot a five-frame bracketed set with a one-stop bracket interval every five minutes, double-checking that I had some detail in the moon by examining the magnified moon image on the LCD. Since nearly all of the image was black, the histogram was useless.

Once I got home, I used Lightroom to select the best exposure from each bracketed set and loaded all the images as layers in Photoshop (Photo>Edit In>Open as Layers in Photoshop). I selected the top layer, then shift-clicked on the bottom layer to select all the layers simultaneously. I then changed the blend mode to Lighten. In this blend mode, Photoshop looks at every pixel in the top layer and compares its brightness to every pixel directly beneath it in the layer stack. It then lets the brightest pixel shine through. Since the moon was much brighter than the background in all but the last few shots, this procedure allowed almost all of the moons to shine through. Achieving the correct density in the land and in the moons that were near the horizon required some additional Curves adjustment layers accompanied by layer masks to confine my adjustments to selected areas of the frame. Although the moon was shining brightly in a black sky at the beginning of the eclipse, the sky was blue and the land was softly illuminated by the time the moon set. I decided to use that twilight rendition of the sky and land behind the series of moons as they changed slowly from the brilliant white of full illumination to the ruddy red of total eclipse, and created the image you see here.

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