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Big Bend '17: No. 9 - Moonset to Twilight

During the transition to a full moon, the moon loses pace with the sun until one rises and the other sets at about the same time. This means that there is very little time when neither the moon nor sun are adding light to the atmosphere, thus limiting the visibility of the stars. However, on our first night out on the Rim, the moon was expected to set just about an hour prior to sunrise, which would give me a short window of only a few minutes where it would be dark enough to see a glimpse of the incredible starscape that Big Bend is known for.

Moonset over the Chisos Mountain Range.

While I love the brightness and clarity of a Super Moon, the increased light is incredibly detrimental to astrophotography, especially rich and detailed starscapes of the Milky way that Big Bend is usually optimal for. In order to still get a few shots of the stars during the single night we had out on the Rim, I looked up the time when the moon would set using the iPhone app PhotoPills, and it happened to set only an hour or so before sunrise. Unfortunately, I would likely only have 20-30 minutes of relative darkness to capture the stars above the desert before the twilight preceding the sunrise would begin to wash out the stars again.

My alarm went off around 05:30 and I fumbled around my sleeping bag in a struggle to turn it off. As I stared up through the fine mesh of our tent, I could see that it was finally a little darker than when we went to sleep, which meant that the moon was finally setting. A windy chill blew through as I fished around my sleeping bag for my jacket and clothes down by my feet. I find it's always warmer to get dressed inside my sleeping bag, though it's not always easy. I slipped my half-awake body out of the tent door and stuffed my cold feet into frozen boots. We layered up, grabbed our breakfast and photo gear, and headed out to the edge of the Rim. Behind us to the Northwest, the moon was slowly setting in the remnants of the high cloud cover that draped the stars in a veil of brilliant ice crystals suspended in the upper atmosphere. It was a strange feeling to watch what seemed like a dim, star-filled sunset behind us, even though we knew the sun would soon be rising on the opposite horizon.

We set up our chairs a few feet back from the edge of the Rim and fired up the camp stove to make some hot tea. The super-light camp chair was a belated birthday gift from Travis that he gave me on our first day in the Chisos Basin, and I really appreciated having it to enjoy the scenery (when I wasn't taking photos on the ground). Travis spent his time looking at star clusters with his binoculars. The binoculars were surprisingly good at isolating the star light and increasing sharpness so we could see more detail in small clusters than with the naked eye. While he did so, I crawled to the edge of the cliff again to place my camera on some rocks and my beanie in an attempt to get some wide angle starscapes using the 20mm f2.8 this time. Given that I had no way to anchor my camera, I just laid on the ground holding the camera strap for each exposure.

As mentioned in a previous post, one of the hardest things about starscapes is getting the correct layout. The viewfinder is only as bright as the max aperture of the lens, but even with an f2.8 aperture there simply isn’t enough light to see stars or the horizon. I generally use the hot shoe on the top of the camera and the front of the lens for rudimentary sight alignment. After that, I estimate elevation angle, and try to keep the camera as level as possible. Sometimes I will use a spirit level on the top of the camera, but the Nikon D750 also has an internal gimbal that can provide a good reference, especially if you set it to one of your accessory buttons (though it will likely blind you in the dark).

Unfortunately, even as we reached the Rim before moonset, twilight was already building in a thin band of light along the Southern horizon. In hindsight, it may have been better to try and catch the stars around 02:00 - 03:00 while twilight was still far off, and the moon was still far enough away from the South Horizon to get a clean photo. There is always a battle between sleep and photography when camping outdoors. On this night, after 13 miles or so of hiking prior, I decided to sleep a little so I would have energy for the last 6 miles of the hike.

During the night, a thin veil of fog seemed to consistently creep in to the valley and replace the haze of dust that existed during the day with smooth, white clouds of moisture that resembled an expansive ocean with small islands peeking out. Ripples of thin, wispy clouds returned to the sky above the desert as the twilight began to overpower the stars closest to the horizon.

We happened to be out during one of the rare moments when Mercury would be visible before the sunrise, just off the horizon, for only moments before the sun would overpower the planet’s small beams of reflected light.

Mercury Rising - It is very difficult to get a glimpse of Mercury because its orbit is so close to the sun that it is almost always surrounded by too much light to be seen. However, every once and a while, Mercury is located on the leading side of the sun that the Earth turns toward, allowing for a brief glimpse of the small, hot, asteroid-pummeled planet just above the horizon. Unfortunately, in mere minutes the glow of the following sun will overpower the shimmer of the Solar Systems first planet, causing it to fade and be lost in the increasing brightness of the Earth's atmosphere once again.

Even with my lens maxed out at 300mm, the small size of the planet and the great distance between the Earth and Mercury made the planet look like nothing more than a small, bright star along the horizon where no other stars were visible.

Mercury hovers just above a dark band of clouds moments before sunrise.

I love the few minutes before the sun crests the horizon. It is a brief moment of twilight that is very similar to that just after a sunset, but with a warmer, softer hue and a haziness that seems to match my own eyes, as if the earth is as bleary-eyed while greeting the dawn as I am.

With the triumphant rise of vibrant twilight, the remaining stars faded again behind the veil of a brilliant, glowing atmosphere. My short opportunity to photograph starscapes on the Rim was already over, and it was now time for the sun to make its grand entrance back to the Chihuahuan Desert.

© 2017-2018 Shaun C Tarpley

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