Dark light of the moon and shifting sunset shadows

The moon just a day past first quarter phase. Though brilliant to the eye, it's nearly 10 times fainter than the full moon. Photo: Bob King

I skied in the woods under last night’s first quarter moon and learned something about trust. In the mix of shadow and low light, I could see the trail clearly only half the time. Along the stretches where darkness or shadow obscured the track, I kept the faith while gravity and momentum did their thing. After an hour of skiing, I’m happy to report there were no falls and only a few moments of trepidation. I learned to really open up my eyes as well as to trust what lay ahead.

A first quarter moon is only 1/10 as bright as a full moon. That doesn’t sound like much, but it’s a surprising amount of light really.

Part of the trail was lit by artificial light. After leaving this cocoon of easy navigation, the woods looks dark indeed, but within five minutes, my eyes were dark-adapted all over again. The trail grew more distinct, peeling birch bark showed shiny highlights under the moonlight, and occasional snow crystals sparkled off to sides.

Tonight the moon will have swelled a bit beyond half and shine a few percent brighter than last night. It’s high up in the south at nightfall just 2 degrees below the Seven Sisters (Pleiades) star cluster. If you block the moon with your thumb and look above and to its right, you might be able to see the cluster with your naked eye. City dwellers, for whom the Pleiades are never visible with the naked eye even under moonless skies, can use the moon as a guide to find them with binoculars.

Tonight the moon will be between the Pleiades and Hyades star clusters. Created with Stellarium

The moon’s high position in the sky reminds us of where the sun once was … and will be again. On and around May 20, the sun will occupy the moon’s spot and remain up for hours just as the moon will tonight. Moonrise this morning was around 10:30 and it won’t set until tomorrow around 2:30 a.m.

Five images taken over 8 minutes of sunset over the Sierra Nevada Mountains. Credit: Andrew Kirk

Yesterday, Andrew Kirk of Bishop, Calif. sent me a fascinating series of photos taken near sunset showing the shadows of mountain peaks cast on dust in the air. Look at each frame closely and you’ll notice at least two things: the shadows lengthen with time, just as your shadow does near sunset, and they rotate about the sun pivoting from right to left. That’s because the sun is not only going down but also moving to the right or north at the same time.

If you watch your own shadow at sunset, it will pivot in the same way as the ones in the photo do. Most amazing is that only 8 minutes elapsed between the top and bottom images. Thanks Andrew for sharing your unique and beautiful perspective on our planet’s daily motion!

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About astrobob

My name is Bob King and I work at the Duluth News Tribune in Duluth, Minn. as a photographer and photo editor. I'm also an amateur astronomer and have been keen on the sky since age 11. My modest credentials include membership in the American Association of Variable Star Observers (AAVSO) where I'm a regular contributor, International Meteorite Collectors Assn. and Arrowhead Astronomical Society. I also teach community education astronomy classes at our local planetarium.

3 thoughts on “Dark light of the moon and shifting sunset shadows

  1. Thank you again Bob, this is a very interesting blog. Concerning the Hyades/Pleiades, both are visible from my Suburban Back Yard either side of the DUE NORTH Meridian on any night of the week (without cloud). Obviously, the rule of thumb is the MOON which would still allow me to see them (unaided) using the THUMB. Naturally, in Adelaide, Australia, the Sketch is reversed with Hyades to the Right & Pleiades to the Left. My Suburbs streets are STAR FRIENDLY but every major road (of which there are many) are emblazoned with YELLOW FOG LIGHTING OVERHEAD. As it is directed downwards, it does not worry me although I am about 150 feet from one 4 LANE ROAD. Like the LMC & SMC, the Pleiades is a bright blotch to the naked eye. They Hyades is far more distinct but seen at its best through a WIDE VIEW EYEPIECE. I favour my SKYWATCHER 35MM 2″ 56 deg for such tasks or slightly more power from my 28 mm 2″ SKYWATCHER 56 deg. Both are magnificent for STAR FIELDS. Thank you again Bob, for you wonderful writings, I never did try to Ski which is probably just as well. I have been VERY lucky so many other times.

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