Moon to Pass in Front of Star Cluster

Most people are unaware of the moon?s rapid movementagainst the background of the stars, but this week early risers will get thechance to witness this as the moon moves past a series of celestial milestones.

Once a month, the moon makes a complete trip around theEarth, going through its sequence of phases.

Back on July 22, the moon was new, in fact located exactly betweenthe sun and the Earth so that its shadow fell across India and China, causingan eclipse of the sun witnessed by millions of people. By July 28, it hadtraveled a quarter of the way around the Earth, reaching first quarter. On Aug.6 the moon was full, having traveled half way around the Earth, and this week,on Thursday, Aug. 13, it reaches the three-quarters mark: last quarter.

As the moon wanes towards new moon on Aug. 20 it passes aseries of markers. Most spectacularly, on the morning of Friday Aug. 14 itmoves directly in front of the Pleiades star cluster. This cluster is thebrightest ?deep sky object? in the sky, readily seen with the naked eye even inthe city. With binoculars it is a glittering jewel box. As the moon passes infront of it, the stars of the cluster will be covered by the leading edge ofthe moon (on the lower left) and uncovered by the trailing edge (on the upperright). Watch it for a few minutes, and you should see this celestialdisappearing act.

The following morning, Saturday Aug. 15, the moon will havemoved well away from the Pleiades, and will be surrounded on three sides: thePleiades to the upper right, the planet Mars to the lower left, and the brightred giant star Aldebaran to the lower right.

By Sunday morning, the moon will have passed Mars and willbe heading towards the brilliant planet Venus, to the lower left. The followingmorning it will almost have reached Venus.

This gives us a special bonus, the opportunity to observeVenus in a full daylight sky. Venus is frequently visible in broad daylight,but you need to look in exactly the right place. Your eyes also need to befocused far enough away to see Venus; most of the time when our eyes arerelaxed they focus fairly close to us.

By starting with the moon, you get a double boost: youreyes will be focused correctly on the moon, and the moon will show you whereVenus is in the sky. Specifically, next Monday morning look for the Moonbetween 9:30 and 10:00 a.m. local time. Face due south and sweep from thesouthern horizon up towards the zenith overhead. The moon will be quite high inthe sky, close to overhead. Once you locate the moon, Venus will be about 4degrees below and to the left of the moon, a bit less than the width of thethree fingers of your hand at arm?s length.

Spotting Venus in daylight isn?t easy, and it may help touse binoculars at first. Once again, the moon gives you a target to focus on,and then you just need to move the binoculars slightly down and to the right tospot Venus. Usually once you can see it with binoculars, it?s much easier tosee with the naked eye.

By the following morning, the moon will be well past Venusand sinking towards the rising sun, but you may still be able to catch aglimpse of it.

As you watch the moon on its travels across the dawn sky,be sure to pay attention to its rapidly waning crescent, which narrowsnoticeably from night to night.

If all these early mornings are too much for you, or yourview is blocked by clouds, you can readily view this whole journey with StarryNight.

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Geoff Gaherty
Starry Night Sky Columnist

Geoff Gaherty was's Night Sky columnist and in partnership with Starry Night software and a dedicated amateur astronomer who sought to share the wonders of the night sky with the world. Based in Canada, Geoff studied mathematics and physics at McGill University and earned a Ph.D. in anthropology from the University of Toronto, all while pursuing a passion for the night sky and serving as an astronomy communicator. He credited a partial solar eclipse observed in 1946 (at age 5) and his 1957 sighting of the Comet Arend-Roland as a teenager for sparking his interest in amateur astronomy. In 2008, Geoff won the Chant Medal from the Royal Astronomical Society of Canada, an award given to a Canadian amateur astronomer in recognition of their lifetime achievements. Sadly, Geoff passed away July 7, 2016 due to complications from a kidney transplant, but his legacy continues at Starry Night.