Waning Moon Occults the Star Aldebaran


The last-quarter Moon will pass through the Hyades star cluster this weekend, on September 4-5, and for observers in eastern North America and western Europe, the Moon will pass in front of the adjacent bright star Aldebaran, the brightest star in the constellation Taurus. During this occultation, the star will disappear behind the lit edge of the Moon, then reappear nearly an hour later, rather dramatically, from behind the dark edge, apparently reemerging out of nowhere. If you’re not in the right place this month, the video above shows Aldebaran as it emerged from behind a waning crescent Moon during an occultation last month.

The occultation of Aldebaran is visible at night when the Moon is rising low in the east as seen from the far northeastern U.S., most of Ontario and southern Quebec, and the Canadian Maritimes. The midwestern states will miss the beginning of the event, but can see Aldebaran emerge just as the Moon rises. In the U.K., Ireland, Spain and Portugal, France, and Germany, the event occurs just after dawn. So with a pair of binoculars or a small scope, you might see the star pass behind the Moon in daylight. Some examples of timing for major cities:

  • Toronto: Disappears at 12:05 am EDT, reappears at 12:40 am EDT; the timing for Montreal is similar
  • Boston: Disappears at 11:57 pm EDT on Sept. 4, reappears at 12:42 am EDT
  • Washington DC: Disappearance not visible; reappears at 12:40 am EDT
  • London UK: Disappears at 5:50 am BST, reappears at 7:07 am BST (sun rises at 6:18 am BST)

For precise times of the beginning and end of the occultation for hundreds of locations, check out the table on this webpage.

The Moon rising near the Hyades early on September 5, 2015 from Ottawa, Canada

The Moon rising near the Hyades early on September 5, 2015 from Ottawa, Canada

During an occultation, the Moon and star appear to move together westward through the sky, as always, as the Earth rotates. But the Moon also moves relative to the star itself, about a full Moon-diameter eastward each hour, as it revolves around the Earth. That’s why we see the Moon pass across the star over the course of an hour or so.

This is not a particularly rare event. In fact, the Moon occults Aldebaran every month this year as seen from somewhere– but not everywhere– on Earth. These occultations are part of a series of 49 such events that will extend to September 2018. But it’s still strangely compelling to see our little part of the cosmos in motion during the course of an hour. You don’t even need a telescope.

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