This evening, the Moon will pass just north of the star Aldebaran.
The Moon was at its exact First Quarter phase only six hours earlier, so it appears D-shaped.
In the picture, it is shown at twice its real size, so it appears to touch the star. Actually, it passes 1.6 degrees north; that is, the center of the Moon passes that far north of the star, as seen from the center of the Earth. The Moon is displaced somewhat south by our viewpoint in a northern latitude; but since the Moon is really only half a degree wide, it misses the star.
However, from farther north, the Moon appears farther south. So it actually “occults” (hides) the star, as seen from Alaska just before sunset and, after sunset, from the Canadian north coast and archipelago, Greenland, Iceland, much of the Arctic Ocean, and part of the northern coast of Russia.
Part of the page of occultation diagrams in Astronomical Calendar 2015. The blue circles are hourly stages of Aldebaran’s “shadow” cast by the Moon. To the right is shown the Moon as seen from the Earth, with the star’s trail south of it as seen from the center of the Earth. Aldebaran’s magnitude is 0.8, and it and the Moon are at this time 92 degrees east of the Sun..
If you do have the luck to live in Greenland or Baffin Island, you will see the brilliant star – the pale orange “eye” of Taurus the Bull – appear to drift toward the southern part of the Moon’s dark advancing edge, and wink out of sight as it hits that edge. It will pop back into view out of the bright edge after a short time, or a longer time if you have the luck to live even farther north in that icy part of the February world. Let us know whether you feel the temperature drop yet another fraction of a degree below zero when deprived of Aldebaran’s light!
This is (as explained in the “occultations” section of the Astronomical Calendar) one of a series of 48 occultations of Aldebaran. Today’s event is the second in the series, which started on Jan. 29. This Taurus part of the sky is in the northern region of the ecliptic, but the Moon’s path through it at this time is slung south of the ecliptic itself; Jan. 29 was when precession pushed it southward enough to intercept the star. The Aldebaran occultations will happen at every Moon-passage – a bit more often than once a month – until 2018 Sep. 3. Not all will pass over convenient parts of the Earth in darkness, but the next one, on March 25, should be excellent for Alaska.