Aldebaran Crossing

On the evening of March 4, 2017 (5 Mar 2017 UT), the Moon passed in front of the 0.9-magnitude star Aldebaran (α Tau).  Currently, this is the brightest star the Moon ever crosses (excepting the Sun, of course).  The favorable first quarter moon (with the nearside 45.9% illuminated) was at a respectable altitude of 31° in the western sky at the time of the dark-limb disappearance.  It is not an instantaneous event.  The effects of diffraction plus the non-zero angular size of the star ensure that the star disappears over several frames of a high-speed camera, as shown in my video recording of the event below.

At my location, the middle of Aldebaran’s disappearance occurred at 3:56:00.570 ± 0.001s UT.  Thin clouds and wind interfered with a pristine recording of the event, but even so you can see in the trace below that the star took about nine frames (~0.017s each) to completely disappear.

The bright limb reappearance was more difficult to time.  In my video recording below, you can see that Aldebaran first reappears at about 4:29:39.828 UT.  By that time, the Moon had descended to an altitude of 25° and was accompanied by a fair amount of atmospheric turbulence.

All in all, I felt very lucky to have observed both of these events.  Soon afterwards, it completely clouded up!

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