Occultation miss

Posted on January 31, 2010 by Steve

My lifetime to-do list includes seeing a total solar eclipse. I made it to Paris for the August 1999 event but was foiled by clouds. March 2006 provided another chance, but I couldn't mobilize travel to Turkey. So the North American eclipses of 2017 and 2024 are already in my calendars. These long-term appointments help to balance my usual last-minute planning, I think.

Meanwhile, lunar occultations provide a compensating, if pedestrian, alternate spectacle. When it first occurred to me that the moon would eclipse stars now and then just as it does the sun, I imagined how cool it would be to see a bright star wink out behind a nearly invisible new moon, only to reappear some minutes later. Unfortunately, while this sort of alignment does occur regularly, it usually happens during daylight hours when stars are inconveniently invisible.

Predictions and timetables for lunar occultations are not easy to come by (at least compared to those provided for solar eclipses and Iridium flares). The International Occultation Timing Association provides some resources. Amateur observers make scientifically valuable contributions by providing accurate times when an asteroid or the moon blot out a star from their location. The forums at the pessimistically named Cloudy Nights site have been most helpful, especially posts by Curt Renz, who generates astronomical diagrams on his site worthy of the Mayday Mystery (examples: 1 2 3 4).

A news alert recently informed me of a major North American event, the occultation of the Pleiades. This famous cluster, also known as the Seven Sisters, has been celebrated since ancient times. God, played by a whirlwind, taunted Job saying "Can you bind the cluster of the Pleiades, or loose the belt of Orion?" It also features in the Subaru logo.

That evening, I parked on a rural side road on my way home from work and used an app (screenshot) to help time the alignment. The moon was several diameters from the cluster (labeled M45), and I estimated that I had half an hour or so before the event. When I got home and checked again, I confirmed the sad news that the app seemed to be indicating: the moon was actually moving away from the cluster. Of course! The moon drifts eastwards along the ecliptic (towards the cluster in my screenshot), but so do all the stars in the background, since it's mostly the earth's rotation causing the apparent movement. The moon's orbit causes it to lag behind the stars. The occultation had occurred the night before, and I had missed it!

Oh well, it was a familiar feeling. I relived the moment in Celestia (screenshot) and rechecked my 2017 calendar.
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