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How often, on avera...

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# How often, on average, does the moon occult a first magnitude star?

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Community Member

from a single location on Earth? Any first magnitude star, but I'm looking for the average "time between" VISIBLE occultations (eg, not during daytime).

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Community Member

Using the data on this site http://www.ast.cam.ac.uk/~ipswich/Occultations/Occultation_intro.htm I can do a back of the envelop estimation.
There are only four 1st magnitude stars (Aldebaran, Regulus, Spica and Antares) that can ever be occulted by the Moon.
Up to 5th magnitude there are 306 stars, 13 of which are occulted in 2009. So an occultable star has a 4% chance to be occulted in any given year.
That means that there is a chance of 1 - 0.96^4 = 15% that a 1st magnitude star is occulted in any given year: roughly once every six years.

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Community Member

Building off of ronwizfr's excellent work, your next step will be to determine how many of those occultations will occur at night. This is going to depend on your location on Earth; if the occultation occurs in the summer and you live at the north pole, you're not going to see it. Now only 50% of the planet is in daylight at any given time, and regardless of where you live, you spend half of your year in darkness, so my guess would be that only half of those occultations are going to occur when you personally are experiencing night. If I'm thinking correctly (and I make no guarantees of that!) then occultations by the Moon of 1st magnitude or brighter stars that are visible TO YOU PERSONALLY should only happen once every 13 years or so. Again, don't quote me on that.
I hope that helps. Good luck!

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Community Member

On average, a couple times or more a year. Antares will be occulted on August 27th, one of a dozen times this year that it will be covered by the Moon from some location on the planet. Regulus was occulted a couple times in 2007. Because occultations are not visible world wide, where you live and watch matters. For an individual spot on the globe, it could be many years between actual bright star occultations, though the moon would appear to be close on many more occasions.

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Community Member

well, it must be possible to calculate it, if you know:
- the 4 known bright stars in the neighborhood of the ecliptic
- yes, the moon goes ALONG this ecliptic, sometimes she is a few degrees above the ecliptic, sometimes a few degrees beneath it, due to the known period of 18.6 years of the floating orbit of the moon. So for Regulus e.g. she could just pass it above or beneath it.
You must know then that this 18.6 movement is so slow that for Antares e.g. the moon many times goes thru it, a series started in 2005 and it will end in 2010.
- and also filtering the day/night events out like you want

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