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Monday, June 17, 2013


Occultation of alpha2 Librae (Zubenelgenubi) by the Moon, June 20, 2013

The evening sky facing east in Sydney on June 20 at 5:05 pm AEST showing the waxing Moon just about to cover alpha2 Librae (Zubenelgenubi). (similar views will be seen from other locations at a similar local time eg 5:08 AEST Canberra). The inset shows a telescopic view of the Moon at 5:05 pm AEST, with alpha2 Librae about to go behind the Moon.

The waxing Moon passes in front of the bright alpha2 Librae in the constellation of Libra on the evening of June 20. Alpha2 Librae, which rejoices in the name Zubenelgenubi, is a bright white star readily visible to the unaided eye (magnitude 2.8). The occultation will be seen from eastern Australia and South Australia. Everywhere else will see a nice, close approach. Appearance and disappearance times are shown in the table below.

CityDisappears Dark LimbReappears Bright Limb
Adelaide-17:23 ACST
Brisbane-18:05 AEST
Canberra17:08 AEST18:08 AEST
Darwin-1deg at twilight
Hobart17:28 AEST18:07 AEST
Melbourne17:13 AEST18:03 AEST
Sydney17:06 AEST18:10 AEST
Perth-1deg at twilight

 With the Moon not far from Full (23 June), this event is really best seen with binoculars or a small telescope (especially for the reappearance of the star on the bright limb of the Moon). If you have a tripod or other stand for your binoculars, it will be much easier to observe. Set up about half an hour before the occultation to watch the star dissapear (so you are not mucking around with equipment at the last moment).

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Oh, oh, oh, I saw it! I went outside and noticed the mist and cloud had utterly disappeared, and as I looked at the brilliant moon, I could see something going behind it. My first occultation, and an accident at that!

No scope, my K7 batteries are charging, and the iPhone couldn't capture the detail. Ah well. I got to see my first occultation!

Even knowing what it was, I was startled. It was in plain sight, the moon swallowing a star! How excited and disturbed we must have been, to see this thousands of years ago...

Thanks for the reminder!
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