Occultation of Propus

24 April 2019

Occultation of Propus (η Geminorum), 11 April 2019, 21:32 (ACST)


This was a totally unexpected happening. Dean and I had made camp at Acraman Bore (with the permission of the station manager, of course). We were relaxing after dinner, and watching The Emu come out to our south. I was using my binoculars to wander around the sky when I noticed the Moon was especially close to a reasonably bright star. Were we were going to see an occultation?

An occultation is when the Moon moves in front of a star. It actually happens all the time, but it's hard to see unless it's a bright star, as the Moon blots out everything around it. But usefully, when the Moon is a crescent, it has dimly lit side. This dim light is reflected off the Earth, and you can see stars right up to this side.

Interestingly, this means that occultations normally only happen in the week or more prior to the full moon, as during this time the Moon appears to move towards its dim side. After the full moon, when the dim side of the Moon is the trailing edge, you get what are called "revelations", when stars pop out from behind the Moon. I've seen one of these in my life.

The star was Eta Geminorum, commonly known as Propus, which is magnitude 3.3. Stellarium told me that, yes, it was going to occult, and it was going to happen at 21:42. Hastily, I set up my 500mm bird-watching lens on a tripod and started taking wobbly photos.

Things happened faster than expected, and Propus disappeared 10 minutes earlier than Stellarium suggested. Perhaps I had the wrong location set in the software, or perhaps it's just not that accurate.

Interestingly, in the last four seconds before occultation (we were watching like hawks) Propus reddened noticeably. Yes, the Moon DOES have an atmosphere - it's just very thin.

When I got home I roughly aligned the photos in Photoshop and created this animation. The first photo was taken at 20:26, so it covers a little more than an hour.