A Cosmic Do-Over : The Purbach Cross Redux

[Rotorhead]

For those who missed the Purbach Cross a few meetings ago at the clubhouse, you missed a really neat event, and one which is not well-known even among regular lunar observers. This is probably because it is a fleeting event, lasting about 15-20 minutes when we last observed it, and because it is not always well positioned for observation from any given point on the earth.

Well, tonight appears to be one of the favorable nights for us, and for those who missed it, Mother Nature is giving you a do-over. For those who saw it, but had to elbow each other out of the way at the only two telescopes available at the club meeting it is a chance to observe the phenomenon on your own.

The calculation that I have read http://blogs.mysanantonio.com/weblogs/a ... _spot.html indicates that at 0234 UT on 11 June (2234 Local {10:34 pm} on 10 June- that's tonight...) the Cross will appear. If our last apparition is a guide, it will be long gone by 11 pm.

Note that the Purbach Cross is an optical illusion, and is simply the illumination at lunar dawn - between the craters Purbach and Werner - of an intersecting crater rim complex that will suddenly be shown as a bright cross in the dark shadow to the west of the terminator. It was really a neat sight and one that impressed all who saw it at the club. Read the report of March 15 on this section of the BB for a summary, and the Spring 2008 Observing Report section for April 14...

Go for it, and let's see if one of our great club lunar/planetary imagers can catch a shot of it. I predict that a good photo would be almost automatic APOD material....

[Mark G]

I can't make it tonight but I sure hope others do Your link also showed the other upcoming dates to this neat event. Have fun & I hope an APOD image is caught tonight.

[Galactus]

Thanks, Bob for the heads up. I enjoyed observing it at the club and hope that I can see it frommy location--I think trees here may be a problem. Perhaps I will try another observing location in the front yard...

[Rotorhead]

I was quite surprised to see the Cross as soon as I focused the moon in the ep at about 9:36. One leg of the X was a bit ragged, which I don't remember from the previous sighting, but it gradually filled in about 90% by 10:30. The moon was 'boiling' in the turbulent air tonight, and seeing was very spotty. We have quite a few 'halos' on the brighter stars here, and that is reducing the seeing, also. However, quite a few minor craters were visible in the area, and I reviewed some of my Lunar II Club targets while watching the X fill in.

I'll be surprised if anyone was able to image this target due to the turbulence, but maybe someone got lucky...

[Pete]

The Purbach Cross 10 Jun 08

I had a Conservation Commission meeting tonight but got home around 21:45 hrs – hopefully in time to catch this lunar illusion. Opening the observatory requires only a few seconds but the 14â€

[Rotorhead]

Very nice, Pete! The seeing really was lousy last night, and the turbulence was a killer. I'm glad you persisted in getting a couple of shots off, and I think you caught it exactly, despite the challenges.