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Mooning

Late Breaking News: Venus Disappears!

Dave North


I don't know if a July 31 event is appropriate for the August mooning column, but I didn't really think to put it in last month, what with the Wandke Moonwriting Marathon in full tilt... but we have a nonOccultation of Venus happening under nearly impossible conditions on the last day of July.

The moon will be very near the sun, and it will be just barely sunset when the semi-occultation takes place. From here, we'll get a partial, which means we'll see Venus go a bit dim, and if we get impossibly good seeing, catch the change in shape.

However, Sacramento should be in the path the full occultation of the planet. If you think it's worth the drive to get there before sunset, head east and north a bit (anywhere on the Sacramento line should do it).

The moon will be only a few degrees from the sun, though, so this will be both very hard to see (perhaps out of the question) and very, very dangerous (because telescopic observation means creeping so close to the sun).

If you're not an ace solar observer, and familiar with all the techniques and safety drill, I wouldn't try until the sun is below the horizon. This gives you almost no time to line up and watch, but better to miss this difficult shot than cause eye damage - keep your eyes and you'll be able to see Venus transit the sun before too terribly long...

I'm sure by the time July 31 rolls around, there will be plenty of "event noise" and advice floating around, for those who want a more thorough picture of what to expect.

August is traditionally a good time to start the hunt for Orientale, but we're moving out of the cycle of best presentations. Still, the theoretical best presentation is often not the one that produces the best image.

On the evening of August 14, from sunset to midnight, there might be a good view of the terminator opening up Orientale. The shadowline will be moving to within a few degrees of the limb as the night progresses, and there is a moderate libration of Orientale toward us.

Actually, that's a pretty good terminator position, but a pretty poor libration. How those two will combine in the eyepiece is never obvious ahead of time (at least to me) so I'd suggest looking.

If you don't follow what I'm up to here, it's actually pretty simple.

The idea is to catch Mare Orientale while the moon is tilted to show it best; turned toward us ... at the same time the terminator is crossing it. This really only happens once a month, just before full moon.

I got started on this annual chase some years back as an experiment to determine whether or not the moon was in fact a dull object near the full phase (or at it).

The lesson I learned from my experiment? Nope. There are tons of things to look for near full - just different things and different challenges.

Lately, the challenge has been the seeing, and that's not likely to get better any time soon.

Why not?

This is the season of the Low Moon. It appears highest in the sky when it's new (when we can't see it). From there, it declines each night, and by sunset is pretty low up until first quarter because it has been setting with the sun.

Then, by the time it starts to be near the meridian at sunset, it has gone lower in the sky due to the ecliptic inclination.

The weird effect is best seen in late June, July and early August new moons (which we have sort of this month): it will seem as if the moon always starts the evening at about the same elevation, and that won't be very high in the sky.

So, as usual, that dang ecliptic gyps us again. Low in summer, and high in winter when we can't see through the clouds!

Whoever designed this system didn't have astronomers in mind.


Mail to: Dave North
Copyright © 2000 San Jose Astronomical Association
Last updated: July 19, 2007

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