SJAA

Ephemeris

Mooning

David North


Continuing with my theme of "when does the moon get high" we'll take a look at an early moon this month - only four days old.

Normally, this means the moon will soon set and there is not much you can see. But on this day in which it will be at it's highest northerly declination, it will still be 40 degrees above the horizon at sunset. That means it will linger for hours, but also that you only have a very short window for good viewing. Tonight, sunset viewing is a must.

Sunset is a fine time to view the moon. Often, the seeing is very steady, and it's easily dark enough to see our bright companion.

Why look at the moon at a time like this, when it's just going to drop down into the soup in an hour? Because this particular part of the moon is usually seen after full moon, as it rises late at night or in the early hours of the morning when it is well placed. But at those times, the light comes from the other side. This month, we can see it in morning light!

So off we go. What is there to see?

Something of a treat, actually. The libration at this time will be fairly strong, showing much of the eastern limb that isn't often visible ... and that will be just a setting for the action along the terminator. A frame for the painting. You'll be able to see large sections of the Humbolt Sea and Mare Smythii, along with very good views of Mare Spumans, Anguis and etc. Make sure you take a look along the limb.

Along the terminator (where night meets day) you'll probably notice a large crater at sunrise in the north: that will be Hercules. Right next door in very good light you'll see Atlas, a cracked and beaten crater with some very nice rilles in the floor.

Further south, the area of Mare Crisium will be in very good light. In particular, you'll be able to inspect the area known as Palus Somnii, just to the west of Crisium (which Jay Freeman thinks of as a teddy bear, but reminds me of the head of a flatworm). Try to memorize what you see of Somnii in this light, when you can make out the topography somewhat - then compare it a few nights later when you can see the odd rays from the crater Proclus, which seem to go everywhere but over Somnii. This is its defining trait. (It's presumed the ray structure is due to the angle of impact of the object that formed Proclus).

Where Mare Fecunditatis is near the terminator, you may have a shot at some nice rilles that edge the mare. The easiest is the Goclenius complex, which are long and multiple and may show well. Near the middle of the moon you may also see the fingers of Rima and Rupes Cauchy emerging from shadow, which should be the name of a sitcom couple...

Perhaps the most spectacular view will be near the south, where the terminator will cut across Janssen (with the monsters Fabricius and Metius just north). Janssen has a messy floor with a large rille system inside that should show well in any scope. This is one of the neatest craters on the moon!

You'll probably have only about an hour or less to drink it in, so come thirsty and don't waste a minute.

In other news, full Moon will be May 30, and the strongest libration of the northern limb will be only two days before that. This means the 28th (and even the night of full) should be very good for taking a look at this area, and if you're lucky you'll catch the terminator crossing an edge-on crater. This gives you an excellent shot at seeing the terraces from the side...

For those of you who go to the seashore, May 15th will see the second strongest tides of the year, due to a particularly close perigee. So be careful.


David North; last updated: October 04, 2007 Prev Next