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Dark Side Of The Moon

Dave North

We're always talking about the terminator (where light meets dark) with an eye toward what's on the lit side. And that would be perfectly normal this month, when the first quarter Moon is highest for the year.

So look at the first quarter Moon! Start around the 7th and pay special attention over the next week.

But while you're looking, don't forget there's another side to the Moon... and that should be particularly obvious in the days before full.

Usually, however, the dark side isn't dark.

Since the Moon will be riding high, you'll be able to see the most commonly known (and beautiful) of the dark side phenomena: earthshine.

That's when the unlit portion of the Moon is visible as a ghost image, due sunlight reflecting off us (primarily the clouds).

The notso-darkside.

Earthshine is at its best right at new moon, but unfortunately you generally can't see it then because of the glare of the sun. So the best date in any lunation is a factor of elevation, elongation and atmospheric conditions on the other side of the planet (and this side too) ... which means I can't say which day will be best. Just look!

Okay, what's next?

When there is a substantial dark side, whether there's earthshine or not, you'll still be able to make out some features.

If the east is lit, look especially for Aristarchus: the brightest spot on the moon. Even when there is only the slightest light, you can often spot it.

On the other hand, when earthshine is strong on the west, you'll find Grimaldi obvious as a dark spot.

Sometimes you can see slight rays from Tycho, and usually the larger maria can be made out, contrasted against the softly brighter highlands.

But what if the dark side really is so dark you can't see it?

That usually means it's small, and the bright side is washing it out. But there will still (usually) be something to see "over the terminator."

High spots.

In fact, some of the most spectacular views of the moon require a high feature on the far side of the terminator, barely lit by grazing sunlight.

You can see distended "horns" at the tips of the moon now and then, or more typically a mountain or the rim of a crater just off the terminator, making a dot or smile in the darkness.

And not to be missed is the every-other-month Sinus Iridum hanging off the edge: its rim is tall enough that it becomes a huge semicircle cutting into the darkside, with an amazingly complex floor drifting off into night.

There are, of course, other things to see on the Dark Side Of The Moon, and if you take some time to look at it, you'll find 'em.

Oh, and there is one bright-side thing I'd like to mention (other than - to repeat - the wonderful first quarter views we might get). On the 17th, the eastern limb (Crisium) will be strongly liberated toward us, just before full moon.

This is a good opportunity to see the eastern Maria. Not much rim wall detail will be visible, as the light will be too high. But the contrast of light to dark should be fairly good, so the maria will stand out nicely.

The next night (if my numbers are right) the terminator will be slightly on the area you were looking at, and you might get a look at some very amazing rim walls: this should be a fascinating comparison with the view the night before.

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

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