SJAA Ephemeris November 2002 | SJAA Home | Contents | Previous | Next


Don’t forget your Leonid cap

Dave North


This month's question: what do you do when the Leonids are gonna be good, but it's a full Moon?

As the song goes: Look Away.

However, it will be just about as full as a Moon can get without actually being completely full, which isn't the most fun. What's worse, there won't be any strong librations ... which can make a near-full Moon an outstanding view ... but won't.

So what to do?

First, don't go anywhere unless you need to get away from clouds, or nearby lights are almost blinding. No matter where you go, there will still be plenty of light from the Moon. This does make choosing your observing site a little less demanding.

On the other hand, if it looks like there will be low clouds (but not high) it will mean going up some nearby mountain.

If you have to do that, you'll probably end up with some time on your hands, and might even want to look at the Moon.

Early on, it will be low in the sky. Probably you'll be getting to your observing spot later, since there's not much point in freezing all the way from sundown. If you are there early, though, you'll start with a deeply colored Moon near the horizon at dusk. That's a fine view in and of itself!

But the seeing won't be good. When that's the case, you're better off at low power, which means you'd better have a Moon filter.

I know, I'm no big fan of them, but this is one of the times when it would be handy. Best for such a situation is the standard old grey filter, usually sold (for example, by Orion) as a ... Moon Filter.

The dual polarized adjustable whammy filters tend to have internal reflections and flaws that kind of mess up the view, in my opinion, and I don't like them.

But there's a particularly good reason for a Moon filter tonight: you do not want to completely blast out your dark adaption to the point where your eyes are too dazzled to see a long burner if you hear the beginning of the oohs and ahhhs...

Full Moon is particularly good for observing rays (the bright radial lines coming from younger craters) and maria near the center of the disk.

(Maria are the darker areas, the remains of huge and ancient impacts that filled eventually with basalt, which is darker than the typical "highland" material).

The contrast between the dark "seas" and light "highlands" is particularly sharp at full Moon.

As the Moon comes up the seeing will improve. This gives you a chance to try some more detailed observing.

The first thing to do will be to take a look at the western (left) side, where there should be a terminator still.

Of particular interest will be the outer ring of Mare Orientale, the freshest of the huge impacts. The 'shock rings' are apparently a kind of resonance effect from such a huge impact, and if you've seen pictures of Orientale "from above" you know it looks like some weird dartboard or "bullseye."

The libration toward us will be quite weak — only a couple of degrees — or this would be a pretty good night for observing Orientale.

However, if you're up long enough, you should be able to glance now and then with any reasonable power and watch the light change.

Also of interest will be Bailly in the south (a huge old crater) and probably Pythagoras in the north. The latter has a way of presenting itself as an "edge on" crater, where you can look across it and see the terraces of the far wall.

It's hard to predict when the light will be exactly right for this to be really cool, but Leonid night offers an opportunity.

There may be several lesser 'edge' craters too, so they might be fun.

About now, however, you should be getting near midnight (and the Moon seemingly nearly overhead). At that point, you should pretty much be paying attention to large areas of the sky. The radiant has risen and you're moving into prime time.

So here are a couple of clues:

Often, the most spectacular meteors are early on, since they have a better chance of 'planing' the atmosphere, or even skipping. This makes for long or multiple trains all the way across the sky, which is wonderful. This is less likely later.

And here's a very useful tip: take a baseball cap (or something like it, with a bill). You can rotate it around and up or down to just block out the glare of the Moon, which will help.

If we get lucky and the moisture (and/or dust) level is low, you may find the Moon is not particularly a problem for meteor watching, especially if the action is fifteen or more degrees removed.

On the other hand, if there are hazy clouds, it may blow out just about everything.

A moderate hope would be only the dimmest meteors are washed out, and who cares about those except Peter and Mike? We're there for the Big Bada Booms!


Previous | Contents | Next