David North

In the Bay Area, our Christmas present is to miss a series of interesting occultations: everything from Mars to asteroids will be occulted somewhere this month, but we won't see any of them!

Librations? When the east limb is best exposed, it's unlit. Same for the west. Both the maximum northern and southern librations work out to be somewhat unfavorable also.

In other words, there ain't nothing special happening up there this month.

Except for one thing: the unofficial beginning of Moon Season.

Why is that?

First, you have to understand I don't often (maybe a couple of times a year) get up in the morning and set up a telescope. I do most of my observing in the evening, before hitting the sack.

So if you want to see the moon in the evening, it is of course at its best when it is high in the sky.

This month, it will be at its highest just after full moon, and at its lowest a couple of days after new. That means the favored "first quarter and later" views will be at about an average declination, increasing each night.

The effect this has on average seeing is stunning! I'm sure you've heard me talk about this before, but usually in the context of one month or another being best... but that's something of a white lie.

Starting as early as December, we start getting very favorable views of the nights just before and after full moon, which have the virtue of always being clear (just ask any DSO addict). Then, as the first half of the year progresses, the prime days move forward from full toward first quarter in the late spring.

So why is it not so good after the maximum declination has moved forward beyond first quarter? It's simple, really: the first quarter moon is at it's maximum elevation right at sunset, so if you're ready you can get the ideal views just before dinner... but before first quarter, the maximum elevation is before sunset!

Now I know you can look at the moon in daylight, but it really doesn't show well until dusk has set in (full darkness is not necessary, and it gets much better right after sunset...)

But wait, there's more! What if you do want to view the moon in daylight, or before sunset?

Get a polarizing filter.

No, I don't mean a dual polarizing filter such as Orion sells to adjust the throughput, though if you get one of those you can unscrew it an have two single polarizers... I mean just one.

Reflected light is polarized, as is some of the scattered light from the sun. If you stick a polarizer on your eyepiece and spin it around, you'll find a position where the contrast on the moon improves quite a bit.

Another experiment I've run with polarizers? It seemed to me they should be able to limit the effects of bad seeing, artificially improving "steadiness."

Results have been uncertain. Sometimes I think it works, and others I think it does nothing. Nor have I been able to determine what conditions might attend either result.

But it's a fun experiment, and relatively cheap. If anyone else cares to give it a shot, I'd like to hear what results you get... send me a message (

David North; last updated: October 03, 2007 Prev Next