How Bright is the Moon? or Neptune?

We have an unusual occultation in June: on the night of the 22nd-23rd, the moon occults Neptune. Neptune is already behind the moon when they rise at about 11:30; watch for its reappearance about midway up the moon’s dark limb at 12:59 am. Unfortunately, the gibbous moon is only a few days past full, so it might not be easy to see dim Neptune with all that lunar glare.

How hard will it be? Just how bright is the moon compared to Neptune?

I had hoped to be able to give you a number; but it turns out the moon’s surface brightness isn’t an easy number to find. Of course, it varies over the surface of the moon – the terminator is usually not as bright as areas away from the terminator. It also varies with the moon’s phase: a full moon is brighter than a first quarter moon, not only in total brightness (duh, the full moon has twice the area lit up) but also in surface brightness: the first quarter moon is only 8% as bright as the full moon, not the 50% you might expect.

Why is that? There are several effects going on. The main reason appears to be related to shadows. Put simply, when we look at a full moon, we don’t see any shadows since the sunlight is coming from pretty much our direction. When we look at a first quarter moon, the light is coming in from the side and there are lots of shadows. So for any given area of the surface, we’re seeing less of it illuminated than we would when the light is more direct. (It turns out that effect happens on more than one level: not only are there shadows from major features like mountains and crater walls, but also on a much smaller scale in the way the light is reflected from the top few millimeters of the lunar regolith, the deep fine powder that covers the moon.)

Some people also claim that there’s an effect from millions of tiny glass beads in the lunar regolith, formed from millions of meteor impacts. These glass beads act like retro reflectors, reflecting most of the light that hits them back in the direction it came from. But the retro reflection is very directional; in fact, it may be so directional that we can’t see the effect at all from Earth. There’s only one time when we’d be enough in line with the sun to see the effect from the glass bead retro reflection: during a lunar eclipse, when we’re blocking most of that sunlight! Anyway, I haven’t been able to get good numbers on how this works, so I can’t say with any confidence whether it’s part of the brightness we see or not.

Anyway, this was getting interesting but not helping in figuring out whether that Neptune occultation should be visible. So let’s try another approach. How does Neptune’s surface brightness compare to Saturn? I’ve seen several lunar occultations of Saturn, and while it’s always striking how dim the planet looks compared to the moon, it’s easily bright enough to see, even against the moon’s dark limb. But Neptune’s albedo (the amount of light it reflects) is a bit less than Saturn’s, plus it’s three times as far from the sun as Saturn so it’s getting nine times less light to reflect.

So it’s pretty dim, but don’t give up all hope. I’d recommend fairly high magnification, and scanning along the moon’s dark limb while keeping the illuminated part out of the field as much as possible. If you stay up for it, let me know how you did!

Meanwhile, there’s some other stuff going on in the shallow sky too.

Jupiter rises in the early evening and transits a couple hours after midnight, moving toward its opposition next month. Unfortunately, it’s quite far south and so never gets higher than about 30° – so you probably won’t see a lot of detail this month.

Mars is visible in the evening sky, setting around midnight. It moves from Cancer into Leo during June, and ends the month right next to Regulus, nearly the same brightness but a very different color. They should make a striking pair.

Meanwhile, Saturn spends the month about 5° east of Regulus, showing a slim ring tilt of 8.5°.

Mercury emerges into the dawn sky during the last third of June, but it remains close to the sun and fairly difficult to spot. Venus is even harder to spot: on June 9 it passes directly behind the sun, marking the midpoint between the Venus transit Jun 8 2004 (which wasn’t visible from San Jose) and the one on June 6 2012 (which will be).

Last but not least, Pluto is at opposition this month, on June 20th. That means it’s the beginning of “Pluto season”, the best time to hunt for this distant planet. As veteran Plutocrats know, the trick is to get a really good star map that shows stars at least down to Pluto’s magnitude (13.9 at opposition). But you also have to be able to find the field that the finder chart is showing, which can be challenging all by itself! Planetarium programs aren’t always right in their Pluto predictions – its orbit is much more complicated than those of the other planets – so if you have more than one program, compare their output, or get a copy of the usually reliable RASC finder chart (it’s on page 213 of this year’s Observer’s Handbook, and I think the SJAA might still have a few left for sale).

It’s in a tricky field, though – the star-packed center of the galaxy in Sagittarius, just north of open cluster M23 and due west from open cluster M18. Does that make it harder (too many stars to confuse with Pluto) or easier (lots of field stars to compare)? You could make an argument either way, but most people would probably say it’s harder with that many field stars. On the other hand, it has its compensations: if you get frustrated in the Pluto hunt, there’s lots of eye candy nearby to cheer you up, so take a break, go look at some Messiers and go back, refreshed, to the Pluto hunt.