Wednesday, December 06, 2006

Coronas, Haloes, Moon and Saturn

Session name: 20061205.2015

In the sky, this session was a repeat of Saturday night though the persons and activities around the scope were very different. On both nights nearly Full Moons dominated the sky. Both nights clouds came and a huge 22° radius halo circumscribed the Moon. Saturday's was fainter and incomplete from the 1 ~ 4 o'clock section. I thought I might have seen a disconnected arc bowing in a direction away from the halo. This observing session's included a corona was apparent tight up surrounding the orb. It exhibited a straw yellowish color which was ringed with a purplish fringe. Charlie mentioned seeing, I think, as many as 5 concentric rings.
I' ve come to learn that halos are formed by light passing through ice crystals and coronae differ by light passing through water droplets and being refracted.

Field measurements were performed with extended thumb & pinky held at arm's length. The measurement could be approximated by the separation of some stars and Moon. About 8:10pm on Saturday, Algol was on the northeastern perimeter. Whereas this session's halo hosted a other bright, familiar stars just inside its rim: Betelgeuse and Bellatrix in the southwest and Castor and Pollux to the east southeast.

I arrived at Turtle Pond Observatory (TPO), having told the others that this is where I was going to observe. TPO has the advantage of a lower horizon to the northeast and east, and I wanted an early start on Saturn. But when I arrived the three street lamps on the liitle cul de sac are overwhelming. It didn't take long after looking northerly to see that TotL is substantially darker. There was a time when we put towels on the lamp - and this session I ensured I had Bulldog clips - but tonight I chose not to. Instead I walked the equivalent of 5 city blocks to the north end of the Great Lawn.

And on both evenings broke the park curfew observing Saturn rise in the east, unbothered by any patrols. The police did come by and waved to us on Saturday but this observing session, Charlie and I didn't see many people after the 7-dogs and their owners had visited. In addition to the normal features observed: rings, equatorial bands, polar caps, shadow, and division; there is one that I haven't made peace with. On the northern face of the planet, Saturn's rings cross the disk. A dark edge, which I assume to be the C-ring, contrasts with the planet's creamy white "surface". On the outer edge (more northerly) the ring casts another dark line, at least I am guessing it is the ring. Can't figure out why that would be dark, I don't think it to be shadow - particularly comparing to how the planet's orb casts its shadow on the rings.

Charlie arrived when I was completing snapping Moon. We would spend a good amount of time concentrating on the evening terminator of the waning 15d3h Moon, especially along the southeast. There was one crater, Boussingault, that we observed for a bit of time. At the time of observation we didn't identify it and incorrectly I assumed we were looking nearby Schickard. Charlie pointed out the two-toned floor crater in the southwestern quadrant, in addition to the nearby crater Schiller.

Back at Boussingagult, he crater exhibited an obvious interior concentric ring, a bit off center. I considered this a feature of heavily terraced walls, walls that slumped and there were pockmarks, or a string of craterlets, on the southern edge of this slumped wall. On the northern side, a craterlet in the rim was observed pretty easily.

The following picture shows the southeastern quadrant we were looking at.

Click on the southeastern quadrant pictured above for a labeled view.

We also cruised along the northeastern quadrant where the terminator provided more fascinating views. Most of the passersby who stopped and had some looks commented mostly about these obvious features. It was pretty hard not to notice crater Humboldt with its central peak and crater walls still catching light, but ready to fall in the precipice of darkness.

Click on the northeastern quadrant pictured above for a labeled view.

Maps of Moon available through LPI
Images of crater Boussingault from Digital Lunar Orbiter Photographic Atlas