5 September 2007

It didn’t look like it was going to be a good observing night so I stayed home last night. I probably should have gone out.

When I got home there were clouds on both the north and south horizons. AccuWeather’s combined radar and satellite loop was showing a band of clouds moving toward the area from the NW. NOAA was saying there would be 50% cloud cover at dusk and that it would only drop to 40% by midnight.

I went up to MetOval to do some reading in astro-archeology. While reading about Moon alignments I remembered Peter and my discussion the other night about lunar libration and his statement about it being irregular. I decided to try to graph libration.

I started looking up libration values for the month of September in Virtual Moon Atlas and when I was part way through the month I came to New Moon and decided that basing my chart on the lunar month would be better than basing it on the calendar month so I made the chart below for libration during Lunation 1048.

The first thing I noticed was that the libration cycle appears to be longer than the lunar cycle by about two days. The next thing was that the points seem to bunch up at the ends of the ellipse and spread out in the lower left quadrant, but don’t spread out on the opposite side of the ellipse.

I decided to extend the plot to an arbitrary length of five lunations to how quickly the ellipses rotate around the central axis and to estimate the length of a lunation “saros.”

The chart got very busy so I had to drop out the individual data points so that I could see the trend.

The obvious thing here is that the shape of each lunation cycle is different from each of the others. That is the variability Peter was talking about. Also, there isn't the same amount of overlap on all of the ellipses.

Five lunations is not a long enough run for me to tell what is going on, but any longer and I wouldn’t be able to tell what is going on for all the clutter on the chart.

I went back to my original chart and added in markers for New Moon and Full Moon to give a bit of a frame of reference. Then I added the apogee and perigee markers to see if there was any relation between those distances and the spacing of the points. Apogee falls in the nether land on the far side of the ellipse where the data points are neither close together nor far apart. Perigee falls in the area of the cycle where libration is changing the fastest. I can’t draw any relationship there.

It looks like it could be a Kepler’s second law situation (equal time-equal area) except that then the flat portion of the back of the ellipse should have widely spaced data points like the front does.

It is higher math than I want to deal with so I am going to leave it at the pretty pictures.

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Observing Location	MetOval
Observational Period	2040-2050 EDT
Atmospheric Conditions	Cloud Cover Scattered Clouds Temperature 71°F Wind Calm Humidity Moderate Feels Like Comfortably cool The clouds have been filling in and clearing out. Fortunately they were mostly clear up to the north. Transparency Fair Seeing
Instruments	Naked-eye - Charlie
Observing Party	Charlie Ridgway

Target Iridium 66 Flare Constellation UMa Category Satellite Time yyyymmdd.hhmm 20070905.2044 EDT Comments I had given up on this flare when I saw it much farther to the east than it should have been. It wasn’t anywhere near Polaris. It was very bright and had a yellow tinge to it is it faded. Mag.: -7 Alt.: 25° Az.: 358° There is a string of these flares that are all Mag -4 or brighter that occur every day for about a week a little earlier and a little higher in the sky each day.