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Last night, while waiting for the Space Shuttle  Discovery to fly over
Chicago, I shot some lunar distances between the Moon and  Jupiter. I didn't know
where Jupiter would be in relation to the Moon, but my  online calculator (which
I can access from the lakefront with my cell phone)  gave a predicted
distance of 15 degrees. I pre-set my sextant and checked that  distance on both sides
of the Moon. Nothing... Even though it was early in  twilight, I still
expected to see it. Then it dawned on me -- I had entered the  wrong date.
Recalculating gave a distance of just over 5 degrees. And sure  enough, there was
Jupiter: 5 degrees away and directly ABOVE the Moon. For  finding GMT, this
geometry would not be ideal, but for testing my sextant, it's  as good as any. Here
are my sights (e is the calculated error, all times are US  Central Daylight
Time):
20:39:19   5d 18.8'    e=0.2
20:42:45   5d 19.2'    e=-0.1
20:46:23   5d 20.1'    e=0.1
20:50:54   5d 21.2'   e=0.4
The average error on  these sights is 0.15 minutes of arc. My sextant is an
"International Nautical"  (Tamaya-alike, similar to an M.A.C.) with polarizing
filters and a 7x  "monocular" telescope. Jupiter is great for lunars since the
planet's disk is  about half a minute of arc across. One can split it neatly
on the Moon's limb.  These sights were also a nice opportunity to check for
"shade error" in my  polarizing horizon filter. At its lowest density setting, I
could swing it in  and out and directly compare the alignment. There was no
shade error as far as I  could tell. IC for these sights was +0.7'

An hour earlier, I did some  Sun-Moon lunars:
19:41:50  115d 52.6'    e=0.0
19:43:34  115d 53.0'   e=0.0
19:45:20  115d  53.0'   e=-0.4
19:47:30  115d 54.0'   e=0.1
With  a distance this large, I was able to confirm that my table-top
telescope  collimation had worked correctly. The Sun and Moon images separated
symmetrically at the extreme left and right edges of the telescope's field of  view.
The Sun's altitude was as low as 6 degrees, so these sights had to be
carefully corrected for temperature and pressure as well as flattening of the  Sun's
semi-diameter.

It's interesting to note here that the apparent  lunar distance in this last
set of sights increased by only 1.3' in 5min 40  seconds while the true lunar
distance increased 2.7'.

-FER
42.0N  87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars


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