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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: A Lunar from the 1840s
From: Frank Reed CT
Date: 2004 Dec 10, 02:09 EST
From: Frank Reed CT
Date: 2004 Dec 10, 02:09 EST
I suggested a few days ago that the first part in puzzling out this lunar
observation is determining the year. The navigator noted that it was an
observation taken at 5P.M. on February 9th in 140W longitude, but
there's no year given so how do we figure it out?
It's a safe bet that the date of this lunar obseravtion would be
within a few years of the date of the logbook so that puts in the 1840s. Next,
we need some piece of very specific almanac info that can fix the date. Consider
how a lunars calculation works... we measure two altitudes and the lunar
distance itself. We perform some calculations on the measured lunar distance to
clear it of he effects of parallax and refraction. Then we compare it with the
tabulated lunar distances taken from the Nautical Almanac. That means that in
any reasonably complete lunar calculation, near the end, you'll find transcribed
a distance from the almanac. So take a look at "section 6" from that old lunar
observation (see the previous message in the thread for the other
sections):
"
[Section 6: left column]
105..57..45 [could be 51 or 52 or 57]
000..02..45
----------------
105..55..00
000..13..08 [??]
000..15..21 [carried over from right]
----------------
106..23..29
000..03..14
----------------
106..26..43
000..27..48 [from log calculation below]
----------------
105..58..55
104..56..18
----------------
001..02..37"
105..57..45 [could be 51 or 52 or 57]
000..02..45
----------------
105..55..00
000..13..08 [??]
000..15..21 [carried over from right]
----------------
106..23..29
000..03..14
----------------
106..26..43
000..27..48 [from log calculation below]
----------------
105..58..55
104..56..18
----------------
001..02..37"
Everything "adds up" as corrections applied to the number at the top. That
is, you get from one line to the next by addition or subtraction to the number
above. That's the clearing process leading from 105..57..45 to a cleared
distance of 105..58..55. But then where does the number 104..56..1
come from?? It doesn't follow from any of the other numbers on the page, so
that's it! That has to be the tabulated lunar distance from the Nautical
Almanac. Next, we can either go dig up an old almanac or use a modern
calculation and find a distance within half a minute of arc of this distance.
The chances of a coincidental match are very low. And sure enough,
predicted lunars based on modern ephemeris data (my web tools again) yield a
distance of 104..56..08 at 0 hours GMT on February 9, 1843. That's gotta be it.
This was one of the predicted geocentric lunar distances bracketing the time of
this observation. It's certain then. This lunar observation dates from 1843..
Next step: where were they?
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
[ ] Mystic, Connecticut
[X] Chicago, Illinois
