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I don't have a 2015 Almanac, but here's an example from 2013. At
longitude W097 36.0, what time is the Sun's meridian passage on Sep 27
local date?

1. In the daily pages for Sep 27, look for the greatest Sun GHA that's
less than the west longitude. In this case, it's 92 17.8 at 1800 UT.

2. That GHA is 5 18.2 less than longitude. In the increments and
corrections table, sun and planets column, look up that angle and find
21m 13s.

3. Thus meridian passage occurs at 18h 21m 13s UT. Again, that's *2013*
Sep 27.

The USNO MICA program says I missed by .07s.


If the longitude is east, convert to the equivalent west longitude by
taking the explement (subtract from 360). For instance, E097 36.0 is the
same as w262 24.0. Following the procedure I outlined above, opposite 5h
in the Sun GHA column find 257 15.0. That's 5 09 short of the desired
angle, or 20m 36s. So transit time is 5h 20m 36s UT.


In some cases the conversion to local time will put you in the wrong
date. The remedy is fairly intuitive. For instance, if longitude is w179
00.0 and local time is 12h behind UT, when is meridian passage on 2013
Feb 10 local date?

In the daily pages Feb 10 tabulation, the greatest Sun GHA less than
longitude is 176 26.9, which is opposite 0h UT. Oops. Obviously, after
you subtract 12h from UT to convert to local time, you're in the
previous day.

So go to Feb 11. Opposite 0h find 176 26.8, which is 2 33.2 less than
the longitude. That corresponds to an increment of 10m 13s. So meridian
passage is Feb 11 0h 10m 13s UT, or Feb 10 12h 10m 13s local time.


Well, that's how *I* would calculate meridian passage with the Almanac.
But the original poster said, "specifically referencing the data located
on the right hand bottom of the far right hand page," and, "The reality
for our class is that our sights require time to the nearest second."

Assuming that means transit time to 1 second (far in excess of
navigational requirements), we have to utilize the equation of time.
 From the 2013 Almanac:

      0h UT  12h UT
27  8m 55s  9m 06s
28  9m 16s  9m 26s

Those times are on the Greenwich meridian. The table is not shaded,
which means apparent time is ahead of mean time.

At W097 36.0 meridian passage is about 18h UT on the 27th. Thus we have
to interpolate between 27th 12h and 28th 0h. The desired time is (97.6 /
180) of the way from the former to the latter. Note the EoT increases
only 10s in that period, so no great accuracy is necessary. With a slide
rule I get an increment of 5.4s, so EoT is 9m 11.4s. I.e., the Sun is 9m
11.4s past the meridian at noon mean time on the observer's meridian. So
meridian passage is at 11h 50m 48.6s

But civil time is based on the 90° W meridian (? - observer's longitude
is near a zone boundary), which is 7°36' to the east. The "conversion of
arc to time" table on page i gives the increments as 28m (for 7°) and 2m
24s (for 36'). So the zone (standard) time is 11h 50m 48.6s + 28m + 2m
24s = 12h 21m 13s. That agrees with my calculation via GHA, but it's a
lot more trouble.

   

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