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Brad wrote-

| Rather than turning LAT into LMT and then adjusting LMT for the
difference in longitude from
| the central meridian, just use LAT directly.

WHY? Where has equation of time got to?

LAT was determined by observation of the Sun, so apparent time.

Frank's lunar predictor provides GMT, Greenwich Mean Time.

Before you look at the difference between those two times, you have to put
them on the same time base. Apparent time and mean time differ, through the
year, by a variable amount, of up to a quarter of an hour either way, the
equation of time. If you don't allow for the equation of time, you will get
the wrong answer.

======================

The other question, that Kermit keeps mentioning, relates to delta-T, due
to slowing of the Earth's spin. Bayly's own calculations were correctly
based on the almanac of his day, and required no correction for delta-T.

Modern back predictions of the positions of astronomical bodies should
always include an allowance for the slowing of the spin, and should say so.
If they don't allow for delta-T, they should say so, too. I presume that
Frank's lunar calculator allows properly for past delta-T, which is not
hard to do. The difficuly, with delta-T, is when you want to predict into
the future.

As I see it, Frank's lunar predictor for 1773 is intended to provide GMT
values that relate to the time shown by a clock running on Mean Time at
Greenwich, in 1773. Bayly's local apparent time is to be converted to local
mean time, and the difference between those mean times is his longitude. No
allowance has to be made for delta-T.

Please argue back if you disagree.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

   

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