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Mike Burkes has been perceptive enough to note discrepancies between
predictions in "The Online Nautical Almanac" for dates in the 1700s, and
has drawn this to my attention.

You can access this online almanac via-



and then typing in the date that interests you in a box at the bottom of
the page.

I have looked into the matter and discovered that the predictions show a
jump of one day (plus an hour) between 0h and 1h on 28 Feb 1800. For
earlier dates than that, the data printed is relevant to the day before
that implied by its date-label. Data relevant to the day of 27 Feb 1800 is
missing altogether, and in its place is the data for 26 Feb, and so on.
After 1h on 28 Feb 1800, the predictions correspond correctly with the
date, but before that, it's all a day out. Users beware!. To get the right
results before 28 Feb 1800, you should obtain the predictions, not for the
date you really want, but those marked for a day later. And there's no way
(except by interpolation) to get predictions for the day 27 Feb 1800.

To show the "jump", Sun declination is predicted as S 8deg 30.0', for 0h on
28 Feb 1800, but an hour later it has suddenly changed to S 8deg 06.4'
(quite impossible in an hour!)

Of course, 1800 is one of those years which divide by 4 yet which isn't a
leap year. The Online Almanac program gets that non-leap-year right for
1800, in not printing output for 29 Feb, and getting the day-sequence
correct as Thursday 27th, Friday 28th, Saturday 1st March. But somehow the
computation of Julian Day Number must have slipped here by a day, for dates
preceding that leap year.

This is a bit ironic. I think it was Frank Reed who pointed out that when
Hamilton Moore produced his "Practical Navigator", a few years prior to
1800, but with predictions spanning that period, he failed to allow for
1800 being a non-leap year. That allowed Bowditch, in his own first
edition, to scoff at all the errors that ensued in Moore's predictions, for
dates after 28th Feb 1800. Of course, Moore and Bowditch were predicting
ahead from an earlier era, whereas the Online Almanac is working backwards
to 1800 from a later era.

The web page referred to above states clearly that "valid dates are in the
range 1950 to 2050", so those using the almanac outside that interval do so
at their own risk. Yet I have found the predictions to be usefully accurate
right back to the early 1800s.

I know that Ken Muldrew has been investigating David Thompson and other
travellers in Canada around this period, and one of his postings
specifically refers to data from the Online Nautical Almanac for October
1800. This is close in time to the moment when the Almanac goes haywire so
Ken needs to take care that none of the data he uses (or has used) relates
to that period before 28 Feb 1800.

I am notifying Omar Reis (at omar@tecepe.com.br, in Brazil) about these
difficulties that historical researchers face, and will pass on any
reactions from him. I suspect that he never envisaged his predictions being
used for the purposes we are putting them to, so could quite legitimately
respond by pointing out that we are working outside his stated validity
period of 1950 to 2050. But I hope he will be more constructive than that.

George

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01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
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