Hello again.

Casio produce several watches, for example the G=Shock GLX-5600, which
display a little moon phase image and a tidal graph divided into six sections
(high tide section, two falling tide sections, low tide section and two rising
tide sections).

When you set up the watch correctly with the year, month, day and time, it
works out and displays a pictorial moon phase and a numerical moon age. When you
set up the correct longitude and the lunitidal interval (the constant interval
between the time the moon crosses your harbour’s meridian and the time of the
next high tide at that specific harbour) for the harbour or beach you are
interested in, it works out the state of the tide and displays it on the graph.
HOWEVER – and it is a big however – the watch manual states that the watch
performs a rough calculation of moon age using integers only and therefore the
moon age could be out by +- 1 day. It then states that as the tidal graph is
based on the moon age, it could also be out by +- 1 day.

In practice, the numerical moon age displayed by the watch stays the same
all day long. 21.3 on one day, 22.3 the next day, 23.3 the day after and so on.
The tidal graph behaviour is more intriguing. Say high tide at your selected
harbour is at 12:15. An ideal watch would highlight the high tide section just
before 11:15 and move on to the first falling tide section just after 13:15.
Casio watches insist on changing tide graph sections on a whole round hour,
which means they work in two hour jumps, and once a day make a 3 hour jump to
catch up with the moving moon. So if high tide is indeed at 12:15, the watch
could either start displaying the high tide section at 11:00 and move on at
13:00, or it could start displaying it at 12:00 and move on at 14:00. There is
no telling in advance. Occasionally, you will hit a three hour interval so for a
12:15 high water time the high water section would display between 11:00 - 14:00
or between 12:00 – 15:00. I use the high water section as an example – all this
is true to all the other sections as well.

The manual does not mention any of this – all this is based on observing an
actual watch over a few days and comparing it to the tide at Brighton Marina in
the UK (longitude just a few miles off the Greenwich meridian, lunitidal
interval of 11:05). Does anybody have an understanding of what exactly is going
on mathematically inside this little watch’s mind? Does anybody know of a watch
that does calculate moon age properly using fractions and therefore displays an
accurate tidal graph? Having invested all this effort by putting the moon phase
and tidal graph into their watches, I am amazed that Casio let them down so
badly by using integer calculations only.