A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Zvi Doron
Date: 2012 Mar 16, 15:06 -0000
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.