Shows
high, low and half tides and the hours in between are marked. Please
note that unlike many of the other tide clocks featured on Amazon, this
model is one of the few with the hours correctly scaled on the clock
face. As, to follow the cycle of the tides, one cycle of the clock must
take 12 hour 25mins, it follows that the time period between the 'High'
and 'Low' points on the clock face and the next [5hr] point will be 1hr
12.5mins while the remaining graduations should be 1hr apart. As can be
seen in the photograph, the spacing of the graduations on the clock face
correctly indicate this.
The moon is the major cause of the
tides. The lunar day (the time it takes for the moon to reappear at the
same place in the sky) is 24 hours and 50 minutes. Most places have 2
high tides and 2 low tides each day. Therefore this clock is designed so
that it's hand rotates once ever 12 hours and 25 minutes, twice each
lunar day.
There are actually 2 tidal cycles: a twice
daily cycle and a once daily cycle. On a tide when the 2 cycles help
each other, high tides will be higher and low tides will be lower. On
the next tide, when they conflict, the tidal ranges will be smaller. The
relative strength of these 2 cycles varies from week to week and place
to place.
The sun also affects the tides, but has less
than half the influence of the moon. When the sun, moon and earth are
lined up, as they are at the time of a new moon and full moon, their
influences combine and high tide is higher than normal and low tide is
lower than normal. When the sun and the moon are at right angles, as
they are at the first and last quarters of the moon, the sun cancels
some of the moons effect and the range of the tide is smaller than
usual. Also, at these times the sun will make the tides somewhat earlier
or later than average. This is why it is important to set your tide
clock on the day of a full moon as the moon has the dominating effect on
the tides.
Abnormal atmospheric pressure can
temporarily affect the time and height of tides. Difference of 1 inch in
barometric pressure will cause a 1 foot difference in sea level. Strong
onshore winds will also cause a temporary increase in sea level. Both
these effects will change the times of low and high tides as well. Tides
in the lower portions of rivers will be affected by changing volume of
river flow.
Your tide clock always stays in exact step
with the moon, but these other factors can make the day to day tides a
little earlier or later than the tide clock shows. Therefore, to
minimise variation, place the AA battery into the clock on the day of a
full moon at exactly the time of the high tide according to local tide
tables and with the hand on the clock pointing to “High Tide”