Tides
Tides are the periodic rise and fall of the
ocean surface due to gravitational pulls
of the moon and the sun on the Earth.
Positions of the moon and sun relative
to the Earth affect the time and height of
the tides.
Gravity is the
force that
causes tides.
(Newton said
that gravity is
stronger
when objects
are closer
together).
The tidal bulges move around
the Earth every 29 days, just as
the moon does (lunar month),
therefore, the tides rise 50
minutes later each day because
the moon rises 50 minutes later
each day.
The moon has a direct effect on
the tides.
So, if the moon rises
at 6:00 tonight, what
time would high tide
be tomorrow night?
6:50
The sun also has an effect on tides, but since
the sun is further from the Earth, the effect it
has on the tides is less than the moon.
Direct high tide occurs when the moon pulls the water
on the side of the Earth nearest the moon. (So the moon
pulls on the water more strongly than on the Earth itself).
Indirect high tide occurs because the moon pulls more
strongly on the Earth’s center than it pulls on the water.
High and low tides occur about 12 hours from the last
high or low tide.
change in tides
Spring Tides occur at the full or new moon phase,
when the sun, Earth, and moon are aligned. During
the Spring Tides, we get especially high tides and
especially low tides.
They occur twice a month. (Once for the full moon and
once for the new moon = 2 times).
Neap Tide occur at the quarter moon phase. The sun and
moon are pulling at 90 degree angles to each other, so the
sun’s tidemaking effect is subtracted from the moon’s
tidemaking effect. The result is that we get hardly any
change between high and low tides.
These Neap Tides occur twice a month. (Once for the 1st
quarter moon, and once for the last quarter moon = 2 times).