The High and Low tide are caused by the gravitational forces between the Earth and the Moon. The
Moon attracts every piece of matter on earth. Since the waters on the Moon side are attracted more
strongly, they bulge towards the Moon, causing a High Tide. The waters on the opposite side of the
Moon, bulge away from the Moon - also causing a High Tide. Low tides occur at about right angles to
When the Moon is in its first quarter or its last quarter, the Sun's gravitational pull is in
perpendicular direction to that of the Moon. The Sun pulls water away from the areas of High Tide
to the areas of Low Tides, resulting in lower High Tides and higher Low Tides. These are called
When the Moon is between the Sun and the Earth (at New Moon), the Sun's gravitational pull is in
the same direction as the Moon's. During these days, the High Tides are higher and the Low Tides are
lower than they would be with just the Moon's pull alone. This is called Spring Tide.
The same thing happens when the Moon is on the opposite side of the Sun (Full Moon). The two
gravitational forces work together to make high High Tides and low Low Tides.
The Moon does not stay put, but rotates around the earth at a rate of about 12o a day, or one rotation
a month. The rotation is in the same direction as the earth's spin, so by the time the earth has done
one rotation, the moon has shifted 12o further, and it takes an extra 50 minutes for the Moon to
be in the same position relative to a point on the Earth. Therefore, the tidal cycle is not 24 hours long,
but 24 hours and 50 minutes. Because of this, High and Low tides are about 50 minutes later every day.