I dislike the word Supermoon very much, it’s in no way descriptive and somewhat childish.
The tides are the rise and fall of the oceans, caused by the combined forces of the moon and sun along with the rotation of the earth.
The earth and moon are both substantially large objects in space and have a gravitational pull that is significant enough to keep the moon in orbit around our planet. The gravitational force of the moon also attracts the surface of the ocean, forming a bulge.
As both the earth and moon are rotating in space, the centrifugal force generated causes another ocean bulge to form on the far side of the earth.
Therefore, the oceans of the earth are not uniform but form two bulges on the earth (See image below).
The size of the tides depends on where the moon and sun are in relation to the earth.
Aside from simply ebbing and flowing, the height of the tide is changeable depending on the position of the moon. When the earth, moon (full and new moon phases) and sun line up (called syzygy) the gravitational forces combine and cause high tides to be higher and low tides to be lower, these are Spring Tides.
When the moon is in its first and third quarter phases (image below) the suns gravitational pull is perpendicular to that of the moon, stealing water created by the moons pull and lessening the height of high tides and increasing low tide level.
Now, these are the celestial forces we’ve all come to know and love which work in cycles over a short period of time. However, the moons orbit around the earth is not circular but elliptical, where sometimes it’s closer to the earth and sometimes farther. This is the reason the moon sometimes appears larger or smaller in the night sky and can appear a great deal large, 12 – 14%! Today, March 19th 2011 the moon will appear it’s largest in nearly 20 years.
Today, March 19th 2011 the moon will appear it’s largest in nearly 20 years, which some people fear will cause an increase in earthquakes and volcanic activity.
As we’ve seen above this lunar perigee will give us above average high tides and lower lows but by only a few centimetres, not even 3cm! Though the moons gravity does affect us, it’s relatively weak and no match for earth’s internal processes which do cause earthquakes and volcanic discharges.
Today we will experience a high tide higher than average and a low tide lower than average (twice), but don’t cower and hide from the geologic forces the pseudoscientists would have you believe are a’comin.
Forget all that, go look at the moon, it’ll be big and cool!
A small aside: Strictly speaking, perigee and apogee are the terms given to the positions of the earth in it’s orbit in relation to the sun for example, celestial bodies have their own personal suffixes, the moons being lune. Today the moon will be in perilune.