Tides are caused by Gravity pulling on the bodies of water on the earth and upon the Earth itself. There are 2 gravitational bodies that affect the tides. The sun and the moon.

The Moon is much closer to the Earth so it has a much greater influence upon the tides.

Notice that when the Earth, Moon, and Sun are all in a line (Full and New Moon Phases) the high tides are MUCH higher than at other times. These are called SPRING TIDES

Also, when the Moon and Sun are at right angles to each other the high tides are lower than at other times. These are called NEAP tides


image from Astronomy 161 Dr. Steven Gaunt University of Tennessee
Gravity pulls on the Earth with different levels of strength. The parts closer to the Moon over head are pulled on harder than those parts further away.

image from Astronomy 161 Dr. Steven Gaunt University of Tennessee
A Tidal Bulge (High Tide) is produced on the side of the Earth DIRECTLY below the Moon. Another is produced on the EXACT opposite Side of the Earth.

The bulge under the moon is produced due to gravity "stretching" the water, pulling it toward the moon. The bulge produced on the opposite side of the Earth is made because the moon tries to pull the Earth out from under the water!


image from Astronomy 161 Dr. Steven Gaunt University of Tennessee

This image show the 3 possible configurations of the Earth, Moon and Sun. The First shows a Neap Tide. Gravity of the Moon and Sun pull at right angles to each other. As a result the high tides at this period of time will be the LOWEST high tides that occur during the month. The Moon is in a quarter phase in this configuration.

The second image shows the arrangement during a New Moon phase. Notice that the Moon and Sun are on the same side of the Earth. The high tides produced at this time will be the highest that can occur during the month.

The third image shows the arrangement during the Full Moon phase. Notice that the Moon and Sun are again in line with the Earth. This arrangement will produce high tides that are higher than the average high tide of the month.

During both the FULL MOON and the NEW MOON phases the high tides will be higher than the average high tide for the month. These are called

SPRING TIDES

Back to Astro-Web: Earth, Moon & Sun     Back to Science Rocks! Home Page