The Heavenly Tug o' War

The Heavenly Tug o' War The relative positions of the moon, sun, and earth affect the strength of the tides.

The following items all affect the ocean tides:

EARTH

Rotation

As the earth rotates, it moves below the bulging tides which are pulled by the moon, causing the tides to seem to move around the earth.

MOON

Position

As the moon orbits the earth, the ocean bulges beneath it pulling the tides. This is the major force on the tides.

Distance

Because the moon's orbit is an ellipse, its distance from the earth varies. At perigee, when it is closest to the earth, it is 222,000 away and the tidal force is 22% more than average. When the distance is greatest, at apogee, it is 253,000 miles away, and the force is 16% less than average.

Declination

The moon is not always directly over the equator. As the lunar plane moves north or south of the equator (±5°), the tides in the northern and southern hemispheres will vary. The force is greatest.

SUN

Position

As the earth orbits the sun, the gravitational force of the sun also affects the tides, although not as greatly as does the moon. When the forces are aligned, that is the sun, moon, and earth are in a straight line, we have SPRING tides, and the combined force is about three times greater than when the sun and moon are at right angles (NEAP tides).

Distance

Because the earth's orbit is an ellipse, its distance from the sun varies by about ±1.7%. In January, when the earth is nearest the sun, called perihelion, the tidal effect is about 5% greater than average. In July, when the earth is farthest from the sun, the tidal effect is about 5% less than average.

Declination

At the summer and winter solstices, the moon is 23° above or below the equator, causing unequal tides in the northern and southern hemispheres. At the spring and fall equinoxes, the tidal effect is greatest as the sun is closest to the equatorial plane.

The combination of astronomical positions that causes maximum tidal forces is when perigee spring tides coincide with perihelion and moon and sun both have zero declination, with the moon, sun, and earth on a common axis. This rare event last occurred in 1433, and will next occur in the year 3800.

Page constructed by Mark Schneider for SCORE

EARTH
Rotation	As the earth rotates, it moves below the bulging tides which are pulled by the moon, causing the tides to seem to move around the earth.
MOON
Position	As the moon orbits the earth, the ocean bulges beneath it pulling the tides. This is the major force on the tides.
Distance	Because the moon's orbit is an ellipse, its distance from the earth varies. At perigee, when it is closest to the earth, it is 222,000 away and the tidal force is 22% more than average. When the distance is greatest, at apogee, it is 253,000 miles away, and the force is 16% less than average.
Declination	The moon is not always directly over the equator. As the lunar plane moves north or south of the equator (±5°), the tides in the northern and southern hemispheres will vary. The force is greatest.
SUN
Position	As the earth orbits the sun, the gravitational force of the sun also affects the tides, although not as greatly as does the moon. When the forces are aligned, that is the sun, moon, and earth are in a straight line, we have SPRING tides, and the combined force is about three times greater than when the sun and moon are at right angles (NEAP tides).
Distance	Because the earth's orbit is an ellipse, its distance from the sun varies by about ±1.7%. In January, when the earth is nearest the sun, called perihelion, the tidal effect is about 5% greater than average. In July, when the earth is farthest from the sun, the tidal effect is about 5% less than average.
Declination	At the summer and winter solstices, the moon is 23° above or below the equator, causing unequal tides in the northern and southern hemispheres. At the spring and fall equinoxes, the tidal effect is greatest as the sun is closest to the equatorial plane.