The Sun’s Effect on Tides
The sun has the same kind of effect on Earth’s waters as the moon does.
However, because it is so much farther away, the sun’s tidemaking effect is
only about half that of the moon. The sun, however, can enhance or detract
from the moon’s effects.
Tides are always high in line with the moon and low midway between
the high-tide points. When the sun is in line with the moon and Earth, as
shown in the figure below, the sun’s entire tidemaking effect is added to the
moon’s. This alignment of the moon, the sun, and Earth occurs at the times
of the new and the full moons. During these times, high tides are especially
high, and low tides are especially low. These tides occur twice a month and
are called spring tides.
New
moon
Sun
Earth
Full
moon
Sun’s and
moon’s pull
SPRING TIDE
Apogee has its roots in the ancient
Greek word apogaios, which means
“far from Earth” (apo, “away” and
gaia, “earth”). Based on this and
what you know about apogee and
perigee, what do you think the prefix
peri– means?
At quarter phases, the moon and the sun are not in line with Earth (see
the figure below). As a result, the sun’s entire tidemaking effect is
subtracted from the moon’s. The outcome is high tides that are not very
high and low tides that are not very low. These tides also occur twice a
month and are called neap tides. Another factor that adds to the tidal effect
is the moon’s proximity to Earth. When the moon is at perigee, the closest
point to Earth in its orbit, the tidal effect is greater, especially if perigee
occurs during the new or full moon phases. If the moon is at apogee, its
farthest point from Earth, the tidal effect is less.
Third quarter
moon
Moon’s pull
Sun
Earth
Sun’s pull
First quarter moon
NEAP TIDE
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Unit 6 Earth’s Oceans