Why does the moon's gravity cause tides on earth but the sun's gravity doesn't?
Category: Space Published: May 9, 2013
The ocean tides on earth are caused by both the moon's gravity and the sun's gravity. In general, ocean tides are not generated by the overall strength of gravity, but instead by the differences in gravity from one spot to the next (the gravitational gradient). Even though the sun is much more massive and therefore has stronger overall gravity than the moon, the moon is closer to the earth so that its gravitational gradient is stronger than that of the sun. Because ocean tides are the effect of ocean water responding to a gravitational gradient, the moon plays a larger role in creating tides than does the sun. But the sun's gravitational gradient across the earth is significant and it does contribute to tides as well.
As detailed in "The Moon Book" by Kim Long, the ocean tides we experience on earth are caused by the sum of the moon's gravitational gradient and the sun's gravitational gradient. When the sun and the moon are aligned, or nearly aligned, their gravitational gradient fields add together constructively, leading to extra strong tides (high tide is extra high and low tide is extra low). This alignment happens when the moon is a new moon or a full moon, which occurs about every two weeks. The moon takes about a month to orbit the earth, hence strong tides occur about twice a month. In contrast, when the sun and the moon are unaligned, their gravitational gradients tend to cancel out, leading to weak tides (high tide is not very high and low tide is not very low). But even when the sun and moon are perfectly unaligned (they form a 90 degree angle relative to the earth), there are still tides because the moon's gravitational gradient is stronger than the sun's. The sun's gravitational gradient never completely cancels out the moon's. The biweekly strong tides are called "spring tides" even though they occur all year long. The name does not refer to the season "Spring", but to the verb "spring" which means to leap forth, because the strong high tides leap higher up the shore. The biweekly weak tides are called "neap tides".
Note that the position of the moon and sun relative to the earth just causes the two-week cycle of strong and weak tides. The daily pattern of high and low tides is not caused by the changing position of the sun and moon, but is directly caused by the rotation of the earth. Because the earth takes one day to rotate about its own axis, the high tide/low tide cycle repeats twice a day. In truth, the tidal bulges of ocean water are fixed relative to the sun and moon, and the earth is rotating underneath these bulges. Just like the earth's rotation makes the sun appear to rise and set every day, it makes the tidal bulges appear to sweep across the earth, when in reality the earth is rotating under the tidal bulge. The reason that there are two high-tide/low-tide cycles every day is because there are two tidal bulges and not one. The fact that there are two tidal bulges is a direct result of the gravitational gradient being the cause of tides and not the overall strength of gravity being the cause. The two-week cycle and the half-day cycle of the tide can be seen in the chart below. Note that the actual tidal patterns from one specific location to the next varies because of local factors such as water depth, shore shape, and ocean currents.