A spring tide is an unusually high tide that ooccurs when the sun, earth, and moon are colinear. I understand why this would be the case when the earth is on the far side of the sun and moon. The cumulative gravitational effect of the sun and moon are additive as both are acting on the earth in the same direction. But spring tides are also deemed to occur when the earth lies in between the moon and sun. In this configuration, I would expect the gravitational attraction of the sun to be diminished by the moon since they are acting on the earth in opposite directions. So the tides should be the lowest, but they are not. Can someone explain this?
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$\begingroup$ This question and answer provide a good explanation to this effect: earthscience.stackexchange.com/questions/16556/… $\endgroup$– arkaiaCommented Jan 4 at 17:43
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1$\begingroup$ Also, a good discussion here: physics.stackexchange.com/questions/121830/… $\endgroup$– Martin FCommented Jan 5 at 23:11
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2 Answers
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Tides are caused by a difference in the gravitational force on the Earth and the ocean. So their magnitude depends on how big that difference is. Their magnitude is not proportional to the strength of gravity here on Earth (of course — you don't feel the dependence of gravity by moon phases, while you do see the tides).
The gravitational force on the surface of the Earth depends on the distance to the other celestial body (the Sun of the Moon, doesn't matter). If you look at the difference between gravity at the surface of the Earth and its center, the difference will be greatest at two points — closest and farthest from the body. The direction of that difference will be away from the center of the Earth. So if all three are collinear, the tidal effects of the Sun and the Moon add up regardless of their order.
This is explained in much more detail here.
Maybe the following thought experiment clarifies it the best. Imagine the Earth without the Sun but with two Moons, which are equal and always opposite. Their gravitational force will cancel out at the center of the Earth, but their tidal effects will be amplified.
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$\begingroup$ Thank you anatolyg. The critical missing piece from my understanding was the realization that the tides represent the behavior of a fluid. If i were simply measuring gravitatonal force, the findings might fit w my initial understanding. $\endgroup$ Commented Jan 7 at 13:21
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To slightly correct your definition of a spring tide, it is not one that is especially high, but one that has the biggest range between low and high tide (as opposed to a neap tide, which has the smallest range between low and high tide).
As per here, spring tides occur after every full and every new moon (so, twice per lunar cycle, or roughly twice per calendar month).
In the case of your latter example, the low tide is lower than normal even though the high tide is not as high as in your first example. Hence, the difference betweeen the high and low tides is still larger than average. You will find that one spring and one neap tide per month is larger than the other (growing up by the sea, I'm sure they had different names but I can't find a reference to that now (it is not the same as a perigean spring tide), so possibly this was a local thing).
