Hydraulic Architecture, Volume 2, Part 2

CHAPTER I. ON THE EBB AND FLOW OF THE SEA.

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...and full moons, compared to the tides of the immediate quadratures. The great tides of the equinoxes are produced by the sum of the actions of the two planets In the 18th century, the sun and moon were often categorized as "planets" in the general sense of celestial bodies that moved across the sky relative to the stars. toward the equator or toward the middle of the earth, and the effect diminishes as the two planets increase their declination. There is even a difference between evening and morning tides when the two planets are near one or the other tropic. Assuming it is summer, the action of the sun and the moon during new moons occurs by day on a part of the sea that is closer to us, whereas it is quite the opposite if the sun and the moon are toward the other tropic.

But what produces an incomparably more significant effect on the tides is the change in distance from the moon to the earth. This planet sometimes moves away from us and sometimes approaches; we do not occupy the center, but rather one of the foci of the ellipse it describes around the earth. When it passes at the furthest end of the major axis of its ellipse, it is at its apogee The point in an orbit where the celestial body is at its furthest distance from the Earth., and it acts much less upon the Ocean. On the contrary, it is capable of great action if it is in its perigee The point in an orbit where the celestial body is at its closest distance to the Earth., or at the other end of its major axis—an end to which we are considerably closer—and the sea also rises much higher at that time.

Never are the tides of the equinoxes greater than when the moon is near its perigee or at its shortest distance from the earth. Not only is the flow The incoming high tide. stronger at that time, but the ebb The outgoing low tide. is as well, and one can see uncovered an infinite number of places that are usually hidden; this is what must be precisely observed in the establishment of works that are appropriate to be built at sea, in order to take advantage of the moments when it recedes the furthest to found and set the buildings one intends to construct. It would be desirable for the Royal Academy of Sciences The French national scientific institution, which published astronomical and navigational data. to be willing to mark in the Knowledge of the Times Original: "connoissance des tems." This was the official astronomical almanac published in France for navigators and scientists. the location of the moon's apogee and perigee; this small addition would be of extreme utility, since it would serve to make known in advance or to foresee the days of stronger and weaker tides.

We have already specified all the conditions of the former; it remains only to add that the weakest tides of all occur around the solstices—that is to say, toward the months of June and December—when the moon is at the same time in its apogee or at its greatest distance from the...