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I've read that during the equinoxes, tides are at their maximum. Shouldn't this only apply to latitudes near the equator? I mean, when the solar declination is near to 0º and we have a full or new moon, the combined effect of the Sun's and Moon's gravity should create the highest tides, but only near the equator (see in wikipedia the article Tides, which says that equinoctial tides are maximum)

Is this true for other latitudes?

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  • $\begingroup$ Could you please provide a source for this? It really doesn't sound correct. $\endgroup$
    – Phiteros
    Jul 28, 2016 at 20:34
  • $\begingroup$ Google equinoctial tide $\endgroup$
    – James K
    Jul 28, 2016 at 20:36
  • $\begingroup$ In wikipedia, for example, says that equinoctial tides are "maximum"... $\endgroup$
    – Carlos Vázquez Monzón
    Jul 29, 2016 at 12:38
  • $\begingroup$ I'm curious as to what you would accept as a valid answer, as "The effect is not real," does not seem to be acceptable. I posted an answer with data from one station that seemed to indicate that this was the case (it seems to me if the effect was real, that that would be enough), but that answer was downvoted without comment, so I deleted it. I have since looked at four other stations, and get the same results, absolutely no maximum ranges between high and low tides anywhere near the equinoxes. $\endgroup$
    – BillDOe
    Aug 2, 2016 at 22:29
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    $\begingroup$ According to Webster, one of the definitions of "Equinoctial" is, "Occurring on or near the equator." With that in mind I looked at the 2016 tide tables for Guayaquil, Ecuador, and the year's max range did, in fact, occur near the equinoxes. The tides at Fortaleza, Brazil were about a month off, but still pretty close. It seems reasonable that Equinoctial tides probably occur near the equator, since the equator, sun, and moon all lie near the same plane around the equinoxes. This is conjecture (or hypothesis) which is why it's not being offered as an answer. $\endgroup$
    – BillDOe
    Aug 6, 2016 at 16:35

2 Answers 2

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Only a syzygial tide during/near an equinox is the strongest. This means, there must be either new or full moon. In general, syzygial tides are strong because three bodies (Earth, Moon, Sun) align near one line, and tidal effects of Moon and Sun on Earth become (nearly) collinear and sum to the maximal possible magnitude. The Moon’s orbit is inclined to the ecliptic by about 5° only (cos 5° ≈ 0.996), so two tides are aligned almost perfectly on a Moon’s syzygy during whichever season. The difference is that, during/near an equinox, this line also lies in the equatorial plane and rotational motion of Earth’s surface/hydrosphere/crust can direct the stuff to move along this tidal line to a maximal range possible. At least, on the equator.

As for variations in terrestrial latitudes, namely mid-latitudes vs equator, comparison between syzygial tides during solstice and equinox far from the equator is a mechanical, not astronomical problem. Tidal bulge should be stronger during solstice, but projection of velocity to the tidal line will be greater on equinox. A detailed analysis of particular body of liquid (and its shore) is required to compare.

In contrast, a quadratural (where Earth–Moon and Earth–Sun lines are perpendicular) tide during an equinox is as weak as any quadratural tide. The solar tide basically cancels a part of the lunar tide. A quadratural tide during an equinox might even be the weakest, since lunar tide outmatches the solar one, but on an equinox and Moon’s quadrature simultaneously the Moon definitely lies away of the equatorial plane.

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The following is a plausibility argument based on the symmetry of an assumed circular orbit of the earth around the sun.

The earth's rotational axis is inclined to the ecliptic plane by an angle of about 23.4°.

For this reason idealized tidal bulges induced by the sun would travel around the earth's equator at the vernal and autumnal equinox whereas they would travel around circles of 23.4° northern and southern longitude at summer and winter solstice.

For symmetry reasons the tidal effects induced by the sun must have either a maximum or a minimum at the equinoxes. From this we conclude that also the combined tidal effects of sun and moon have either a maximum or a minimum at the equinoxes (on average).

Given the higher proportion of land to water at 23° northern latitude and a similar proportion at 23° southern latitude compared to the equator, it seems reasonable to assume a maximum at the equinoxes.

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  • $\begingroup$ This is all conjecture. I have five data-points that say there is no equinoctial tide, and this answer cites no sources. $\endgroup$
    – BillDOe
    Aug 5, 2016 at 21:56
  • $\begingroup$ Shunning the concept of Moon–Sun syzygy is an unacceptable omission. And very astonishing, given this comment. $\endgroup$
    – Incnis Mrsi
    Sep 10, 2016 at 17:54

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