StarainBoy
Well-known member
In High Tide but No Green Grass, I wonder how spring tides end up in spring and not in January.
in january the sun lies below the equator so the suns gravitation vector is pulling slightly south at the equinox it's on the equator so the gravitational forces are pulling in the same direction, straight out from the equator, well that's my guess anyway
but it doesn't answer the question that he asked though, i don't think.
that just tells us why we get tides, rather than why we get the biggest tide when the sun isn't at it's closest point to the earth.
The moon orbits the sun in the plane of the ecliptic so when the sun is in the southern hemisphere the moon is in the northern so exert their greatest force on the respective hemispheres, dragging water away from the equator and evening out the tides globally. At the equinoxes the equatorial bulge caused by the spinning of the planet is added to by the gravitational pull of the sun and moon aligned over the equator.
although the explanation is far from clear i think it can be found in the NOAA website.
Spring tides do not happen around the solstices because the Lunar Declination means that when, at this time, sun earth and moon are aligned at syzygy the moon is 5 degrees off the ecliptic plane thus not pulling exactly in line with the sun. Additionally in the summer months we are further from the sun (Aphelion 2 July) so spring tides tend to be on the winter side of an equinox when we are closer to the sun (Perihelion 2 Jan.)
However, the earth's axial tilt is means that only at the equinoxes is the sun directly over the equator. Within a 28 day period around each equinox the moon will also cross the equator thus syzygy can coincide with the moon pulling directly along the ecliptic plane so you have your spring tides.
I think this perfect syzygy (i.e. sun, moon and earth not only in line but also with moon on the plane of the ecliptic) does not have to occur over the equator. The 2 March spring tide was well before the equinox so presumably the moon passed behind the earth very close to the time it crossed the plane of the ecliptic. Can't have been exact though as this would have caused an eclipse of the moon.
the attached sketch might help. [note i have not even attempted to include the affects of lunar apogee/perigee]
I've done some more digging and the precession of the Moon’s orbit means it works its way round the Earth in 18.6 years. Right now it crosses the ecliptic in January and July. In a couple of years it'll be November and May. But the spring tides will keep happening somewhere near March.
Dang! I thought we’d cracked it then!
Right now it crosses the ecliptic in January and July.