David Madore's WebLog: Happy summer!

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Entry #0066 [older|newer] / Entrée #0066 [précédente|suivante]:

(Saturday) · Summer Solstice (2003-06-21T19:10:27.32Z) · Last Quarter

Happy summer!

—or merry winter, if you live south of the equator, naturally. Today at 19:10:27.32 Universal Time Coordinated (that's 19:10:59.32 International Atomic Time, or 19:11:31.50 Terrestrial Time: see my Time page to know more about these various time scales and their differences), the plane defined by the Earth's center, the Sun's center, and the Earth's instantaneous rotation axis will be orthogonal to the osculating plane of the Earth's orbit (I hope I didn't get this wrong…).

So here's the hour of summer or winter according to your place of residence from a random selection of cities in the world:

This represents the same instant everywhere in the world, of course. At that moment (the solstice), the Sun will be directly overhead a point of the Earth's surface, whose coordinates (approximately calculated—I don't have the patience to work it out to within a meter or so) are: 107°11′ of longitude West of Greenwich, and 23°27′ of latitude North, this seems to be on the West coast of Mexico; for obvious reasons, it's on the tropic of Cancer, in fact, that's the definition of the tropic of Cancer (though it may not be exactly on the tropic, since the tropic is defined with respect to the mean ecliptic and inclination of the Earth, and the solstice corresponds to the instantaneous ecliptic and inclination, so the phenomenon of nutation slightly perturbs things).

The summer solstice shouldn't be called the summer solstice, because it's only summer in one half of the world: it should more accurately be called the solstice of Cancer (and the other one the solstice of Capricorn, and the vernal equinox the equinox of Aries, and the autumnal equinox the equinox of Libra), because it's the starting point of the astrological constellation (month) of Cancer. By “astrological” I don't mean that I adhere to astrology in any way, I just mean that the twelve “astrological constellations” of the zodiac are equally spaced along the ecliptic, and Aries always starts at the vernal equinox, whereas the “astronomical constellations” of the zodiac are unevenly spaced (and are thirteen in number, anyway, not twelve, the thirteenth being Ophiucus, the Serpent-Bearer) and have fixed absolute directions which means that their position relative to the equinox varies since the equinox precesses (the vernal equinox is now in the astronomical constellation of Pisces).

The Moon's last quarter, incidentally, takes place today around 14:46 Universal Time (to within a precision of a few minutes, I won't be bothered to place that to the exact second, nor to find out the position on the Earth's surface that is exactly below the Moon at that point).

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