Sky Watch by J. Alex Knoll
Wobbling into a New Season
Without Earth’s tilt, every day would be equinox
Friday evening around 10 the waning gibbous moon rises in the northeast amid the stars of Taurus. The brilliant red heart of the bull, Aldebaran, shines about a dozen degrees in the moon’s wake. But dancing around the moon itself are the sisters of the Pleiades star cluster. The brightest of the bunch, Alcyone, is nigh under third magnitude, while the others are dimmer still, bordering on the realm of the naked eye. But binoculars, or better still a small telescope, will illuminate the six remaining sisters and their father, Atlas, as well, all less than one-half degree from the moon’s orb.
Monday, September 22 marks the first day of fall with the autumnal equinox. On this date the sun hovers above the equator, bathing northern and southern hemisphere alike in 12 hours of daylight and 12 of night.
Were it not for Earth’s 231⁄2-degree tilted axis, the sun would always face the equator, and there would be no changing seasons. As it is, the earth spins like a top but wobbles from north to south as it faces the sun over the course of the year.
For six months, the sun graces one hemisphere with more light than the other. For us, that peaks on summer solstice, when the noontime sun hovers its farthest north, above the Tropic of Cancer, which crosses the tip of Florida. By contrast, at winter solstice, the sun reaches its southward peak above the Tropic of Capricorn, which bisects South America and Australia. By no coincidence, both tropics are at 231⁄2-degrees latitude.
On the two equinoxes, however, the equator faces the sun due-on, with north and south pole aligned upright at right angles to the sun.
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