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A Tilt for All Seasons

Earth’s 23.5-degree axis gives us summer, winter and everything between

As evening twilight settles Thursday, look to the western horizon for the nascent crescent moon. Above it are Venus and Jupiter. The bright star Regulus is up there, too, forming a line with Venus and Jupiter, each roughly a dozen degrees from the next. Keep an eye on the two planets as they inch closer together over the next two weeks before a spectacular end-of-month conjunction when they are within one-third degree of one another.
    Friday and Saturday the moon, Venus and Jupiter form a loose triangle. By Sunday the moon is far to the left of the planets but is just a few degrees below Regulus and the Sickle of Leo. The Sickle, which looks like an inverted question mark, outlines the head of the lion.
    While Venus and Jupiter glimmer above the western horizon, Saturn shines off to the east. By midnight it is at its highest almost due south. Don’t confuse its steady golden glow for the brighter red star Antares, which is a dozen degrees below. With greater magnification there’s no confusion: Saturn’s rings appear as a distinct bulge with even binoculars, and seen through even a modest telescope the rings themselves come into view.
    You’ll need binoculars to spot Mercury, which is emerging from the sun’s glare before dawn low in the east-northeast. Early Wednesday morning provides the best view of Mercury as it reaches its greatest eastern elongation, its point farthest from the sun in our sky, when it will peak 22 degrees above the horizon and remain visible 45 minutes before obscured by daybreak. Mercury is bright, but a couple degrees below it the star Aldebaran shines brighter still.
    Sunday at 12:38pm marks the summer solstice, when the sun reaches its northernmost position in the sky. ­Because the earth spins at an odd, 231⁄2-degree angle as it orbits the sun, the Northern Hemisphere is currently positioned to receive far more sunlight than the Southern Hemisphere. On the summer solstice, the sun appears to stand still above the Tropic of Cancer, which straddles the earth 231⁄2 degrees north of the equator. After the solstice, the sun will slowly shift southward from day to day until six months from now it is at its southernmost extreme over the Tropic of Capricorn, which is 231⁄2 degrees south of the equator. The sun shifting from one extreme to the other is what causes our seasons here on earth. Were the planet to spin upright like a top instead of tilted, the seasons would never change, with perpetual summer along the equatorial band and growing ever darker and colder the farther north or south you went.