This is the month of the equinox, the onset of spring, and one of our very few remaining celestial celebrations. In most developed countries, equinoxes and solstices are the only astronomical cycles that are still commonly recognized. Throw in the moon’s phases and you’ve got about as much as the public comprehends of the rhythms of the sky. The rest of the cosmos seems a vast, patternless puzzle.
Of course, it’s not so. The clockwork mechanism of the solar system coaxes the planets into periodic configurations; the stars shift reliably westward by two hours per month, producing seasonal constellations. These were familiar phenomena millennia ago. The ancients tracked numerous solar, lunar, stellar, and planetary cadences, including many patterns far more recondite than the yearly spring and fall equi- noxes. But most people today typically regard celestial events as random, or--because they do not recur at the same time each year--impossibly complex.
If we can overcome being intimidated by nonannularity, we too can learn to appreciate the timeless choreography of the heavens. Venus, for example, has dominated the predawn sky for the past six months and will soon be gone. Next year during the same season there will be no trace of it. If Venus blazed as a morning star every winter and as an evening star every summer, we would all know the pattern from childhood. But since it does not conform to our 12-month calendar, few of us are aware of the simple cadence that Venus follows in its alignments with Earth and the sun: About every 19 months Venus floats in the eastern sky as the morning star. Its evening appearances follow the same schedule, as do all other Venusian events. The dazzling maximum brilliance achieved by the evening star in July of 1991 will be repeated in February of 1993. Nineteen months--not so difficult.
Even greater correspondence appears when larger time frames are considered. By chance, 13 Venusian years are nearly a perfect matchup with eight orbits of Earth. So after eight years Venus reappears the same month in the same constellation, sporting the same brightness, phase, compass heading, height--everything. Hence the winter apparition of the evening star in 1985 will be impeccably reproduced next year.
Jupiter and Saturn, too, pursue uncomplicated cycles. Every year each planet arrives at a point called opposition, when it's closest to Earth. At this time they appear biggest and brightest and they’re visible all night long. You don’t have to be an astronomer to keep up with them. Jupiter’s cycle is 13 months, meaning that oppositions happen one month later each year. Jupiter’s favorable position in May 1994 is thus followed by June 1995, July 1996, August 1997, and so on. Saturn’s oppositions are two weeks later each year. Another striking pattern is the 26-month period separating Martian oppositions, when the Red Planet dramatically brightens into eye-catching brilliance. Knowledge of even a few of these celestial waltzes confers a lifetime of enjoyment.
And then there’s the orchestral pause when both poles of Earth tilt neither toward nor away from the sun, producing a worldwide symmetry of sunlight and darkness. It’s the one we’ll celebrate this March 20--the vernal equinox.