In ancient times, sky watchers kept a close eye on the planets, taking special notice of when they came close to one another. Just as we pay attention when world leaders gather, our forebears saw such “meetings of the gods” as full of import.
Nowadays, planetary conjunctions are a bit like your car’s odometer rolling over a big number: fun to watch, but of no particular significance in the big scheme of things. And, like those mileage milestones, the rarer the conjunction, the more important it seems.
Since planets take longer to orbit the sun the farther they are from it, the intervals between outer planet conjunctions are longer than inner planets’, just as the hour and minute hands of a clock only line up every 65 minutes or so, while the second and minute hands line up every 61 seconds.
Jupiter and Saturn (the outermost naked-eye planets) orbit in 30 years and 12 years, respectively, and line up roughly every 20 years (the last time was in 2000; the next will be in 2020). Mercury and Venus (the innermost planets) orbit in 88 days and 225 days, respectively, lining up every 143 days, on average.
This week, the inner solar system (Mercury, Venus, and Mars) is all together in the pre-dawn. Venus is the brightest, rising in the ENE just before 5:00 AM. Dimmer Mercury follows, breaking the horizon just after 5:45 AM, with ruddy Mars on its heels just before 6:00 AM. Keep watching Mercury and Mars: On Saturday morning less than half the width of a pencil at arm’s length will separate them. Finally, a razor-thin waning crescent moon insinuates itself into the scene on Monday, creating a vertical line (from top to bottom) of Venus, the moon, Mars, and Mercury.
Next column: Farewell to Cassini.