This month’s sky antics are bunny-quick. Right now, the sun changes its north-south motion, or declination, at the year’s fastest rate, rising two sun-diameters higher every three days. Such a swift ascendancy explains the sun’s increasingly long path through the sky in March, and also why each day offers about three minutes more sunlight than the day before.
Celestial movement is obvious rather than conceptual. Unlike the mysterious content of dark matter or sausages, it’s in your face for ready inspection. And March offers a slew of objects whose varying motions are readily noticeable.
Take meteors, for example: their differences become apparent once you’ve observed a few. (Under ideal dark skies, that shouldn’t take long, since five or six appear per hour every night.) These fragments of asteroids or comets typically travel somewhat faster than Earth’s speed of 18.5 miles per second. However, if they’re catching up with us—as are meteors visible in the evening hours—they seem to lope lazily across the heavens. Meteors that hit us head-on race much faster, like summer’s zippy Perseids—best seen just before dawn, when you’re on the forward-facing side of our planet.
The fastest meteors (except for a few oddballs arriving from beyond the solar system) travel in long, looping parabolic orbits. When they meet us straight-on, their speed—up to 26 mps, the solar escape velocity at Earth’s distance from the sun—adds to ours, and they enter our atmosphere at a sizzling 44.5 mps, appearing and vanishing in less than a second. You can observe such ultrafast meteors every November 17 during the Leonid shower.
Nearly all other meteors travel slower than the Leonids. And some travel much slower: on the twenty-fifth of this month you can see the molasses of meteors, in a minor shower called the Camelopardalids. Overtaking us, they chug along at around 7 mps. That’s as slow as the Apollo astronauts traveled when returning from the moon.
Objects we encounter could not be slower. Seven miles per second, in fact, is the speed an object would display if it weren’t moving relative to Earth but merely dropped in, pulled by our gravity alone. Yet there are slower bodies to be seen. The moon, for one, creeps along at less than 1 mps. Watch it move against the background stars and you’ll see what a lazy 2,200-mile-per-hour motion looks like. The moon’s diameter is also about 2,200 miles, so it sweeps through one of its own widths each hour. No other celestial body performs this coincidental synchrony.
There’s more March motion madness. The fastest and slowest naked-eye planets, Mercury and Saturn, now hover side by side. The best showing of these strange bedfellows occurs March 20, about a half hour after sunset. Just above the sunset point (precisely due west, since it’s the equinox), as high as a clenched fist at arm’s length, sit two little stars more clearly seen through binoculars. Way off in the distance, the leftmost, Saturn, plods at just 6 mps. Near its right is Mercury, zipping along at 30 mps.
Their contrasting paces have been noticed for millennia. They’ve even entered our language: sad, sluggish people are saturnine; mood-changing characters, mercurial. The ancients couldn’t have known, but those planets’ speeds represent how fast each world has to move to balance the sun’s gravitational pull at its location. Add or subtract velocity from either planet and it would be flung from its present orbit.
This month’s idiosyncratic tempos are reassurance that all is right and stable in the solar system. The dance goes on.
