Astro Bob: Orion returns to keep a promise
If you're up late watching the Perseid shower this weekend, give a wink to the big hunter. He's baaaak!
Unable to sleep and lured by thoughts of catching sight of Orion's return to the morning sky, I got up around 3:15 a.m recently and drove to a nearby field. All was still and silent. As roving fingers of fog felt their way along the tall grass, I soaked in the stars before dawn could pluck them away.
I also hoped to see some Perseid meteors and wasn't disappointed. Even the aurora poked around low in the northern sky, with pulsating patches of green light topped by faint purple plumes. Gazing across the south I spotted Mars near the Pleiades and the glaring light of Jupiter. One of the reasons the largest planet looks so brilliant this season is because it's in Pisces, a constellation with exactly zero bright stars. With no competition it's all the more impressive.
But seeing those three stars in Orion's belt creep up from the trees felt like the glee of getting tickled as a kid. Such a wonderful sight in early August when the sun still beats down and crickets chirp. Where I live in Duluth, the Earth spins at 713 mph (1,147 kilometers per hour). That's a spectacular velocity, yet it took more than 10 minutes for all three of them to clamber through the branches and rise into the clear, egged on by Earth's rotation.
Orion takes a "vacation" every year from May through July. During this time both sun and constellation occupy the same region of sky, so the hunter's up only in the daytime. If you could zoom to the moon in June, you'd see the Orion all day long because there's no atmosphere to paint the sky blue. Air molecules scatter the blue portion of the white light streaming from the sun and spread across the sky, hiding the stars.
We rarely think about it, but Earth's orbital motion causes the sun to move eastward in the sky about 1° per day. Of course, it's not really moving — it only looks that way. We're the ones on the move. The sun follow a path called the ecliptic, which is the plane of Earth's orbit extended into space to the sun and beyond.
Only after the sun has sallied off a sufficient distance to the east does Orion finally escape its glare and return to view in the dawn sky. The Hunter and all his winter friends continue to rise earlier and earlier as the sun speeds east until they show up as soon as evening twilight come mid-December.
In this way, the celestial sphere goes round and round, year after year, century after century — a merry-go-round from which it is neither desirable nor possible to step off. As always, enjoy the ride.