by The Unknown Astronomer
I hope you all had unclouded views of the gorgeous conjunction of Venus, Jupiter, and the moon last weekend, and that you continue to observe Jupiter in decline and Venus ascendant. Of course, the two planets are not really approaching and receding from one another in space. What is actually going on is more a race than a dance.
Jupiter, Venus and Moon, Dec 1, 2008. Image from Starry Night Pro Software
Picture a circular track with the sun in the center. The innermost lane is occupied by the planet Mercury, whizzing around the track like Jimmie Johnson at Dover Downs, and completing its circuit in a mere 88 days. Venus is in lane 2, making its journey in a zippy 243 days – not exactly NASCAR, but at least a turbo-charged Mustang. Earth orbits in lane 3, getting once around in a respectable 365 days – the Mustang minus the turbocharger. By the time we get to lane 6, way out beyond Mars and the asteroids, it’s taking 12 years to complete one circuit. That’s Jupiter – a Yugo driven by your grandmother.
So, what we have been observing over the last couple of weeks is a nightly snapshot of speedy Venus overtaking plodding Jupiter, from the perspective of lane 3.
Another effect of where we all are in space (or on that track) is the sometimes peculiar look of Venus through a telescope. Right now it appears to be a tiny half-moon! Indeed, like the moon, Venus (and Mercury) has phases, and for the same reason: since it is inward toward the sun from our perspective on Earth (lane 2 as seen from lane 3), the planet will at times be only partially lit, appearing as full, half, or crescent as it approaches and overtakes the Earth.
Now, how do you follow an act like a planetary conjunction? WIth a meteor shower! The Geminids, so named because they appear to radiate from the constellation Gemini, peak on the night/morning of Dec. 13 – 14. These are bright, medium-fast meteors that can be colorful: In addition to the usual white, I have seen distinctly yellow and green ones, and they can also be blue or even red. You can start looking a little north of east any time after 8:00 pm. Gemini will have just cleared the horizon by then, so the best time to observe is realy after midnight, when Gemini is high in the sky. There is one major problem with this year’s shower, however – we will be just one night removed from the largest, closest, brightest full moon of 2008, which will blot out all but the brightest meteors. Ah, the moon – beloved of poets and romantics, bane of astronomers…
No matter which category you fit into, if it’s clear, brave the cold anyway – at it’s height, this shower can produce a meteor a minute!