Stellar Vista Observatory Sky Report for Dec. 6 – 12

We’ve enjoyed seeing brilliant Venus in the evening sky since June, but that’s about to end, and during the next four weeks you can watch it depart, then return to view in the morning sky late in January.

Here’s what’s happening: Venus travels on an orbit that is inside ours, so as it orbits the sun we see it alternately to the left of the sun and to the right, but always in the sun’s vicinity. When it’s to the left of the sun it’s in our evening sky; to the right, the morning sky. This month it catches up to and passes the earth, moving from left to right of the sun.

Because it’s at its closest when it passes the earth, its relative motion against the stars is at its greatest and in only weeks it moves from the evening sky to morning. The accompanying diagram shows its motion relative to the other planets and horizon and sun at one-week intervals for a month. Enjoy it while “the Evening Star” remains to grace our evening sky.

Any telescope will show that Venus has phases, like our moon, as Galileo discovered with his ultra-primitive telescope that magnified 20 times in 1610. (You can buy a far better telescope today for under $50.) When Venus is on the far side of the sun it looks “full,” and when between the earth and sun it looks “new.” Presently it’s a thin crescent that grows thinner by the day as it moves more nearly in line with the sun.

Jupiter and Saturn are also moving, but much more slowly. Mostly what we notice is that they set half hour earlier each week (along with the stars behind them) because of our earth’s motion around the sun.

Notice that Mercury appears in the evening sky as the month ends, briefly swapping places with Venus; more on that in future Sky Reports.

Our moon passes the planets this week. It’s especially close to Venus on Monday the 6th when they’re separated by less than 3 degrees and can be seen together in any binoculars. The moon is an ultra-thin crescent only 11% illuminated by the sun and is only twice as bright as Venus.

On the next night it’s 5 degrees from Saturn and on Wednesday night, its roughly between Saturn and brighter Jupiter. Then look for earthshine on the moon.

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