Monday, December 17, 2012

Chiefly Astronomical

The sun shone furiously over a  huge shoulder of cloud this afternoon: cars kicked bursts of water up from the drenched streets, like December pedestrians panting puffs of steam into the frosty air. Every drop and cloud of spray sparkled. Blaze, old girl, one more time!

A couple days ago the streetlights came on at four in the afternoon. Real cold has started to filter into the house. I start reckoning the time to the solstice. But the sun is still slipping away from us, dallying with New Zealanders, Chileans, South Africans.

Last night, I came out of the house where I'd been doing a massage, and Auriga was high in the sky, in a deep recess between curtains of cloud, and a planet of incredible brightness was burning, right up there beside it in Taurus, totally outshining Aldebaran. To my chagrin I couldn't say what planet it was: it seemed too yellow for Venus, but too bright for Jupiter. Altogether it was an eerie sky, with its unclaimed planet, and the folding wings of cloud around it. Auriga hangs like an interdict upon my hopes, at the best of times: neither it nor any of the winter constellations have ever had any love for me.

And now the sun has been buried. Everything is dark again, and a steady rain is falling. Cold. Last night as I washed up for my massage I held my hands in the hot water a long time before they finally warmed up.

According to Sky and Telescope, that planet was indeed Jupiter. No explanation of its brilliance, though. Maybe sometimes it just gets tired of being the runner-up to Venus.


christopher said...

Perhaps it is the Sun that is brighter at times. This can actually happen.

Jupiter indeed emits more energy than it absorbs and that is because it is well on the way to being big enough to turn itself into a star. Stars run on their own gravity to initiate the processes of fusion.

But even so Jupiter does not have enough energy to emit the photons we see it by. The brightness of Jupiter happens because it is close enough, reflective enough and big enough to reflect the sun back to us as only Jupiter can.

Venus is brighter because it is always closer and is also reflective enough and big enough in being closer.

Dale said...

:-) I was assuming -- too readily, now that I think of it! -- that the extra brilliance was only apparent: the optics of night sky and contrast. It was very bright indeed. I've seen Venus cast a shadow: Jupiter seemed that bright, that night. (Though of course in urban Portland looking for a shadow would have been hopless :->)

christopher said...

There is an apparent brightness difference that also figures in that Jupiter on the same side of the sun in its orbit as we are in ours is significantly closer to us than when Jupiter is on the other side of the sun. Under the right conditions of angularity that same closer Jupiter can be high enough in the sky during the evening or morning hours and then be brighter.

Jupiter orbits Sun in most of twelve years and thus spends about one year in each of the twelve regions of the sky we know as the traditional astrological signs, moving at speed forward about two thirds of the year but apparently stationary, backward and stationary again, one third of the year. Jupiter is brighter then because it is literally also closer to our earth which is in between Jupiter and the sun rather than on the opposite side of the sun.

Or say it another way, you can actually add the distances of Jupiter to the sun and earth to the sun or take away the distance of the earth to the sun from the sun to Jupiter and the difference then is the closest while the former is the furthest distance of earth to Jupiter.

On top of that, you can add in the aphelions and perihelions of each planet and the planetary latitudes as well and those also can add or subtract from Jupiter's apparent brightness. All of these variations happen in some combination in a twelve year span and thus an apparently brighter Jupiter is normal in human experience.

Dick said...

I retain my ignorance of which heavenly body is which and where so that those who can conjure so graphically with their names and stations will continue to thrill me!

Dale said...

This from

"Jupiter reaches opposition to the Sun on December 2, when it is closest to Earth and at its largest and brightest. The gas giant rises shortly after sunset, remains visible all night and looks stunning through a telescope. It shines at magnitude -2.8 and resides in Taurus, close to Aldebaran, the brightest star in the constellation and one of the brightest in the nighttime sky."

Dale said...

So I was right, it was very bright. -2.8! Not brighter than Venus, but bright enough to laugh any star to scorn.

christopher said...

Thumbs up! :D