[another guest post, this time at boingboing.net ]
Back in the good old days everyone knew how many planets there were, then scientists came along and screwed everything up. How could something that was always a planet suddenly not be one? It made no sense. Chaos ensued, people protested, and scientists were thrown in prison.
I'm not making up that prison part, either.
It was dangerous being one of the first scientists to go against the traditional view of what was and was not a planet. But, regardless of the danger, 467 years ago, Copernicus stood firm. "The Sun and the Moon are not planets", he declared. Two of the seven known planets gone like that.
The other five known planets were doing fine though, it's just that they now orbited the Sun instead of spinning around the fixed Earth. Those five -- Mercury, Venus, Mars, Jupiter, and Saturn - remained. But in perhaps the biggest change to the word "planet" in human history, the Earth under our feet was declared to be on par with those other five. Galileo - defending these ideas with observations of the actual night skies through his newly acquired telescope - was imprisoned by the church.
Those seven initial planets - the "wanderers" from which the original Greek word came - are still out there and still part of our every day experience. The names of the seven days of the week come from the original seven planets. Sunday is obvious. Monday pretty easy. Satur[n]day? Check. The others are a bit obscure from old English and Latin, but they're all there.
Before artificial lights, I suspect that most people had seen most of the seven planets. These days? Almost no one.
From now until the full moon I'm going to try to get you all of them but one (pesky Mars is on the other side of the Sun right now; sorry).
First, the easy one. Step outside. Look up. See that big yellow glowing thing in the sky? It's the Sun. Ancient planet number one.
Planet three - Mercury - is the hardest one of them all. So hard, in fact, that only a tiny fraction of humanity alive today has ever seen it. Mercury is currently not in the easiest place to see, but we're going to try, for the sake of the planets.
First, remember that Mercury is the closest to the sun. It travels in a circle (mostly) around the sun at a distance of about 40 sun diameters. It's hard to really look closely enough at the sun to get a good feel for its diameter except right when it is setting. But when it's setting, measure the sun's diameter (for me it's about the width of a pinky; you could measure the moon instead: they look, coincidentally, about the same size). Now estimate about 40 diameters and draw a mental circle around the Sun. Mercury is in there somewhere. Always. We just need to find it.
Now, early Decemeber 2010, Mercury is sticking out to the east of the sun. That is, after the sun sets, Mercury will still be up. But remember, it will never be further than that circle you drew, so, as you can see, it will set very quickly after the sun sets. You need to work fast.
Go outside right as the sun has set and look west. Bring binoculars if you have them, but if you don't know need to worry. You will need, however, a very low horizon. Hills, houses, or trees are possibly enough to consign you back to the vast majority of people who have never seen Mercury. (Do you live right on the ocean in California? You're in good shape. But if you live right on the ocean in California you probably don't need me telling you that.) Look low to the southwest. Really low. Like maybe only a few hand widths up from the true horizon. From my house, a low Mercury in the southwest looks much like airplanes taking off and landing from LAX, except that Mercury doesn't move. Airplanes, if they know what's good for them, do. Even for me, knowing what I'm looking for, it's not always obvious. Whenever I think I have Mercury I always have to line it up with a tree or a bush and watch for 20 or 30 seconds and see if really is it.
Don't worry about stars or anything else in the sky. If you are looking only a few handwidths up and you do see something and it doesn't move, you have found it. It's too far away and too small to see much of anything, but if you did bring your binoculars out now is a good time to use them (and then don't forget to tour the moon, later).
If your weather is bad or your horizon is high, don't fear. You have about a week to keep looking before Mercury's orbit carries it too close to the sun to see. If will emerge in a month of so on the other side of the sun, but, then, you won't be able to see it at sunset. You'll have to wake up before dawn.
And no one wants to do that (except for you, dear reader, when you are going to want to do that next week to see Venus and Saturn at the end of our tour; so get your beauty rest now).