Did anybody catch Mercury for the first time last night? I had just enough hazy cloud on my western horizon last night that Mercury was lost in the much. If you missed it, keep trying. And if you still can’t find it, don’t fret: your assignment for tonight is much, much easier.
The planets all travel around the sun in flat disk. Since we sit inside this disk too, when we go outside and look for planets they will all lie along one giant circle around us. Planets move slowly, so waiting for one of them to trace out the giant circle can take a while, but the Moon takes only a month to circle around us, so we can use it to trace the paths of the planets in the sky.
If you’ve been watching the moon the last few days, you have seen it climbing in the evening sky still growing towards its first quarter (which comes up on Monday – so quickly! Wasn’t it a tiny sliver just days ago?).
The earthshine is fading away, as the view of the Earth from the Moon is also moving from full to third quarter.
As the moon has moved eastward, it might have been hard for you not to notice the incredibly bright star that the moon has been getting closer and closer to. It will be at its very closest on Monday night. That star is a great marker for helping you really visualize how fast the moon is moving across the sky. On Monday night, if you look right and sunset and then again a few hours later, you will even be able to notice the different positions in a single night.
That super bright star, though, is more than just a convenient sign post. And it’s not a star. It’s Jupiter. Jupiter! I think so many of us have gotten used to the fact that NASA and others provide us so many beautiful pictures of planets from spacecraft and telescope that we have forgotten that these things are really there, up in the sky, night after night after night.
Now that you know where Jupiter is (and, again, don’t worry if you don’t see it tonight; it is going to be the brightest thing gracing our evening skies for the rest of the year) you have a chance to see one of the most spectacular sights in the sky. Go back in and grab some binoculars. If you don’t have binocular go back in and call your favorite present-giver and remind him or her that binoculars really would make the perfect present for you. Go back outside with your binocular and find a place where you can hold them good and steady. I like to lean against a wall, but you can try lying on the ground or setting them on a fence or anything that works for you. Now find Jupiter.
If you can get your binoculars steady enough, the disk of Jupiter will come into view. And it will clearly be a disk. Strung out in a line beside the disk will be four little orbs. Stars? Nope. Moons. These are the four moons of Jupiter that were first discovered by Galileo.
On the left, close together are the oddly magnetic Ganymede and the icy ocean filled Europa, close on the right is Io, the most volcanically active place in the solar system, and furthest of all on the right is Callisto, which is, well, just Callisto.
Come back tomorrow and everything is different. There are only three moons. Io and Europa have swapped places, Callisto hasn’t moved much, and Ganymede is now so close to Jupiter that you probably won’t be able to see it. The next night? All different again.
If you have been paying extra close attention you might even notice that the line that the four moons make basically points in the same direction as the line that our moon is tracing across our sky. Those moons of Jupiter are in the same disk as the planets of the solar system.
This amazing sight – Jupiter and its moons dancing across the sky – is, to my mind, one of the most wonderful things you can see in the solar system, on par with the Grand Canyon or Iguazu Falls or eruptions on Kilauea. Chances are you’ve never seen it, but it’s just outside your door. It’s free. Go outside. Look up!