We are in for some cloudy nights here in Chicago, so let me shine the spotlight on Jupiter.
Jupiter is the largest planet in the Solar System, and it is in fine view in the evening sky all winter. Indeed, the only thing brighter in the night sky right now is the Moon. Jupiter is over 11 times the width of the Earth and its clouds reflect almost half the light it gets from the Sun. Both of these factors contribute to how bright Jupiter is.
In the diagram below, you can see that the Earth’s current position is sort of in between the Sun and Jupiter. This is why Jupiter has such a prime position in the night sky right now. Contrast this with Mars and Venus; from where Earth is right now, you have to look almost directly at the Sun to see either one. This is why, for now, you can only see Venus just before sunrise and you can only see Mars just after sunset.
Jupiter takes just under 12 years to orbit the Sun. In other words, as Jupiter circles the Sun once, we go around almost 12 times. Earth is like a minute hand to Jupiter’s hour hand (that is, if the hour hand were longer than the minute hand). We are currently moving away from Jupiter. In the coming months, it will get slightly less bright as the distance between it and the Earth increases. As we move around the Sun, Jupiter sets earlier and earlier. By the middle of spring, the Sun will be almost directly in between Jupiter and us, so we will only be able to see the giant planet just after sunset.
I have always had a special fascination with both Jupiter and Saturn. One reason is that the Voyager spacecraft visited both planets while I was in elementary school, around the time when I was interested in all things space. In 1979, photos of the giant planet and its Great Red Spot—a storm wider than the Earth that we have been observing for more than 200 years—were beamed back to Earth and printed in newspapers and magazines. Later, the Galileo spacecraft would orbit Jupiter from 1995 to 2003, even dropping a probe into the Jovian atmosphere.
Jupiter has its own menagerie of moons orbiting it; over sixty have been identified to date. Galileo discovered the four largest moons—Ganymede, Callisto, Io, and Europa. He saw them as four tiny little companion stars to the giant planet. When the Voyager spacecraft flew by, I felt like I had discovered four little alien worlds.
Io was the color of a pizza and had spectacular erupting volcanoes of sulfur. If you could smell Io, it would smell like a rotten egg. But because it orbits within an intense radiation belt, you would be fried before you had the chance.
Europa looked a little like a cue ball, ivory with a few stains. It is made almost entirely of water ice, and there is a lot of evidence that underneath the surface of ice lies an ocean of liquid water. Jupiter’s gravity is so strong that as Europa orbits, Jupiter squeezes the little moon like a hand squeezing a racquetball. This heats up Europa’s insides enough to keep the water from freezing. If there is any life in the Solar System outside the Earth, it is probably swimming in Europa’s ocean.
So enjoy the view of Jupiter this winter, and know that when you are looking at it, you may also be looking at a place that harbors life.