Jupiter is back
Published on Jun 17, 2008 at 1:39 pm.
2 Comments.
Filed under astronomy, skywatching.
Of course, Jupiter didn’t leave the Solar System, become invisible, or any such thing. But, we haven’t seen it for a while in the evening skies. And, observers who have stayed up late or got up before dawn have been seeing Jupiter. It just hasn’t been visible at sunset in quite a few months.
Since ancient times, people have looked at the sky and observed that some “stars” appeared to wander from constellation to constellation. These wandering “stars” were called planets. Jupiter is one of these planets. It takes about 12 years to move around the sky. But, people have also observed that night after night, the entire sky shows a subtle shift to the west when observed at the same time of night, about 1 degree per day. This is due to Earth’s motion around the Sun.
Since Earth moves around the Sun faster than Jupiter, it is no wonder that we left it behind. It was prominent in the sky last summer. But, Earth moved on. Eventually, by winter, the planet was hidden behind the Sun. (For my readers in the southern hemisphere, reverse the terms “summer” and “winter”!) But, you might wonder, “Where is Jupiter?” You might say to yourself, “I saw it about sunset last year at this time. Where is it now?” After all, it’s been a year, and Earth is back to where it started. So, what has happened to Jupiter? Well, quite simply, it moved.
Remember, planets move around the Sun. Jupiter is a planet that is farther from the Sun than Earth. So, that means that once in a while, Earth gets between Jupiter and the Sun. When that happens, Jupiter is said to be in opposition. That means that it appears opposite the Sun as seen from Earth. Jupiter was last at opposition on June 6, 2007. On that date, Jupiter rose at about sunset and set about sunrise. It was up all evening. It was visible as soon as the sky got dark enough to see it during twilight.
But, one year later was June 5 (Remember that this year was leap year, so one orbit of the Earth was 365.26 days later, and that was June 5). But, Jupiter was nowhere to be seen. You had to wait a bit over two hours after sunset for Jupiter to rise. What happened? Well, remember, Jupiter moves, too. Since it takes Jupiter almost 12 years (11.86 years, actually) to orbit the Sun, in the year that it took Earth to go around the Sun, Jupiter had moved about 1/12 of the way around its orbit, so it was no longer opposite the Sun from Earth in early June.
One orbit of Earth around the Sun did not result in another opposition for Jupiter (or any other planet). So, Earth must move a bit farther around its orbit for it to line up again so that Earth is between the planet and the Sun. Thus, the next opposition of Jupiter will not occur until July 9, 2008. So, oppositions of Jupiter occur at intervals of just over 13 months. This time interval, from opposition to opposition, is called the synodic period of a planet. Jupiter’s synodic period is about 399 days.
Since Jupiter will be rising shortly after sunset in early July, and it will be progressively rising earlier as the month wears along, then it will be a favorite target for amateur astronomers all month long, and for the next several months, too. July will be particularly good, though, because we will be closest to Jupiter at that time, so it will look slightly larger in the telescope than normal. But, because Jupiter is so far from us to start with, this effect is far less noticeable than it is with Mars.
But, you don’t have to wait until July to see Jupiter. You can still see it now. You’ll just have to stay up a bit later. For my part of Texas, Jupiter is currently rising just after 10pm. At the top of this posting is a view of what the sky would look like at almost 11:30 at night on June 19 facing to the southeast. I picked that night, two nights from now, because the Moon will be very close to Jupiter then. It will be just a little to the west of Jupiter. The following night, it will be just a little to the east of Jupiter. That will make it appear a little down and to the left of Jupiter as seen from Texas. In fact, on June 20, if you are in the right part of the world, the Moon will be passing Jupiter at a distance of only about 4 times the Moon’s apparent diameter. But, that happens just after dawn on that date, as seen from here. Observers in the Pacific, Australia, or Asia will see that happen, though.
-Astroprof
Skyview produced using Stellarium software
MsLaurel on June 21, 2008 at 6:50 pm: 1
I’ve enjoyed reading this. You make Jupiter fascinating.
wheny on September 7, 2009 at 4:22 pm: 2
where is jupiter right now.and what planet is visible in the night sky at about 10 pm