Our planet Earth orbits the Sun once every year whereas it takes Jupiter about twelve years to complete one orbit. This means that once every thirteen months Earth catches up to Jupiter and passes it in this planetary race around the Sun. When Earth “passes” Jupiter, we say that Jupiter is at “opposition”. This year’s opposition is happening today.
At opposition, Earth lies almost directly between the Sun and Jupiter. As a result, Jupiter is as close to Earth as it ever gets. Today, it is 3.97 astronomical units (au) away. One “astronomical unit” refers to the distance from Earth to the Sun. Hence, to say that Jupiter is 3.97 au away is to say that it is just a little bit less than 4 times further from us than we are from the Sun.
Earth orbits the Sun at a distance of 1 au (by definition). Jupiter orbits the Sun at a distance of 5 au. As shown below, depending on where Earth and Jupiter are in their orbits around the Sun, the distance from Earth to Jupiter varies between 4 au and 6 au. It is 4 au at opposition and 6 au when it is behind the Sun.
Now is the best time to view Jupiter in a small telescope. Shown at the top of this page is a picture I took of Jupiter just a few days ago. In this picture, Jupiter’s apparent size is as big as it ever gets for the particular telescope and camera I used.
In terms of “angular size”, Jupiter is currently 49.6 arcseconds in diameter. An “arcsecond” is a measure of angular size. We are all familiar with the concept of “degree”—there are 360 degrees in a full circle. An “arcminute” is a smaller measure of angular size. There are 60 arcminutes in one degree. An arcsecond is even smaller. There are 60 arcseconds in one arcminute. Hence, 49.6 arcseconds is the same as 0.83 arcminutes. To put this angular size into more familiar terms, note that the Moon is about 31 arcminutes in angular diameter. Hence, the Moon currently appears about 37 times bigger than Jupiter.
To appreciate the changing apparent size of Jupiter, it is interesting to compare pictures taken with the same equipment at different times. Last August, I took a picture of Jupiter with the same equipment that I used a few days ago. Shown below are the two pictures side-by-side. Jupiter today is clearly bigger than it was back in August. When I took the August picture, Jupiter was 4.50 astronomical units away. Hence, its apparent size was about 15 percent smaller then than now as the picture shows.
Even though Jupiter is at closest approach right now, it will be an excellent evening object to view over the next several months. Currently, it rises in the east exactly when the Sun sets in the west. About an hour after sunset it is the blazingly bright object low in the east. Most people will think it is just another airplane off in the distance but those of us who know better can tell our friends that, no, this bright object is the majestic planet Jupiter.
Over the next few months, we will slowly draw further and further ahead of Jupiter in our orbit around the Sun and Jupiter will start to look smaller in the telescope. But, this regression will take place gradually and therefore Jupiter will continue to be spectacular for at least the next few months.
More detailed information on the equipment used can be found at http://www.princeton.edu/~rvdb/images/NJP/jupiter.html.