Saturn at Opposition
Published on Feb 24, 2008 at 4:40 pm.
1 Comment.
Filed under astronomy, skywatching.
About a week and a half ago, I wrote about Neptune being in conjunction with the Sun. That was when Neptune was on the opposite side of the Sun from Earth. Today, I am writing about another planetary alignment. Saturn is in opposition. That means that Saturn is on the opposite side of the sky from the Sun, as seen from Earth. Another way to look at this is from outside the Solar System. There, you would see Saturn and Earth on the same side of the Sun, forming a line with the Sun, Earth, and Saturn.
Any planet that is farther from the Sun than Earth (that is the definition of a superior planet) will at some time be in opposition. Earth will be between it and the Sun. Now, the planets orbits are not exactly in the same plane, so it is very rare for the Sun, Earth, and the planet to form at actual line. In other words, the planet at opposition will actually be just a bit north or south of the point directly opposite the Sun in the sky. Still, the moment that the planet is most nearly opposite the Sun is defined as the opposition.
Earth orbits the Sun in one year (about 365.26 days). Saturn orbits the Sun in about 29.46 years. These are what we call the sidereal periods of these two planets. So, one year from now, Earth will be back in the same position in its orbit as it is now. Saturn, however, will have moved. But, it will have only gone about 3.4% of the way around its orbit (remember, it takes 29.46 years to go all the way, and it has only been one year). So, Earth has to go just a little farther to catch up with Saturn for them to be in opposition again. It takes about 378 days to go from one Saturn opposition to another Saturn opposition. We call this the synodic period for Saturn.
When a planet is at opposition, it is about the closest that it gets to Earth. For Mars, that is a big deal, since the range of distances is quite great. When Mars is at opposition, it looks much larger than it does when it is far from opposition because it is so much closer then. But, Saturn is so far away from Earth to start with that the percentage difference in distance between opposition and conjunction is only about 11% difference. That means that Saturn looks only a bit larger at opposition than at conjunction (if you could even see it at conjunction). That means that through a telescope, you never see a huge difference in size in Saturn, unlike with Mars, and even to a lesser degree with Jupiter.
Being opposite the Sun in the sky, though, means that at opposition Saturn will rise and set opposite the Sun. In other words, Saturn will rise at about sunset and set about sunrise. Therefore, once it gets dark enough at the end of twilight to see stars and planets with your naked eye, Saturn will already be up. Until today, Saturn has risen after sunset (albeit, only slightly after sunset the last week or so). When at opposition, planets are highest in the sky near local midnight. So, the best time to observe Saturn right now is a few hours before or after midnight, when it is very high in the sky. You can see it in the early evening, but you will be looking through far more air, so the image won’t be as good in the telescope.
-Astroprof
Saturn image courtesy NASA, JPL
Heather on February 24, 2008 at 5:44 pm: 1
Thanks for the clear explanation. I’m new to astronomy (relatively) and keep hearing/reading terms like opposition and conjunction without having a clue. It’s nice to finally feel “in the loop”