When a planet is at opposition, it rises at sunset, climbs high in the sky in the middle of the night and sets at sunrise. This is notable because, at opposition, an outer planet — a planet with an orbit larger than Earth’s — is closest to Earth and best situated for viewing.
Presently, you will find Saturn above the horizon in the southeast a couple of hours after sunset, rising higher and higher as the night wears on. In the early morning hours, you will find Saturn above the horizon in the southwest, eventually setting as the dawn sky brightens.
To the unaided eye, Saturn appears as a bright star; through a small telescope, Saturn’s rings are clearly visible.
Italian all-around top-notch scientist Galileo Galilei was the first person to see Saturn’s rings when he turned his small telescope skyward in 1610, although he didn’t know what to make of what he saw. Drawings in Galileo’s hand of his blurry view of Saturn show Saturn with flanking blobs of light that don’t look much like rings.
Dutch astronomer Christian Huygens is widely believed to be the first to propose, in 1655, that observations of Saturn’s appearance could be explained by a thin ring surrounding the planet.
If you read my article in March 2010, and are now reading this article in April 2011, you can figure out something important about Saturn’s orbit: Saturn was exactly at opposition on March 22, 2010; and again on April 3 of this year.
Let’s visualize what has happened in the past year and 12 days.
On March 22, 2010, Saturn stood exactly opposite the sun from our perspective. So, on a sheet of paper, with a dot representing the sun marked in the center, a line drawn straight out to the right would pass through Earth and Saturn (with Earth between the sun and Saturn).
This past Sunday, Saturn stood again exactly at opposite the sun from our perspective. Since the last opposition, Earth has orbited the sun exactly once, plus a bit more — 12 days more.
Why wasn’t Saturn at opposition again on March 22, 2011? Because, in the time it took Earth to orbit the sun once, Saturn made a small amount of progress along its own orbit about the sun.
Saturn orbits the sun much more slowly than Earth does, and it is Saturn’s orbital period that we can figure out from these observations.
Since it took 12 days more than a year for Saturn to drift from opposition 2010 to opposition 2011, we know that Saturn has drifted just about 12 degrees (of 360 degrees) around its orbit. This is so because the Earth moves 360 degrees around the sun in 365 days — or about 1 degree per day.
Saturn has made it only 1/30th (12/360) of the way around its orbit in a year’s time. Therefore, Saturn must go around the sun once in about 30 years.
According to NASA’s Saturn Fact Sheet, Saturn’s orbital period is 10,759 days — or 29.456 years.
Our calculation is pretty good!
Doug Furton is a member of the physics faculty at GVSU. Send questions and suggestions to firstname.lastname@example.org. An archive of some of his “What’s up” columns is available online at http://gegenschein.wordpress.com.