This week the planet Saturn reaches opposition, placing itexactly opposite the sun in Earth?s sky.
The exact time of opposition is Monday March 22 at 1h.Universal Time, which converts to Sunday, March 21 at 9 p.m. Eastern DaylightTime.
Being in opposition has several effects. Mainly, it meansthat Saturn rises at sunset and sets at sunrise, so is visible all night longeverywhere in the world. At sunset Saturn is rising in the east, and at sunriseSaturn is setting in the west. At local midnight, Saturn is high in thesouthern sky in the northern hemisphere, or high in the northern sky in thesouthern hemisphere.
Most astronomers count their first view of Saturn through atelescope as one of the astronomical highlights of their lives. Until you actuallysee Saturn?s rings with your own eyes, it?s hard to believe that something soodd and beautiful can exist.
How much optical power does it take to see the rings?Although some people claim to have seen them with less, a magnification of 25times is a realistic minimum. At that magnification they will be tiny butunmistakable. Seen in a good telescope at 150 times magnification, the ringsbecome fascinating in their detail.
Oddly enough, opposition is the worst time to look at therings, because the straight-on sunlight lacks shadows. The best views of therings come a month or two before and after opposition, when the sun is shiningat an angle. This casts a shadow of Saturn?s globe on the rings, and a shadowof the rings on the globe, enhancing the three-dimensional effect.
There are other things to look at besides the rings. Firstis detail within the rings. A dark band, called Cassini?s Division after itsdiscoverer Giovanni Domenico Cassini (1625?1712), divides the dusky outer ringfrom the brighter inner ring. Sometimes a faint inner ring, known as the CrepeRing, can be seen.
While Jupiter has four brightmoons, Saturn is blessed with a greater number and variety of moons. Thebrightest is Titan, the only moon in the solar system large enough to have adense atmosphere. Images of Titan from the Cassini spacecraft are showing us astrangely familiar landscape of hills and lakes, except that those lakes areliquid methane rather than water. Titan is readily visible in small telescopesas a pinpoint of light.
With care and increasing aperture, more moonsof Saturn are visible. Most of the larger moons move in the same plane asthe rings, tilted relative to an observer on Earth. A planetarium program willshow you the exact positions of the moons on
a given night.
Saturn?s moon Iapetus is particularly interesting. Itsorbit lies in a different plane from the rings and other bright moons, and itis often far from the planet. A chart from a planetarium program is essentialfor differentiating it from background stars. But its strangest feature is howit changes in brightness from one side of its orbit to the other. When ateastern elongation, as on March 30, Iapetus will be magnitude 11.9. When atwestern elongation, on May 7, it will be magnitude 10.1, nearly two magnitudesbrighter.
The varying brightness of Iapetus was a mystery until theCassini probe sent back detailed images of the moon which show that its leadinghemisphere (facing towards us on March 30) is blackened by material swept up asit circles Saturn, while its trailing hemisphere is pristine white.
The change in Iapetus? brightness is easily observed if youfollow it over an entire orbit, 79 days.