This week the planet Saturn reaches opposition, placing it
exactly 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 Daylight
Time.
Being in opposition has several effects. Mainly, it means
that Saturn rises at sunset and sets at sunrise, so is visible all night long
everywhere in the world. At sunset Saturn is rising in the east, and at sunrise
Saturn is setting in the west. At local midnight, Saturn is high in the
southern sky in the northern hemisphere, or high in the northern sky in the
southern hemisphere.
Most astronomers count their first view of Saturn through a
telescope as one of the astronomical highlights of their lives. Until you actually
see Saturn's rings with your own eyes, it's hard to believe that something so
odd 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 25
times is a realistic minimum. At that magnification they will be tiny but
unmistakable. Seen in a good telescope at 150 times magnification, the rings
become fascinating in their detail.
Oddly enough, opposition is the worst time to look at the
rings, because the straight-on sunlight lacks shadows. The best views of the
rings come a month or two before and after opposition, when the sun is shining
at an angle. This casts a shadow of Saturn's globe on the rings, and a shadow
of the rings on the globe, enhancing the three-dimensional effect.
There are other things to look at besides the rings. First
is detail within the rings. A dark band, called Cassini's Division after its
discoverer Giovanni Domenico Cassini (1625–1712), divides the dusky outer ring
from the brighter inner ring. Sometimes a faint inner ring, known as the Crepe
Ring, can be seen.
While Jupiter has four bright
moons, Saturn is blessed with a greater number and variety of moons. The
brightest is Titan, the only moon in the solar system large enough to have a
dense atmosphere. Images of Titan from the Cassini spacecraft are showing us a
strangely familiar landscape of hills and lakes, except that those lakes are
liquid methane rather than water. Titan is readily visible in small telescopes
as a pinpoint of light.
With care and increasing aperture, more moons
of Saturn are visible. Most of the larger moons move in the same plane as
the rings, tilted relative to an observer on Earth. A planetarium program will
show you the exact positions of the moons on
a given night.
Saturn's moon Iapetus is particularly interesting. Its
orbit lies in a different plane from the rings and other bright moons, and it
is often far from the planet. A chart from a planetarium program is essential
for differentiating it from background stars. But its strangest feature is how
it changes in brightness from one side of its orbit to the other. When at
eastern elongation, as on March 30, Iapetus will be magnitude 11.9. When at
western elongation, on May 7, it will be magnitude 10.1, nearly two magnitudes
brighter.
The varying brightness of Iapetus was a mystery until the
Cassini probe sent back detailed images of the moon which show that its leading
hemisphere (facing towards us on March 30) is blackened by material swept up as
it circles Saturn, while its trailing hemisphere is pristine white.
The change in Iapetus' brightness is easily observed if you
follow it over an entire orbit, 79 days.
This article was provided to SPACE.com by Starry Night Education, the
leader in space science curriculum solutions.
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