results. The primary source of this difficulty is of course due to atmospheric turbulence. Due to their small
angular sizes, high magnifications are required to make surface detail visible. Unfortunately, small and large scale motion in the air is magnified as well. Pockets of air with different densities cause the telescopic image to move in and out of focus, blurring the view. Interspersed with these periods of poor seeing, are
brief intervals of clarity, when the planet can appear to suddenly pop into sharp focus. It is in these spells of
good seeing, that the observer is able to record planetary markings.
magnification of 225 X. The color differences between the two sets is subjective, the redness in the later image
being exagerrated, possibly due to a contrast effect from the prominent polar cap.
Although markings are often subtle and difficult to perceive clearly, the eye -brain system will learn to discern detail better with repeated viewing. Familiarity
with a planet's features will tend to reveal finer details that went unnoticed at first. These drawings are really
integrations of the moments of good seeing, those few seconds when things come into sharp focus. A min- imum of fifteen minutes viewing time would be typical
for each drawing.
belts. These are obvious even in small telescopes. In moments of good seeing, far more detail would be visible in these belts than is shown here.
Also obvious in binoculars or a small telescope are Jupiter's four brightest moons. The positions of the moons changes completely from night to night,
motion can indeed be seen during the course of an observing session. At
times one or more will cross in front of the planet, and a transit, takes place.
At left a moon is shown near the right edge, or limb, of the planet. The black circle serves to indicate the satellite for purposes of the drawing, in reality
the moon appeared as a bright spot against Jupiter. When properly positioned with the sun, a moon transiting Jupiter will cast a distinct shadow on the cloud tops, as in the lower drawing.
as a bulge in the South Equatorial Belt. (south is at the top in both drawings, matching the telescopic view) A product of atmospheric turbulence roughly the
size of the earth, the Red Spot revolves in a counterclockwise direction every six
days. Jupiter itself completes a rotation in just under 10 hours, producing the
oblate shape and allowing positional changes to be seen on short time scales.
Numerous other belts and lighter zones are regularly seen, the level of detail
increasing with scope size and improved seeing.
outer, or A Ring, is separated from the B Ring by the Cassini Division. This dark area is shown
partially resolved in the drawing at left. The A Ring itself is split by the Encke Division, a finer and
more difficult feature. Good seeing and a six to eight inch telescope is usually required to see it.
The B Ring is the widest and brightest. Detail is sometimes observed here as concentric lines,
or radial shadings, called spokes, but these are difficult to see without optimal conditions. The
innermost, or Crepe Ring, is much fainter, best seen against the planet itself. Otherwise it tends to fade into the dark sky background. The orientation of the rings changes regularly over time, appearing edge on every 13.7 or 15.5 years. When this occurs, they disappear from view in even the largest telescopes. The lower left drawing shows their appearance three weeks after the
earth passed through the ring plane in 1995.
