Visual Observations of Jupiter

The surface of Jupiter has alternating dark and bright bands of clouds. The bright regions, designated as zones, are a higher cloud deck consisting of crystallized ammonia. The dark bands, called belts, lying deeper in the Jovian atmosphere consist of ammonium hydrosulfide. Although detailed observations of Jupiter's atmospheric activity are usually thought to require much larger instruments, there is enough visible in a small telescope to make careful observation and for sketching an interesting pursuit.

Observations of Jupiter are best made when the planet is near opposition and has its largest apparent diameter. The most prominent features visible in a small telescope are the north and south equatorial belts and the equatorial zone. Also commonly visible is the narrow north temperate belt. And with very good seeing other narrower, less distinct belts can be distinguished. The equatorial belts which may vary in width and contrast have borders of distinctly irregular appearance. Occasionally festoons, faint intrusions of the belts into the equatorial zone, are vivid enough to be seen. The southern belt has been known to disappear completely.

Because the appearance of the great red spot varies in intensity and hue, it requires excellent seeing to detect with a small telescope. Since it merges with the southern boundary of the south equatorial belt, it is usually visible as a faint oval protrusion into the zone south of it. It is more plainly visible when the south equatorial belt fades in intensity.

When the red spot is visible, a valuable observation is to time its transit across Jupiter's central meridian. If an eyepiece with a cross line reticle is not available, estimate when the spot is exactly at the center of the planet. Start a stopwatch at a standard time signal and stop it when you estimate that the red spot is at the Jovian meridian. Record the elapsed time and the time of the transit. Report your observation to the ALPO or equivalent organization. Predicted red spot transit times can be found in the RASC Observer's Handbook, Sky and Telescope and other astronomical publications.

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