Standing out as the premier object in this month’s Colorado sky is the “Planet King,” the mighty planet Jupiter. Visible in the southeast at the onset of darkness, Jupiter glows as a yellowish-white object astride the stars of central Virgo and is visible nearly all night long. To the naked eye Jupiter reigns as the fourth brightest object in the sky behind only the sun, moon, and the brilliant planet Venus. The planet Mars can occasionally outshine Jupiter, when the Earth swings close to the Red Planet, as it will next year, but even then only for a few weeks at a time.

Jupiter is consistently the most interesting planetary object for observers with binoculars or a small telescope. A pair of binoculars readily reveals four companion “stars” to Jupiter which were first discovered by Galileo early in the 17th century. Galileo named these objects the Medici Stars in honor of his patrons in the city of Florence, Italy. Happily this designation has been dropped over the centuries and these objects are now appropriately referred to as the Galilean satellites.

Galileo observed that over several days these objects could be seen swinging back and forth from one side of Jupiter to the other. From these observations Galileo concluded that these four companion “stars” — Io, Europa, Ganymede and Callisto — were orbiting Jupiter, just as Jupiter, the Earth and the other planets were orbiting the sun. Just as Galileo did 400 years ago, one can easily follow these satellite motions about Jupiter on a night-to-night basis.

By Roger B. Culver

Observations made of Jupiter with a small telescope reveal, first of all, a disk which is oblate in shape. This oblate shape of Jupiter is the result of the planet’s fluid-like density, only 1.3 times that of water, and its rapid rotation rate in which a world 11 times larger than the Earth rotates once every 10 hours. The overall effect of these factors is to impart a distinctly flattened shape to Jupiter’s disk.

When viewed through a small telescope, Jupiter’s disk also exhibits a set of parallel lines which are known to be atmospheric features similar to the jet streams in our own atmosphere. If the evening air is particularly steady, a small telescope might even reveal the famed Great Red Spot in Jupiter’s atmosphere. The Great Red Spot is an oval-shaped cyclonic storm of gargantuan proportions, two Earth diameters long and one Earth diameter wide. Space missions to Jupiter indicate that the Jovian hurricane has wind velocities of over 300 miles per hour. So it has raged for the nearly four centuries we have had telescopes powerful enough to detect it, and so probably it will rage for centuries more.

In short, unlike most of the other planets, the wonders of the Planet King can be savored on several levels of perception and instrumentation during these warm and hopefully clear springtime nights.

Elsewhere in the sky: The planet Mars sinks ever deeper into the evening twilight and can be seen with difficulty to the right of the ruddy Taurus star Aldebaran for an hour or so after sunset during the first week of May.

The planet Saturn rises to the southeast about two hours after evening twilight ends, and glows prominently as a golden-hued object on the Scorpius-Sagittarius border.

The planet Venus is a splendid object in the morning sky and blazes unmistakably above the eastern horizon for over two hours before sunrise.

The planet Mercury rises less than an hour ahead of the sun and may be glimpsed in the predawn twilight during the last half of May

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