Giant Jupiter Shines Bright

Planets are very much in the fore these days, especially with three of them now putting on a show as prominent evening luminaries.

Over in the west-northwest sky at dusk, Mars and Saturn remain a lovely sight in close proximity to the bright bluish star, Regulus in Leo. Meanwhile, emerging into view low in the southeast you?ll note a very bright silvery "star." That star however, is the planet identified with the supreme sky-god: Jupiter.

The heavenly ballet continuously being performed by these "wandering stars" has played a crucial role in the celestial lore of all peoples. It is not surprising that they were regarded as deities. The five naked-eye planets (Mercury, Venus, Mars, Jupiter and Saturn) have been known since antiquity, and it is also interesting that the members of this quintet have all been examined closely by space probes.

A star is (almost) born

Of course, before the advent of the telescope, all peoples regarded planets as a special category of star. And in a strange sense, Jupiter might even be referred to as a stillborn star, for it has the makings (mostly hydrogen), if not the mass of a stellar body.

With a diameter of 88,900 miles (143,000 km), Jupiter is the largest planet in our solar system, a colossal ball of hydrogen and helium without a solid surface. It has a rocky core encased in a thick mantle of metallic hydrogen enveloped in a massive atmospheric cloak of multi-colored clouds of ammonium hydrosulphide.

Its relative smallness, however, prevents the initiation of the nuclear processes that could have turned it into a full-fledged star. Had this been the case, we would have the distinction of living within a binary star system.

At its best

At 4 a.m. Eastern Daylight Time on July 9, Jupiter will arrive at that point in the sky directly opposite to the Sun, called "opposition." It will then be visible all night long at its brightest and (in telescopes) biggest. It rises in the southeast at sunset and burns near the teapot figure of Sagittarius, not far from the south end of the summer Milky Way.

If the planets' paths around the Sun were true circles, the moment of opposition would coincide with our closest approach to Jupiter, 387 million miles (623 million km). That actually will take place 27 hours later.

Opposition occurs when the Earth, moving faster in its orbit than Jupiter, overtakes it. From then on, we?ll leave Jupiter behind. Jupiter is also currently moving toward the Sun in its own elliptical path, while the Earth is arriving at its farthest point from the Sun.

Jupiter will reach its closest point to the Sun, 368 million miles (592 million km), at its perihelion point on September 20, 2010. Earth on the other hand, always attains its maximum distance from the Sun (aphelion) in early July. At 3 a.m. Eastern Daylight Time on July 4, it will be 94.5 million miles (152.1 million km) away.

Check it out!

Once it climbs a reasonable distance above the southeast horizon — say two to three hours after sundown — take a good look at Jupiter through a telescope.

Along with a restless atmosphere it also boasts a retinue of bright satellites. While steadily held binoculars show Jupiter as a tiny disc, a medium-size telescope reveals numerous dark belts, light zones and a wealth of festoons, garlands, ovals and other features extending here and there, as well as Jupiter's most famous feature, its Great Red Spot.

And the smallest telescope — even steadily held 7-power binoculars — will reveal Jupiter?s four largest satellites, each of which is equal to or larger than our Moon. The moons are a telescopic treasure. The four — Io, Europa, Ganymede and Callisto — popularly known as the "Galilean satellites," run a merry race with each other around the planet. Typically at least two or three are visible at any given moment. They can be followed for hours as they speed in front of Jupiter, throwing their shadows on the planet, vanish behind its giant disc, or plunge into invisibility within its dark shadow.

For instance: on the night of July 7, at 11:43 p.m. Eastern (8:43 p.m. Pacific) Time, Io will emerge into view, having crossed directly in front of Jupiter. For the rest of the night all four moons will be visible on one side of Jupiter. Pay especially close attention to Europa and Ganymede for they will be drawing closer to each other. At 2:40 a.m. Eastern Time (on July 8), they will be at their closest, separated by only 8 arc-seconds. That's less than one-sixth of the apparent diameter of Jupiter!

And during the night of July 29-30, it will appear as if Jupiter has gained a fifth satellite. That will be because it will be passing very closely north of HIP93649, a faint background star in Sagittarius. At their closest, they'll appear just 2 arc-minutes apart or about 2-and-a-half times Jupiter's apparent diameter.

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, New York.