After having spent nearly a month without any planets in the evening sky, Jupiter returns to provide us with a bright object to view on hazy, summertime nights. The giant planet is at opposition -- opposite Sun -- about 8 a.m. on Thursday, July 4. This means that Jupiter rises as Sun sets, sets as Sun rises, and that it crosses the celestial meridian, the imaginary line dividing our sky into eastern and western halves, about 12 hours after Sun crossed the celestial meridian.

    When any sky object crosses the celestial meridian it is at its greatest altitude above the south point of the horizon, but this summer that maximum altitude is only a little more than 27 degrees for Jupiter. Jupiter is located in the part of its orbit lying in the direction of the constellation Sagittarius, the same constellation where Sun is located in the middle of winter. Consequently, Jupiter will "swim" in the haze near the horizon all summer.

    Jupiter is also closest to Earth when it is at opposition. Earth and the planet are at the points in their orbits where a plane passed down through Sun and Earth would also pass through Jupiter. Because the distance between Earth and Jupiter is at a minimum the planet appears brightest to us, -2.7 magnitudes, and has its largest apparent diameter, 47.1 seconds of arc, when viewed through a telescope or pair of binoculars.

    A conspicuous feature of Jupiter, when viewed through a telescope or steadily held binoculars, is the shifting array of its four largest moons. Since these were first observed by Galileo, they are referred to as the Galilean satellites. They range in size from larger than Mercury to a little larger than our Moon, and all four are, in principle, bright enough to be seen in a clear, dark sky without a telescope. Except to a few very sharp-eyed observers,however, they are lost in the bright glow of Jupiter without some optical aid. The rest of Jupiter's sixteen satellites are too faint to be seen without a large telescope.

    The three inner-most of the Galilean satellites, Io, Europa, and Ganymede, undergo eclipses by Jupiter as they pass in and out of Jupiter's shadow, and they undergo occultations by the planet as they pass behind Jupiter. They can be observed to transit Jupiter's disk -- move across the face of the planet -- and transits of their shadows can also be observed. These phenomena, along with their constant shifting of position, sometimes on the left of the planet, sometimes on the right, make them fascinating objects to view. Charts showing the positions of the moons as they orbit Jupiter are published in The Astronomical Almanac, The Observer's Handbook, Sky and Telescope, and Astronomy. The times of beginnings and ends of eclipses, occultations, satellite transits and shadow transits can also be found in these publications.

    The Galilean satellites' orbital planes are very close to the equatorial plane of Jupiter. The planet's equatorial plane, and consequently the orbital planes of the satellites, are now nearly edge on to us and to Sun, and will remain so for about 3 years. Callisto, the most distant of the Galilean satellites, will undergo occultations and eclipses by Jupiter, and Callisto and its shadow will transit the planet during this interval.

    This "edge-on" phenomenon for the Galilean satellites occurs about every six years (twice in each Jovian year of 11.86 Earth-years), just as Saturn's rings are edge-on to us twice in each Saturnian year of 29.42 Earth-years. As a result of their orbital planes, which are nearly coincident, being edge on to us and to Sun, the moons can eclipse and occult one another, in addition to being eclipsed and occulted by the planet. Predictions for these events are published in the magazines Sky and Telescope and Astronomy.