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March 31, 2005
Jupiter at its best
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WAUKESHA, WI – Jupiter, largest of our solar system’s planets, reaches maximum brilliance April 3. On that date, the giant planet will reach a point astronomers call opposition. As viewed from Earth, Jupiter then lies directly opposite the Sun. Jupiter will rise at sunset, set at sunrise, and be visible in the sky all night.
Because opposition also is when the planet lies closest to Earth, Jupiter shines brightest and appears largest. April, then, will be the best time for viewing. Furthermore, opposition places a planet due south and at its highest in the sky at local midnight, the position where we view it through the least amount of Earth’s atmosphere. The higher a celestial object is, the less turbulence we’re looking through and the better the view.
This year, find Jupiter nearly 50° above the southern horizon around midnight. Now’s the time to look because next year Jupiter’s highest altitude will be only 35°.
On April 3, Jupiter lies 10° northwest of Virgo’s brightest star, Spica. Throughout April, the planet’s westward motion carries it toward Porrima, Virgo’s second-brightest star, so that by the 30th, Jupiter lies 1.3° south of it. On April 22, you’ll find Jupiter 6° to the upper right of the almost Full Moon.
Throughout April, Jupiter will be the brightest object in the night sky, except for the Moon when it’s visible. Jupiter will appear 25 times as bright as Spica and roughly 130 times as bright as Porrima.
Jupiter is the largest planet in the solar system. Not only would 1,266 Earths fit inside Jupiter, but the eight other planets would only comprise 68.8 percent of Jupiter’s volume.
Jupiter is 318 times as massive as Earth.
Jupiter spins (rotates) so rapidly that it is flattened slightly. Its polar diameter is only 93 percent of its equatorial diameter.
Jupiter is the second most reflective planet (next to Venus). It reflects 52 percent of the sunlight falling on it.
At this year’s opposition, Jupiter’s apparent size in a telescope reaches the largest of any outer planet. Venus can get larger, but only in twilight and as a thin crescent. This large apparent size offers great opportunities for viewing cloud features in the jovian atmosphere. Jupiter completes one rotation in less than 10 hours, allowing the entire planet to be viewed during a single night this month.
Jupiter’s four bright moons, Io, Europa, Ganymede, and Callisto, can be observed easily through small telescopes. Io takes less than 2 days to orbit, so you can see its relative position change in an hour or so – less when it appears close to Jupiter. Our line of sight lies in the plane of the moons’ orbits, so we see occultations (when a moon goes behind Jupiter), eclipses (when Jupiter’s shadow falls on a moon), and transits (when a moon passes in front of Jupiter) at various times.
The solar system’s largest satellite is Ganymede, with a diameter of nearly 3,300 miles (5,270 km).