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October 2008 Feature: Watching Jupiter's Moons at Quadrature


The planet Jupiter will be at eastern quadrature on October 6, at 1 p.m. Eastern Daylight Time (5 p.m. Universal Time). If you had a bird's-eye view of the solar system at this juncture, you'd see the Sun, Earth and Jupiter making a right (90-degree) angle in space. If the planet appears in Earth's evening sky, it's called eastern quadrature. If the planet appears in the morning sky, it's called western quadrature. This diagram helps you to visualize the concept of quadrature.

Finding Jupiter

If you have a hard time finding this blazing world in your southern sky at nightfall, just look for the Moon at dusk and early evening on October 6 & 7. Jupiter will be that overwhelmingly bright point of light nearby the Moon. When any planet resides at eastern quadrature, it crosses your celestial meridian about six hours after the sun crosses your celestial meridian at solar noon.

Jupiter's Setting Time at Quadrature

If, at quadrature, Jupiter resides on the celestial equator, this (or any) planet sets around midnight as seen from everywhere around the world. However, since Jupiter resides some 23 degrees south of the celestial equator at this particular quadrature, Jupiter sets before midnight in the Northern Hemisphere but after midnight in the Southern Hemisphere. At the Earth's equator, Jupiter still sets around the midnight hour. North of the Arctic Circle, you won't see Jupiter at all, because this word stays beneath the horizon for 24 hours around the clock.

Jupiter's Moons

Jupiter's four major moons - Io, Europa, Ganymede and Callisto - are easy to see in a telescope or even good binoculars. Sometimes, you can see all four moons at once. Sometimes, a moon or two may be missing, because these moons routinely swing in front and behind of Jupiter. As you're looking at these moons from the Northern Hemisphere, these moons swing from left to right (more properly: east to west) when crossing in front of Jupiter's disk, and move from right to left (west to east) as they swing behind Jupiter.
When the moon is in front of Jupiter, the moon is said to be transiting Jupiter. When the moon sweeps behind Jupiter, Jupiter is said to occult - cover over - its moon. You can't see any of Jupiter's moons when they're occulted by Jupiter, and it's awfully difficult to see Jupiter's moons when they're transiting Jupiter's disk.

Quadrature & Jupiter's Moons

However, given a decent telescope, it is quite possible to see a moon's dark shadow crossing Jupiter's disk. This is called a shadow transit. At eastern quadrature, the moon's shadow transits Jupiter after the moon itself transits Jupiter. As seen from Earth at quadrature, the Sun's rays are not hitting Jupiter (and its moons) head on, but somewhat askew.
At quadrature, the Sun's rays are about 11 degrees off from hitting Jupiter perpendicularly. As seen from Earth at eastern quadrature, the Sun is to the right (west) of Jupiter, and the shadows fall a bit to the left (east) of Jupiter and its moons. The shadow angles are always most askew at quadrature, whereas at opposition, the shadows point most directly away from Earth. At opposition, it's possible for the moons to occult - cover over - their own shadows!
A western quadrature is the exact opposite of an eastern quadrature. At western quadrature, the moons' shadows transit Jupiter's disk before the moons themselves do.

Transit Cycles

Presuming that you live in the Eastern Time Zone in North America, Io's shadow will transit Jupiter on October 2 from 2:56 p.m. to 5:11 p.m. Eastern Daylight Time. As a rule of thumb, the three inner moons - Io, Europa and Ganymede - return to the same place relative to Jupiter in periods of about 7 days plus 2 hours. That means a week from October 2 - on October 9 - Io's shadow transit will come about two hours later (4:51 p.m. to 7:07 p.m. EDT). And two weeks after October 2 - on October 16 - the shadow transit will come about 4 hours later (6:47 p.m. to 9:02 p.m. EDT).
For the specific occultation and transit times of these Jovian moons, I refer you to this Chasing the Moons of Jupiter article, courtesy of Sky & Telescope magazine.

Io, Europa & Ganymede Locked Into an Orbital Resonance

In a period of a little more than one week, Io circles Jupiter 4 times, as Europa circles Jupiter twice and Ganymede circles Jupiter once. In the meantime, the outermost moon Callisto marches to the beat of a different drummer. But Callisto is expected to join this orbital resonance in a few hundred million years.

copyright 2008 by Bruce McClure

Phenomena of Jupiter's Satellites

September 2008 Feature * November 2008 Feature