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On August 14, 2009 the king planet Jupiter will be at opposition - or 180 degrees from the Sun in our sky. At opposition, Jupiter rises opposite the Sun at sunset, climbs highest overhead at midnight and sets opposite the Sun at sunrise. At or near opposition, Earth comes closest to Jupiter for the year, and Jupiter, in turn, shines most brightly in our sky.
Given a bird's-eye view of the solar system at opposition, you'd see that the Sun, Earth and Jupiter make a straight line in space. Because the shadows of Jupiter's biggest and brightest moons (Io, Europa, Ganymede and Callisto) point directly away from Earth when Jupiter is at opposition, any moon should cross the Jupiter's disk concurrently with its own shadow. Incidentally, Jupiter's shadow also points directly away from Earth at opposition.
Jupiter's Moon Shadows Before and After Opposition
Jupiter's four major moons, which orbit Jupiter above Jupiter's equator, are fairly easy to observe through a telescope. In their outward order from Jupiter, these moons are Io, Europa, Callsito and Ganymede. As viewed from our Earthly terrain, these moons travel from east to west when they circle in front of Jupiter, and from west to east when they swing behind of Jupiter. (As seen from our northern hemisphere, these moons move in front of Jupiter from left to right, and behind Jupiter from right to left.)
Before Opposition, Moon Transits Trail Moon Shadows
As seen from Earth, Jupiter is less than 180 degrees west of the Sun before opposition. That means the Sun is to the east of Jupiter. Therefore, the shadows of Jupiter and its moons point a bit west of the bodies themselves before opposition, causing the moon shadows to transit (or cross) Jupiter's disk before the moons themselves do.
After Opposition, Shadow Transits Trail Moon Transits
As seen from Earth after Jupiter's opposition, Jupiter is less than 180 degrees east of the Sun. In other words, the Sun is to the west of Jupiter. After opposition, the shadows of Jupiter and its moons point a bit east of the bodies themselves. Thereby, the moons of Jupiter transit Jupiter before their shadows do.
This year, in 2009, no major moon transits Jupiter on the opposition date.* However, Io, Jupiter's innermost major moon, transits Jupiter the day before and the day after opposition. Referring to the table below, you can see that Io's shadow precedes Io across Jupiter's disk before opposition. After opposition, the shadow follows the moon. The times are given in Universal Time (UT).**
Io Transits Immediately Before & After Jupiter Opposition
|Date||Shadow Transit||Moon Transit|
|August 13||19:41 to 22:00 UT||19:43 to 22:01 UT|
|August 15||14:10 to 16:29 UT||14:09 to 16:07 UT|
Shadow Play Most Oblique at Quadrature
Any heavenly body that makes a right (90-degree) angle with the Sun in Earth's sky is said to be at quadrature. When the body is 90 degrees west of the Sun, it's called western quadrature. If the celestial object is 90 degrees east of the Sun, it's called eastern quadrature. Click here for a quadrature and opposition diagram.
Distinguishing Western Quadrature from Eastern Quadrature
Perhaps the best way to distinguish western quadature from eastern quadrature is to note the object's position relative to sunrise and sunset. As seen from our temperate northern latitudes, Jupiter appears in our southern sky at sunrise when it's at western quadrature. At eastern quadrature, Jupiter appears in our southern sky at sunset. Take Earth's moon, for instance. At first quarter phase, the moon is at eastern quadrature. At last quarter phase, the moon is at western quadrature.
Jupiter Quadratures About 3 Months Before & After Opposition
Jupiter is at western quadrature about 3 months before Jupiter's opposition; then, about 3 months after opposition, Jupiter reaches eastern quadrature. This year, Jupiter's western quadrature falls on May 16, 2009, whereas eastern quadrature falls on November 10, 2009. If you could look straight down upon the solar system plane at these times, you'd see the Sun, Earth and Jupiter making a 90-degree angle in space.
Quadrature Shadows 11 Degrees Askew of Perpendicular
As seen from Earth, the shadows of Jupiter and its moons point a maximum 11 degrees east or west from perpendicular (pointing directly away from Earth) at quadrature. At eastern quadrature, the shadows point westward of the moons, so the shadows transit Jupiter a maximum amount of time before the moons do.
It's just the opposite at western quadrature. At western quadrature, the shadows point maximumly eastward of the moons, so the moons transit Jupiter a maximum time before their respective shadows do.
The table below gives the moon transit and moon shadow transit times for Jupiter's innermost major moon, Io, on May 15, 2009 (day before western quadrature) and November 10, 2009 (day of eastern quadrature):*
Io at Jupiter's Western & Eastern Quadratures
|Date||Shadow Transit||Moon Transit|
|May 15||13:26 to 15:43 UT||14:47 to 17:03 UT|
|Nov 10||7:46 to 10:03 UT||6:26 to 8:43 UT|
* But the moon Io will eclipse and occult the moon Europa on August 14 (from 3:49 a.m. to 4:01 a.m. Eastern Daylight Time). See page 52 of the July 2009 SKY & TELESCOPE.
** Source: 2009 Astronomical Handbook, pages 237-242
copyright 2009 by Bruce McClure
|Phenomena of Jupiter's Satellites|
|Chasing the Moons of Jupiter|
|Jupiter's 2008 Opposition|
July 2009 Feature * Sept 2009 Feature