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The planet Jupiter will be at opposition on the night of July 8-9. When at opposition, Jupiter stands opposite the Sun in Earth's sky. Jupiter rises opposite the Sun at sunset, reaches its highest point in the sky at midnight and sets at sunrise. At or near opposition, Jupiter comes closest to Earth for the year and Jupiter, in turn, shines at its brightest in our sky.
If we could see Earth from Jupiter during this opposition, we'd see the Earth transiting (crossing in front of) the Sun. A transit is a special type of eclipse, whereby a smaller celestial body - such as Earth - passes in front of a larger celestial body - such as the Sun. On July 9, while Jupiter is at opposition as seen from Earth, Earth is transiting the Sun as seen from Jupiter.
Orbital Paths Not On Same Plane
If the orbital paths of Earth and Jupiter occupied the same exact plane, it'd be so simple. Every time Jupiter returned to opposition in Earth's sky, Earth would transit the Sun in Jupiter's sky.
Alas, it's not that easy. Jupiter's orbital plane is inclined about 1.3 degrees to the ecliptic (Earth's orbital plane). Half of Jupiter's orbit passes to the north of the ecliptic, whereas the other half passes to the south of the ecliptic. Jupiter crosses the ecliptic two times during its nearly 12-year circuit around the Sun. When Jupiter crosses the ecliptic going from south to north, Jupiter is said to be at its ascending node. On the other hand, when Jupiter crosses the ecliptic from north to south, Jupiter is at its descending node.
Close Encounter With Node Neccessary For Transit
For the Earth to transit the Sun from Jupiter, two conditions are neccessary. First, Jupiter has to be at opposition in Earth's sky. Second, Jupiter must be appreciably close to one of its nodes when at opposition. During this 2008 opposition, Jupiter comes close enough to its descending node for the Earth to transit the Sun as seen from Jupiter.
Although oppositions of Jupiter recur roughly every 13 months, more often than not Jupiter does not come close enough to its node for a transit to occur. The last transit took place around 6.5 years ago, when the opposition of January 1, 2002 found Jupiter very close to its ascending node. The next transit will take place about 5.5 years from now, when Jupiter will swing by its ascending node once again during the January 5, 2014 opposition.
Ascending Node in Early January; Descending Node in Early July
In a nutshell, when Jupiter has an opposition in early January, a transit is inevitable because Jupiter is at its ascending node; and when this planet is at opposition in early July, a transit is inevitable because Jupiter is at its descending node. If Jupiter's opposition happens within a week or so of the optimal date, a transit is still in the works, though it's of lesser duration. Transits at the descending (or ascending) node often recur in 12-year periods. For example, the descending node transit taking place on June 9, 2008, caps a series of descending node transits that began on June 24, 1972. The table illustrates:
Descending Node Transits 1972-2008*
|June 24, 1972|
|June 29, 1984|
|July 4, 1996|
|July 9, 2008|
83-year Series Cycle
As you can see from the table above, the opposition of Jupiter comes about 5 calendar days later every 12 years. In 1960, the opposition of Jupiter happened on June 20, so the date fell too early (and Jupiter was too far from its descending node) for a transit to take place. In 2020, Jupiter's opposition will fall on July 14 - a bit too late for a transit to occur. The next descending node transit after this year's July 9, 2008 affair won't be until June 24, 2055, exactly 83 years after the June 24, 1972 transit! What's more, this whole series of transits will repeat in 83 years, as shown in the table below!
Descending Node Transits 2055-2091*
June 24, 2055 June 29, 2067 July 4, 2079 July 9, 2091
copyright 2008 by Bruce McClure* Source: page 323, Morsels IV by Jean Meeus
June 2008 Feature * August 2008 Feature