The Jupiter-Neptune Triple Conjunction,
May to December 2009
by Martin J. Powell
Paths of Jupiter and Neptune, in North-eastern Capricornus, shown at 10-day intervals for the period April 4th to September 11th 2009 (click on the thumbnail for the full-size image, 50 KB). A star chart showing Neptune's general location in the sky from 2006 to 2023 can be found here.
Two of the three conjunctions between these planets took place during the period indicated on the chart, i.e. on May 25th and July 13th (at which times, a line drawn vertically through the two planets show them to be in alignment).
After September 11th (when Jupiter exited the chart area) the position of Neptune is marked by individual points through to October 21st 2009.
Much of the star field in the chart is easily contained within a binocular field of view (which typically ranges from 5° to 9°). Stars are shown down to an apparent magnitude of +8.5. Astronomical co-ordinates of Right Ascension and Declination are marked around the border, for cross-referencing in a star atlas. A Southern hemisphere view can be found here (50 KB). Printer-friendly (greyscale) versions of the chart are available for Northern (23 KB) and Southern hemisphere (23 KB) views.
Click here (34 KB) to see a 'clean' star map of the area (without planet paths); a printable version can be found here (17 KB). The apparent magnitudes of selected stars in the region (down to magnitude +8.5) are shown in brackets in this chart (50 KB).
As Jupiter and Neptune passed each other in the sky during 2009, they underwent a series of three conjunctions (i.e. they attained the same celestial longitudes) over a seven-month period; this is referred to as a triple conjunction, and they are relatively rare events (the previous one between Jupiter and Neptune took place in 1971).
The first of the 2009 conjunctions took place on May 25th 2009, when Jupiter (at an apparent magnitude of -2.4) was moving direct (or prograde, i.e. West to East) against the star background and Neptune (at magnitude +7.9) was approaching its Eastern stationary point. The two planets were separated vertically by 25' (25 arcminutes, or just under half a degree) at this point, and were visible in the morning sky before sunrise. The second conjunction took place on July 13th 2009, when both Jupiter (mag. -2.7) and Neptune (mag. +7.8) were retrograding (moving East to West) against the star background; the planets were separated by 36' (0°.6) at this point, and were visible from the late evening onwards. Jupiter then reached opposition to the Sun on August 14th (having brightened to mag. -2.8), followed only three days later by Neptune (at mag. +7.8). Finally, the planets were again in conjunction on December 20th 2009, when both Jupiter (-2.2) and Neptune (+7.9) had resumed direct motion, the pair then being separated by 33' (0°.55); this conjunction was visible in the evening sky, shortly after sunset.
The next triple conjunction between these two planets will be in 2047-8, when they are both in Aries. Conjunctions of any kind between these two planets (mostly single or double) occur, on average, about every 12.6 years. The next will be on April 12th 2022 (single, on the Aquarius/Pisces border) then on March 25th 2035 (single, in Pisces); both of these conjunctions will be difficult to see because the two planets will be close to the Sun at the time.
A point should be made regarding the planetary conjunction dates mentioned here. There are in fact two ways of defining a conjunction date: one is measured in Right Ascension (i.e. along the celestial equator) and the other is measured along the ecliptic, which is inclined at 23½° to the Earth's equatorial plane (this is due to the tilt of the Earth's axis in space). Conjunction dates measured in Right Ascension (RA) apply whenever two planets attain the same Right Ascension; those measured along the ecliptic apply whenever two planets attain the same ecliptic longitude (which is measured in degrees Eastwards from the First Point of Aries). The conjunction dates determined by each method can differ by up to several days, particularly when conjunctions occur in the vicinity of Pisces and Virgo, i.e. the constellations in which the ecliptic intersects with the celestial equator. Although dates determined using the ecliptic longitude method are technically more accurate (separations between planets can be significantly closer) the Right Ascension method is the more commonly used, and it is the one which is adopted here.
An animation demonstrating the principle of the triple conjunction - and showing how conjunction dates are determined - can be found on the Jupiter-Uranus triple conjunction page.
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Copyright Martin J Powell February 2009
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