[BAA 00458] Conjunction between Jupiter and Neptune

[Rick]

BAA electronic bulletin No. 00458 -- http://www.britastro.org/
Bulletin transmitted on  Sun Dec 20 13:41:08 GMT 2009
(c) 2009 British Astronomical Association

The last of this year's three conjunctions between Jupiter and Neptune occurs tonight, Sunday, 2009 December 20. Neptune will be 0.6 degrees N of Jupiter, and thus in the same low-power telescopic field.

A series of three conjunctions between planets is known as a "triple conjunction" and is quite unusual. The last occurred between Uranus and Neptune in 1993. There will be a series between Jupiter and Uranus in 2010/11 (2010 June 8 and September 19 and 2011 January 04), but then no more until 2037/38 when the pairing will again be Jupiter and Uranus. They only occur for superior planets. The nearer planet passes the more distant in the sky, retrogrades and passes it again, then passes for the third time in direct motion.

The conjunction tonight will be best seen just after dark or in twilight. At 17:00 UT the planets will be about 25 degrees high just west of south. The occasion offers an excellent opportunity to easily identify Neptune. From east to west the Jupiter system will be seen as Io-Europa-Jupiter-Ganymede. (Callisto will be transit across the face of Jupiter and therefore possibly invisible). Neptune will be found approximately five times that system's width to the north of 
Ganymede.

It will also be worthwhile to look at the configuration a day later on December 21 when the crescent Moon will be 4 degrees to the north of Jupiter, so all three bodies might be fitted in the same binocular field. These alignments will provide good subjects for DSLRs and other cameras used with telephoto lenses or small telescopes.

Happy Christmas from me and the Council of the BAA.

David Arditti