For much of 2009 Jupiter and Neptune have been sharing the same space in the sky inside the Zodiac constellation Capricornus. Over the last few months they have moved progressively closer to each other and are now aligned in what is called a "Conjunction." In fact, the year 2009 saw this happen three times in a row, what astronomers call a Triple Conjunction, a rare and beautiful alignment of heavenly bodies.
I had not seen Neptune before this year's Triple Conjunction. A conjunction creates an opportunity to easily locate the less-bright planets Neptune and Uranus. These two are just dim enough to be invisible unless you (a) know exactly where to look, and (b) use optical aid such as a telescope or binoculars. With bright Jupiter as my guide, I was easily able to find Neptune lurking in the neighborhood, a beautiful blue pinpoint of light (see my previous post on the subject, Blue Neptune). The third of the three conjunctions of Jupiter and Neptune is even easier to find this week because the waxing Moon passes just above the duo on the evenings of December 20 and 21. In fact, the Moon makes a splendid guide to the planets each month, having just passed near Mercury a few days ago, and later in the month approaching orange-red Mars and yellow-white Saturn.
For a more detailed view of the paths of Jupiter and Neptune across the night sky check the diagrams on Martin Powell's excellent astronomy website. With these detailed charts, you can get a clear perspective on how these objects slowly make their way from one Zodiac constellation to the next every year. As Jupiter now begins prograde motion away from Capricornus, it will speed across Aquarius and into Pisces during the winter and by spring will be closing in on Uranus, where in 2010 we will have a Triple Conjunction of Jupiter and Uranus. Scroll down this page to see an excellent animation showing the motion of Uranus and Jupiter across the sky in 2010. The heavens never cease to offer up opportunities for discovery. Here's to clear skies!