Skywatchers are hoping for clear evening skies over the next few days. For eight days, starting Saturday, Jupiter and Venus, the two brightest planets, will appear very close to each other in the night sky.
The last time the two planets got this close to each other was almost two thousand years ago. According to earthsky.org, the next close apprach will happen August 27, 2016, so you won’t have to wait another 2000 years if you miss this one.
These two bright planets will be so close that they will appear like a double star. The magazine Sky & Telescope writes that a similar conjunction occurred during the 3-2 B.C. timeframe, which has makes it a potential source for the original “Star of Bethlehem” in the Bible.
Rice University astronomer Pat Hartigan, has a nice write-up about this event, answring some common questions.
Here’s a sample from his site.
How often do Venus and Jupiter line up?
Venus appears to follow the Sun around the sky as seen from the Earth because Venus is an inner planet. Jupiter on the other hand, is an outer planet that moves around the Sun slowly, and as a result appears to circle the sky roughly once a year from Earth. Hence, Venus and Jupiter typically have one conjunction (lining up as seen from the Earth) each year.
How does the June 30, 2015 conjunction rank?
Very highly! For the best viewing you want a close conjunction that is high in the sky. Consider the following plots for a typical southern city (Houston) and northern city (New York). The best conjunctions are the ones in the upper right quadrant. Circles are evening conjunctions, and squares are morning. As you can see, the conjunction of June 30, 2015 is the best one we will have in the evening for quite some time, rivalled only by the one on March 1 2023, which is not quite as close. The June 30 conjunction will be 1.15, 0.65, 0.33, 0.55, and 0.98 degrees, respectively, on June 28, 29, 30, and July 1, 2, all at about 32 degrees above the horizon at sunset in Houston (27 degrees for NYC). Hence, any of these five nights will be better than the best of most other conjunctions. For reference, the size of the full Moon on the sky is 0.50 degrees.