Conjunction of Venus and Jupiter

By Jason Schreiner, Planetarium Coordinator

Our two brightest planets, Venus and Jupiter, will put on a show this month, as they pass within 1.4 degrees of each other in the western sky at sunset.  That’s about the width of your index finger at arm’s length. In astronomy, when two astronomical objects appear to share a part of the sky, we call this a conjunction.

Venus and Jupiter will hover just above the horizon in the west after sunset this month. 

Image credit: EarthSky.org

From our view on Earth, Venus and Jupiter are both brighter than any star. Having them so close together in the sky makes for an exciting spectacle. Why are these two planets the brightest, though? Venus is small and rocky, while Jupiter is giant and made almost entirely out of gas. They’re quite different but share one important similarity: their reflective atmospheres.

Venus is the brightest planet for two main reasons. First, Venus is our planetary next-door neighbor, so we have a very good view of it from here on Earth. Second, Venus has an extremely reflective atmosphere. The measure of a surface’s reflectivity is called albedo. The albedo scale ranges from zero (reflecting no light) to one (reflecting all light). So the higher an object’s albedo, the brighter and shinier it is.

Even though Jupiter is hundreds of millions of miles away, in the outer part of our solar system, it is still the second brightest planet, due not only to its own high albedo, but also its size.