Astronomers use some interesting terminology: phrases such as “in opposition” and “at greatest elongation”. What do these phrases mean, after all? One is of particular note right now: “Saturn is at opposition.”

On April 15th, the planet Saturn will be located directly opposite the sun in the sky. They will be 180 degrees apart. This means that one could draw a line from the Sun, to the Earth, then right out straight to Saturn. This is a good thing for those interested in seeing the ringed world (or any planet in opposition). Planets at opposition are at their closest point to Earth and are at their brightest and largest apparent size.

The opposite is true for planets at “conjunction with the Sun.” Things in conjunction are placed in the same location in the sky from our vantage point. If Jupiter was at conjunction with the Sun, then it would be behind it and invisible to Earth-based astronomers.  The planet Jupiter in conjunction would also be specifically in superior conjunction, which means it’s on the far side of the Sun, since it can never be between the Sun and Earth. Planets like Mercury and Venus can be in superior conjunction or in inferior conjunction, when it is between the Sun and the Earth.

(image Wikipedia Commons)

For those inferior planets like Mercury and Venus, they have no way to reach an opposition position to Earth. They do however reach a position furthest from the Sun in the sky, and astronomers call that the point of greatest elongation. Planets at greatest western elongation appear to rise in the east just before the sun.  Planets at greatest eastern elongation set in the west just after the sun sets.

Other things can be in conjunction as well. A classic example would be the lovely conjunction of the planet Venus and the star cluster, the Pleiades, which occurred last week.  Those interested should definitely see this image of that event: APOD Venus Pleiades Conjunction.