Inferior and Superior Conjunctions

I got some questions after last week’s post about Transits of Venus and Mercury asking for clarification on a term I used — inferior conjunction. I linked the term (as I did here) to an astronomy encyclopedia that has a pretty good diagram and explanation. But based on the questions I got, I decided to do an animation that clearly shows both inferior and superior conjunctions. I used Venus for this demo.

Inferior conjunctions can only happen with celestial bodies positioned closer to the Sun than Earth is. That includes Venus and Mercury, of course, and also any asteroids or comets that stray into this part of the solar system. Objects farther from the Sun than Earth (e.g, Mars) can only experience superior conjunctions, when they pass behind the Sun from our point of view.

The animation shows 1.5 Earth orbits (Earth-years) of motion for Earth and Venus. During this time, Venus goes through its cycle from one inferior conjunction, to a superior conjunction, to the next inferior conjunction.

You may have noticed in the animation that these events happen at positions that are 90° and 180° from the starting point. If so, good eyes. In reality, there is no such pattern. It arose because I rounded the value for the angular speed of Venus to a multiple of Earth’s. They’re close to the actual values but not exact. The inferior-superior-inferior cycle for Venus should actually take 1.599 Earth-years — not the 1.5 seen in the animation.

Vocabulary FYI: 1.599 Earth-years is the synodic period of Venus — the time it takes a planet to return to a specific position in the sky as seen from the moving Earth. You can see in the animation that Venus itself makes a complete orbit in 0.615 Earth-years. That time, which is measured with respect to the non-moving stars, is known as its sidereal period. But more about that in a couple weeks.

Next Week in Sky Lights ⇒ Ring of Fire Eclipse

Q&A: Transits of Venus and Mercury
Ring of Fire Eclipse