In the night between November 30 and December 1, Mars and Jupiter will be at heliocentric conjunction. Is that interesting?
Heliocentric conjunction, what’s that? It’s when a planet overtakes another. As seen from the Sun, they are in the same direction. A routine geometric event, like others that fill the list in Astronomical Calendar 2015 for the last week of November – Mercury at aphelion, Venus at perihelion, Saturn behind the Sun, the Moon passing stars and planets by wide margins.
But perhaps it’s interesting in that it’s rather surprising. If you go out before dawn and look at the scatter of planets still inhabiting the part of the sky to the right of the Sun, you don’t seem to see Mars at any kind of conjunction with Jupiter: they are more than 18 degrees apart, almost a fully stretched handspan. In fact Mars appears nearer to Venus than it does to Jupiter.
Imagine yourself at the Sun, down there below the horizon; then it looks more reasonable that there is a straight Sun-Mars-Jupiter line in space.
But it takes a 3-D picture to show that it’s really so, and to explain why Venus isn’t in the same line.
The planets’ courses in November and December, with sightlines at Dec. 1 from Sun through Mars to Jupiter, and from Earth to the planets. The dashes and gaps in the sightlines are 0.2 astronomical unit (Sun-Earth distance) long. The Sun is exaggerated 5 times in size, Jupiter 50, the other planets 400. The ram’s-horns symbol marks the direction to the vernal equinox point (the base point for celestial mapping).
I have the luck to be sitting on a revolving office chair, though rather a threadbare one. I pivot slowly leftward, holding out a left fingertip at arm’s length, holding out a right fingertip nearer to me and letting it overtake the other one. On the Sun, the Keeper of the solar system sounds a gong as one planet glides past the other.