In the night between November 30 and December 1, Mars and Jupiter will be at heliocentric conjunction. Is that interesting?

Pre-dawn sky 2015 Dec 1The instant of the event is 2 hours Universal Time, so it is Nov. 30, 9 PM, in eastern America, Dec. 1, 2 AM, in Britain.

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.

Mars and Jupiter heliocentric conjunctionThe 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.

6 thoughts on “Carousel”

  1. If my estimation ability is still in good working order, then it would seem that if the Earth happened to be somewhere around the late-February / early March part of its orbit (but no other changes to the planets’ positions), then we would see one of my “holy grail” nighttime vistas, a close conjunction of Mars and Jupiter while they would be very close to opposition (although the Feb / March Martian oppositions are distant ones, like 1980 was). The way you have drawn the stalks on the planets, it gives the impression that both Mars and Jupiter are at this point “above” the ecliptic plane, so they would probably be fairly close in the sky: a beautiful double-planet, sort of an inverse Albireo, magnified by 4 or 5 magnitudes!

    1. I like looking at Guy’s such diagrams. I look into the future too and figure how the Mars/Jupiter conjunction figures in. I get high just lookin out and figurin’ out how we’re all placed on this , well, not quite a circle, but a dance around all the orbs around the sun…. Anyone else counting down the days til the Aug 21 2017 total solar eclipse? Seems like it’s passing right over Guy’s univeristy? Anything to say about that, Guy? Can I bring my dog?

      1. I’ve been making it no secret that I hope to be back in Greenville for the 207 eclipse. Among the phenomena to watch will be the behavior of moths, birds, and exuberant Italian-Americans, and a special study of whether their dogs how at the moon or just busy themselves with the usual other doggy interests.

        1. If I were to draw lines on the map from my location here in Fredericksburg to the 2017 eclipse track, and find the shortest line, it would be somewhere in central South Carolina, so I’ll probably be there to observe it as well. Maybe those of us in the eastern United States should all converge on Guy right after the eclipse and badger him into resurrecting the AC!

          However, let’s not get ahead of ourselves: with our appetites for transits having been whetted in 2004 and 2012, this coming year offers a chance to see the Mercury transit. Despite being 52 years old, I’ve never seen one, so this year I’m determined to do so! I’m sure Guy will cover it extensively in AC 2016.

          1. Yes, it has a page in AC 2016.
            The books reached our distributor (Ed) yesterday and “will start shipping by the weekend”.

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