Mars marks the sky’s halves

Earth’s small brother planet crosses the celestial equator on January 29, at 12 by Universal Time.

The red dot of Mars serves today as an almost exact marker of the point where the ecliptic and the celestial equator intersect.  This – the vernal equinox point, anciently called the First Point of Aries but now in Pisces – is the point the Sun will reach at the March 20 equinox.  We use it as the “origin” or zero point for our system of mapping the sky: both longitude, parallel to the ecliptic, and right ascension, parallel to the celestial equator, are measured eastward from it.

This is how the inner planets are now arranged, so that we see Mars and Venus east of the Sun.

Spatial view, from 15° above the ecliptic plane, of the planets’ paths in January, with sightlines from Earth.  The dashed line and ram’s-horns symbol mark the vernal equinox direction, which at this moment is parallel to the Earth-Mars direction.

Mars will be in the northern celestial hemisphere for the next 270 days, traveling its arc through the constellations of the northern half of the zodiac: Pisces, Aries, Taurus (at the farther end of which it will be highest north, on June 6), Gemini, Cancer, Leo, Virgo, where it will descend through the equator on Oct. 26.


8 thoughts on “Mars marks the sky’s halves”

  1. While checking out Mars and Venus, I tried to visualize the direction from where Earth came. At first I thought it had to be some point between Mars and Venus, but after looking at your solar system diagram, i saw that it would be east of Mars.

    Is the direction from which the Earth has come called the Antapex of the Earth’s Way?

    1. Yes, and I could add that mark into diagrams, will make a note to think about it after all else I unfortunately have to do today. But that antapex will be about 180 degrees from the Sun, whereas Mars is now nearer to its 90 degrees position or quadrature.

        1. Didn’t you know that Earth is falling directly toward the Sun?
          I was in too much of a hurry to get things done before having to depart for half the day.
          What I should have said is that Mars was nearer to 50 degrees in elongation. The antapex, yes, must be around 90 degrees from the Sun.
          I’ve now programmed to plot it and will show it in the future. Added to that Jan 31 picture, it is in Aries,
          30 degrees or so up along the ecliptic from Mars.

  2. Are the lines from Earth to Mars and from the Sun to the first point of Aries exactly perpendicular, or do they gradually converge toward the same vanishing point?

    1. I think the sightlines from Earth to node and from Sun to node are parallel and would not even meet at infinity. Mars happens to lie on the Earth-node sightline, but the sightline from the Sun to Mars is certainly not parallel to the others.

      1. Got it, thanks. The node is a platonic ideal, not a very very distant real object!

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