Where Earth is heading From

If Earth is a car, and we now look out of the rear window, we see the Moon.  It isn’t, however, chasing us like a cop, but is crossing over the road (our orbit) behind us.

And apparently walking backward across the traffic: it’s moving from right to left, but with its D-shaped face it looks westward at the Sun.

All this merely means that it is at its First Quarter position.  The actual moment was near the beginning of this date, 4:19 by Universal Time, which was from 5 to 8 hours earlier in the time zones of the contiguous U.S.A.  So for America it was nearer to the sunset of the 3rd than of the 4th.

Looking at Mars and Venus in our space-diagram of the planets at the end of January, Rick Scheithauer wondered whether Mars, which happened then to be at the point where ecliptic crosses equator, was also near the direction from which Earth had traveled.

This direction is the opposite of what we have called informally the EDOT or “Earth’s direction of travel,” known in astronomical literature as the Apex of Earth’s Way.  So the behind-direction is the Antapex of Earth’s Way.  It’s not easy to think of a homelier term; “rearview point” isn’t too good, so until you suggest something cute I’ll call it antapex.

It has to be roughly (though not exactly) 90° from the Sun, tangent to Earth’s not-quite-circular orbit.  Mars, however, has an elongation from the Sun of 51° on Jan. 30, 50° on Feb. 4, so the antapex is considerably out east from it.  I’ve now worked it into my programming, and this is just the time to show it.   There it is, near the First Quarter Moon.   At that phase the Moon’s longitude is 90° from the Sun, so it would have passed exactly through the antapex, but for its latitude, which at present has it cruising south of the ecliptic.

A planet’s position when it is 90° from the Sun is called “quadrature.”  Mars does not reach either east or west quadrature this year, because it never gets far enough from being behind the Sun.  Mercury and Venus are never as far out as quadrature; the outer planets pass through both quadratures almost every year (Jupiter for example was at west quadrature, out in the morning sky, on Jan. 12).

There is also such a thing as the Apex of the Sun’s Way, and I’ll return to that.


10 thoughts on “Where Earth is heading From”

  1. My main takeaway is that a first quarter moon is always approximately the same direction as the anti Apex of the Earth’s Way. I just looked at this first quarter moon which happened to be quite high in the sky due south. I imagined the ground falling beneath me at 30 Km/sec = Earth’s orbital speed. I almost got virtually dizzy. Thanks for this virtual joy ride.

  2. I especially enjoy observing Jupiter and Saturn when they are near quadrature. The shadows of the planets, of Jupiter’s moons, and of Saturn’s rings fall farthest to the west (when the planet is moving toward opposition) or east (when the planet is moving toward conjunction with the Sun). Jupiter’s moons either disappear into eclipse or suddenly emerge from eclipse when they are distant from Jupiter’s disk. A transit of a moon’s shadow across Jupiter’s cloud tops either precedes or follows the moon’s transit in front of Jupiter. Saturn’s shadow is obvious on the rings, and, depending on the rings’ tilt to our line of sight, the shadow of the rings may be visible on Saturn’s cloud tops.

    (By the way, I’m relieved that the Earth is no longer falling straight toward the Sun! ;-) )

    1. One more pleasant thing about observing a planet around quadrature: it’s highest in the sky around sunrise (waxing quadrature) or sunset (waning quadrature), so you can see it at its best and still get a good night’s sleep.

  3. Hmmm… this does need a good nickname. All I can come up with is the ADOT – Anti Direction of Travel which isn’t very imaginative! I am certain someone else will have a better name.

    These are some of my favorite motions to think about and ponder. If I can see the Milky Way, it’s also fun to figure out which part is the back of the arm in front of us and the front of the arm behind us. It’s too bad so many people can’t see the Milky Way to enjoy sights like that.

    1. ADOT is good, but it doesn’t reference the Earth. EADOT would be more accurate but that sounds just like EDOT.

      I thought of EDFT (Earth’s Direction From Travel), but EDFT isn’t easy to say and I’m not sure it’s grammatically correct.

      EHOT might work (Earth’s History of Travel).

      1. Rick, I was just making a play on the word “antipodes” which means the point on the globe opposite of wherever you are ~ a destination you can never get to! Or perhaps I should say, to avoid being made fun of by Guy on account of my writing, “a destination to which you can never get”.

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