Does the orientation of the ecliptic shift relative to the galaxy?

I got to thinking about this tonight, and I haven’t found an answer. Is the orientation of the ecliptic moving appreciably relative to the galaxy? Is there a point in the moderately far future when the ecliptic would pass through Orion, and would this be before the constellation of Orion itself changes apparent shape much?

The Ecliptic will make a full pass around the galaxy for each full orbit the sun makes around the galaxy. Which takes about 240 million years. The axis of the Earth is pointing quite some way off parallel to the axis of rational around the galaxy, so the Ecliptic will cover a pretty large fraction of the sky over this time. It will certainly intersect Orion at some point. Orion isn’t all that far from the Ecliptic now, but It makes my head hurt to try and work out if intersecting it again is soon, or has just gone, and is a long time to come again.

The limiting problem is likely more the question - will Orion exist as a constellation by the time the Ecliptic reaches it? The proper motion of the stars may have quite scrambled the geometry as seen from the Earth.

Which is what buggers up the astrologers. Anyone who believes in this woo should look at this BBC page to see what sign they were *really *born under…

Our solar system is part of the Sagittarius Dwarf galaxy, a small galaxy captured by the Milky Way. It is diving at an angle across one of the Milky Way’s spiral arms which explains why our local ecliptic gives us a titled view of the Milky Way.


The Sagittarius Dwarf galaxy’s orbit does cause it to cross the galactic plane, but earth was never a part of it. And, in any case, the orbital planes of solar systems don’t correlate with the the orbital plane of the galaxy, so there’s no need to posit a mechanism for why the ecliptic doesn’t line up with the Milky Way.

^I came in here to ask about this after reading Ken’s post. If you want more details, here’s The Bad Astronomer’s take on it.

Thanks D18 for posting a cite that provided a tiebreaker for the two conflicting posts before yours.

Very interesting and thank you.

It sounds like you’re saying that the precessional period of the ecliptic is the same as the Sun’s orbital period. I have no idea why this would be the case, and it sounds extraordinarily unlikely (typically, precessional periods are much longer than the corresponding orbital period).

Ugh, not what I intended to say. The basic idea was to ignore precession, and consider the Earth’s axis as staying roughly pointing in the same direction, and only then consider the rotation of the solar system around the galaxy. Which is why I said sun rather than just Earth.

That’s right. The top of Orion is only about 1 degree below the ecliptic; if we assume that the plane of the ecliptic remains constant (which seems inevitable in the long run), then in a million years the plane of the ecliptic will move more than a degree with respect to the plane of the galaxy.

However I don’t know whether the ecliptic is moving towards Orion or away from it at the present time.

Hmm; if I am visualising this correctly, the Sun (and Orion, and the rest of the local galaxy) is moving towards Cygnus, which means that the plane of the ecliptic is moving towards Orion and will intersect the top of it in roughly a million years. Note that the top boundary of the constellation is quite a long way from the brightest stars in Orion, so you won’t get to see the Sun go in front of Betelgeuse right away. In fact Betelgeuse will probably blow up long before then in any case.

You can’t treat the movement of our solar system through the galaxy as an analog to the movement of planets within our solar system around the Sun. For one, Sol doesn’t orbit a central mass; it is part of a moving disk of matter in which it is essentially embedded. Left alone (frozen, so to speak) we would expect it to maintain the same orientation with respect to the galactic center as it does currently, e.g. with the solar ecliptic at ~60˚ to the galactic plane. In reality, the motion and orientation of the solar system is dominated by the gravitational and magnetic fields of the nearby structures (the Local Interstellar Cloud through which the solar system is currently moving and the G-Cloud Complex which it is moving toward) far more than the mass at the galactic center.

As already noted, the claim that the solar system was from the Sagittarius Dwarf galaxy is untrue, and in fact, a large portion of planetary systems in the observable portion of the galaxy are angled at 30˚ or greater to the galactic plane, which is likely due to the complex turbulence in which planetary systems form. I don’t think there is any consensus about the long term orientation of the solar system except that any precession occurs over spans much longer than hundreds of thousands of years.


I’ve not heard of any value for the precession of the ecliptic, but it seems likely that it is a very slow process compared to the precession of Earth’s poles, which is already a fantastically long cycle.

Is it more or less than half of them? That’s how many one would expect if the orientations were uncorrelated.