When I wrote recently about the pole stars of other planets, I was aware of one thing my sky maps didn’t show—the rotation poles of our galaxy. They weren’t really relevant to that discussion, but I’m now prompted to write a bit about the Milky Way galaxy, and our relationship to it, because I’ve just encountered a rather garbled, misleading and self-contradictory Wikipedia article on the topic—specifically the section entitled Sun’s location and neighborhood. (The last time I referred to a misleading Wikipedia page on this blog, the page was eventually fixed by an editor citing my article. This was gratifying, but it meant my link to the page no longer demonstrated the problem to which I’d objected. So this time I’ve linked to a Wayback Machine copy of the page dated 9 April 2023, which will at least give permanent context to my griping.)
The view of our galaxy at the head of this post is an artist’s impression, sketching in the major features as currently understood. Our sun, marked by the arrow, lies between two spiral arms. The Perseus Arm, which lies farther out than our location, is considered to be one of the Milky Way’s two major spiral arms; and inwards from our position is the more minor Sagittarius Arm. Spanning the gap between these two arms is a more diagonal structure, clearly visible in the painting, variously referred to as the Orion Spur, the Orion Arm, or the Local Arm, the status of which seems to be much debated. Our sun lies within the inner edge of that diagonal structure.*
Our distance from the centre of the galaxy isn’t known with great accuracy. From a couple of recent papers addressing the issue, the distance is somewhere in the region of 7.9 kiloparsecs (25,800 light-years) to 8.34 kiloparsecs (27,200 light-years), which you’ll see from the map above is very roughly halfway from the galactic centre to its rim. The two papers in my link are in much stronger agreement about the angular velocity of the sun in its orbit around the galaxy, which they place between 30.2 and 30.6 km/s/kpc. Call it 30.4, which translates to 9.85×10-16rad/s, or one orbit every 200 million years. But the sun is near perigalacticon at present (its closest approach to the galactic centre) and therefore moving faster than it normally does. Commonly quoted estimates for the duration of a true galactic year therefore lie between 225 and 250 million years.
So much for our radial location within the galaxy. How are we placed relative to the galactic plane? Estimates of our distance from the galactic mid-plane (which we can think of as the galactic equator) vary, but a recent measurement and review places it about 17 parsecs (55 light-years) “above” the plane. I’ll come back to what “above” means in this context later.
The orientation of the solar system relative to the plane of the galaxy turns out to be a little unexpected. In the view below, we’ve moved a short distance from the sun towards the galactic rim, and are looking back on the solar system framed against the nebulae of the galactic core, with the galactic equator running horizontally across the picture.
I’ve marked the constellations in the vicinity of the galactic core for orientation. The four visible orbits are those of the outer giant planets from Jupiter to Neptune.
So the plane of the solar system is tilted at an angle of about 60° relative to the plane of the galaxy. It also, as you see, comes close to aligning with the centre of the galaxy—the mismatch is only about 6°. Since the plane of the solar system remains fixed as it orbits the galaxy, a perfect alignment will occur twice in every galactic year—the most recent happened three or four million years ago.
Now I’m going to zoom in to look at the Earth’s orbit. The plane of the Earth’s orbit is called the ecliptic, and it’s pretty closely aligned with the overall plane of the solar system—close enough that I plan on using it as a reasonable proxy for the solar system’s invariable plane, later.
I’ve included a cartoonishly large Earth to give you a visual cue to the orientation of the ecliptic along the axis perpendicular to your screen—the left side of the Earth’s orbit, as seen here, is farther away from our viewpoint than is the sun, while the right side brings the Earth closer to us. I’ve also marked four key locations in the Earth’s orbit—the solstices and equinoxes. And here’s a coincidence! The solstice points align pretty well with the galactic core. In 2023, for example, the sun will cross the galactic equator at about 11:00 on 22 December, the day of the solstice.†
Now, we know that the Earth’s axis is tilted by about 23½° relative to the ecliptic. So how does the Earth’s tilt interact with the solar system’s tilt? There’s a hint in my previous picture, from the location of the solstices and equinoxes, but we can zoom in farther from the same vantage point to make the situation clearer.
The Earth is rotated in the plane of the image through the 60-degree tilt of the rest of the solar system, but the 23½°-tilt between its equator and its orbit is directed towards us, out of the computer screen. In other words, the Earth’s tilt relative to the ecliptic is almost at right angles to the ecliptic’s tilt relative to the galaxy. This is difficult to depict in a single diagram, like the one offered by Astronomy magazine recently (third image down on the linked page), in which it looks very much as if the Earth’s tilt is in the same plane as the ecliptic tilt. And it’s something that can catch out even the astronomers who wrote the entries for Swinburne University’s online Encyclopedia of Astronomy. For a few years their entry on the Galactic Plane claimed that:
The galactic plane is tilted at an angle of 63 degrees to the celestial equator. Since the ecliptic (the path of the Sun on the sky) is inclined at an angle of 23.5 degrees to the celestial equator, the galactic plane and the ecliptic are nearly at right angles (63 + 23.5 = 86.5 degrees), although this is purely coincidental.
Purely fantastical, more like. The entry was mercifully corrected in 2012; my link takes you to the original version embarrassingly preserved on the Wayback Machine.
But they were correct about the 63° tilt between the celestial equator (the extension of the Earth’s equator into the sky) and the galactic plane. The easiest way to see how all this fits together is not to try to diagram the intersecting planes of the Earth’s equator, the ecliptic and the galactic equator, but instead to plot the position of their north poles in the sky. The Earth has its celestial north pole, near the pole star, Polaris. The ecliptic has its own north pole, corresponding to the rotation axis implied by the movement of the planets in their orbits. And galactic north was defined by the International Astronomical Union, back in 1958, as being the extension of the rotation axis of the galaxy into the northern sky of the Earth. (Which, at last, allows me to say what is meant by the sun sitting about 55 light-years “above” the galactic plane—it lies north of the galactic plane.)
The north galactic pole lies in the obscure little constellation Coma Berenices. And you can immediately see how the angle between the ecliptic and celestial north poles is measured almost at right angles to the angle between the galactic and ecliptic north poles, so that the celestial and ecliptic poles end up at much the same angular distance from the galactic pole.
But there’s one little quirk to this trio of north poles, arising from the way the IAU defines north, which I addressed in more detail when I wrote about pole stars previously. If we looked down on Earth from the celestial north pole, we’d see it rotating anticlockwise; likewise, if we looked at the solar system from the ecliptic north pole, we’d see the planets moving anticlockwise around the sun. But if we were to look at the galaxy from the galactic north pole (the view in the picture at the head of this post), we’d observe it rotating clockwise. Like the planet Venus, then, the Milky Way galaxy turns out to be a retrograde rotator.‡
I can now go back to my original solar system diagram, and add some orientation arrows.
The solar system, which participates in the galactic rotation, is therefore moving in a direction approximately aligned with my horizontal arrow. The extent to which it deviates from that alignment will be my topic next time I write on this topic—at which point I’ll also be able to explain my gripes about the Wikipedia article that started all this.
* For more detail on the layout of the spiral arms, and detailed maps of the solar neighbourhood, visit Kevin Jardine’s jaw-dropping Galaxy Map site, which is a thing of beauty and a labour of love.
† This approximate alignment between the galactic core, the sun and the Earth on the day of the December solstice was of course one of the foundations of a fatuous end-of-the-world scenario predicted for 2012. (To be honest, though, I did quite enjoy Roland Emmerich’s accompanying disaster movie, 2012).
‡ There’s a passage in Larry Niven’s classic science fiction novel, Ringworld (1970), in which he describes how the Puppeteer Fleet of Worlds abandons the Milky Way galaxy and sets out for the Small Magellanic Cloud. Elsewhere in the novel, Niven tells us that the Fleet of Worlds is “moving north along the galactic axis”. Unfortunately, the SMC actually lies south of the galactic equator. Oops. But this would make sense if Niven was using rotational north (that is, the direction from which a body appears to rotate anticlockwise), rather than the IAU’s parochial system based on the orientation of the Earth. (I once actually raised this possibility with Niven, but he responded that the Puppeteers were “taking the scenic route”.)