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| Astronomy Essentials on Jul 19, 2012

Did Earth cross the galactic equator in 2012?

Earth didn’t physically cross the plane of our Milky Way galaxy in 2012. But Earth did cross the galactic equator in 2012. That’s something we do every year – twice.

No, Earth did not pass through the galactic plane in 2012, contrary to what you might have heard. Earth won’t be physically passing through the plane of the Milky Way galaxy for another 30 million years. However, Earth will cross the galactic equator in 2012. As seen from the sun, the Earth does this every year – twice.

Here’s some background. When we speak of the galactic plane and the galactic equator, we’re speaking of two different systems: the real and the imaginary.

The real: our sun and Earth reside in the Milky Way galaxy. If you could see the Milky Way face-on (which we can’t, of course, because we are inside it), it would look round. But if you viewed it edge-on, it would appear flat. The plane of the Milky Way is the flat part containing most of the galaxy’s stars. Our sun lies slightly off-center in the galactic plane. Will we cross the galactic plane in 2012? No. Image via AstroBob, NASA/JPL/Caltech (left) and Ned Wright (right).

The real. When someone says galactic plane they are most often referring to the real Milky Way galaxy – home galaxy to our Earth and sun – spinning in space.

The galactic plane is the actual mid-plane or center line of our galaxy’s huge spinning disk of stars. We are not located on the exact mid-plane of the galaxy. It’s this exact mid-plane people are speaking about when they speak of crossing something.

How far are we from it? You might think that astronomers have this number at their fingertips, but they don’t. We’re at least several dozen light-years from it, maybe more. If you’re interested in how astronomers discuss these things, check out this letter from John Bachall and Safi Bachall printed in the journal Nature in 1985. It suggests our Earth and sun are currently above the plane (to the galactic North) by about 75 to 101 light-years.

The imaginary: our sun and Earth are at the center of a great celestial sphere of stars. Here’s an animated depiction of equatorial, ecliptic, and galactic coordinates on the celestial sphere. Earth would be at the center of all these intersecting planes. The yellow line indicates the galactic equator. When someone speaks of the galactic equator, think of this imaginary system, which depicts the sky as seen from Earth’s surface. Image via Wikimedia Commons

The imaginary. The galactic equator is an imaginary great circle that divides the equally imaginary celestial sphere into two equal halves. The celestial sphere is – of course – a fiction. It’s the same fiction that so confounded the early stargazers, that, as seen from Earth, we appear to reside in the center of a great globe of stars. In modern times, the fiction of the geocentric view of the universe enables astronomers to use a workable coordinate system for mapping the sky. It’s handy, but it’s not reality.

Now let’s define some terms. When someone says galactic equator, they’re usually talking about the coordinate system of astronomers. On this coordinate system, modern astronomers tweak things a bit, to devise a sun-centered way of mapping the Milky Way galaxy.

What’s tricky is that – when you look at the starry sky at night as seen from Earth – the galactic equator closely follows the plane of the Milky Way galaxy. Of course it does, because we’re talking about the real Milky Way in our sky.

As seen from the sun, the Earth crosses the galactic equator twice a year, every year. Nothing special here. Keep moving.

As seen from Earth, the sun crosses the galactic equator twice a year, every year. Keep moving.

As seen from Earth, the moon crosses the galactic equator two (sometimes three) times a month. Are you seeing the pattern here? All of this crossing of the galactic equator is just part of the heavens’ normal motion, really Earth’s normal motion as projected on our sky’s dome as we travel around the sun.

Back to 2012. There’s been much hoopla about the winter solstice sun aligning with galactic plane on December 21, 2012. You, however, now know the reality that, as seen from Earth, the sun crosses the galactic equator twice a year. And the galactic equator on our sky’s imaginary astronomical coordinate system more or less corresponds with the plane of the Milky Way galaxy. So, in this sense, the sun crosses the plane of the Milky Way twice a year (as seen from Earth).

The great circles of the ecliptic and celestial equator intersect at the equinox points. The ecliptic also intersects the great circle of the galactic equator near the solstice points. Note: The galactic equator is not shown on this illustration. Click here to see galactic equator on all-sky constellation map.

Quite by coincidence, the great circle of the ecliptic – the projection of the Earth’s orbital plane onto the celestial sphere – intersects the galactic equator near the solstice points. According to the computational wizard Jean Meeus*, the solstice points were in alignment with the galactic equator as recently as the year 1998 – in other words, they were closer on the sky’s dome then. But in 2011 and 2012, these points – the solstice point, and the point where the sun crosses the galactic equator – are near each other on our sky’s dome.

What is the ecliptic?

It’s true that the sun on the December solstice doesn’t return to the same exact spot in front of the backdrop stars every year. The solstice point slowly but surely moves westward through the stars at about one degree per every 72 years. (For reference, the sun’s diameter equals about 1/2 degree.)

Therefore, the solstice point moves about 30o westward every 2,160 years. By the year 2269, the December solstice point will cross into the constellation Ophiuchus. Then the solstices won’t happen so near the location of the galactic equator in our sky.

Take a look at the sky chart at the very top of this post. If you could see the stars during the daytime, you’d see the sun in front of the constellation Sagittarius on each December 21 solstice. We show Sagittarius as a Teapot on this sky chart, because many people are able to see that pattern. On or near each December solstice, the sun crosses the galactic equator above the spout of the Teapot, a bit north of the galactic center. Can you see that intersection? If you were standing under the real sky on a dark, moonless night, you could see the great boulevard of stars that we call the Milky Way running astride the galactic equator.

Scientific studies indicate that the solar system lies at least several dozen light-years north of the galactic plane, possibly farther. What’s more, we are continuing to travel northward, away from the plane of our Milky Way galaxy, at some 7 kilometers per second. Therefore, we won’t be physically passing through the galactic plane in 2012 or anytime in the near future.

Where is the ecliptic in relation to the Milky Way?

Don’t believe us? Check out this video from NASA.

Bottom line: Earth didn’t physically cross the plane of our Milky Way galaxy in 2012, but Earth did cross the galactic equator. That’s nothing special! As seen from the sun, Earth does this every year – twice.

*page 301-303 of Mathematical Astronomy Morsels

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