Galactic Alignment

Will Earth pass through the Milky Way’s galactic plane in 2012? And if so, what could that mean to Earth?

Much ado has been made of the winter solstice sun aligning with the galactic plane on December 21, 2012. But according to the computational wizard Jean Meeus (page 302 of Mathematical Astronomy Morsels), the solstice points were alignment with the galactic equator as recently as the year 1998.

In other words, the 2012 alignment isn’t unique. Consider the view from our local star, the sun. As seen from the sun, Earth crosses the Milky Way’s galactic plane (also called the galactic equator) twice a year, every year.

Or consider the view from Earth. As seen from Earth, the sun crosses the galactic plane – also called the galactic equator on our sky – twice a year, every year. All of this is just part of Earth’s normal motion, as projected on our sky’s dome, as we travel around the sun and through the galaxy. Quite by coincidence, the ecliptic – the projection of the Earth’s orbital plane onto the stellar sphere – intersects the galactic plane near the solstice points. That’s why these points are so near each on our sky’s dome.

However, 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 30 degrees 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 plane in our sky.

Take a look at our sky chart. 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 plane of the Milky Way galaxy above the spout of the Teapot, a bit north of the galactic center. Can you see that intersection? Also look for the various M-objects along the galactic equator, as shown on our chart. Those are star clusters in the star-crowded plane of the galaxy. -EarthSky.org