It's not the end of the world...

February 10, 2009 7:29 AM

Reading this article (and a USA Today story linked in it) this morning, and I'm curious about an astronomical event mentioned in it.

So, all of the silliness surrounding the end of the world in 2012 seems to be predicated on the fact that the end of this particular "long cycle" in the Mayan calendar occurs on a winter solstice, and the fact that our sun will align with the center of the Milky Way galaxy on the same day. Apparently, this type of alignment occurs only once every 26,000 years.

What does that mean? I'm not a mathematician or an astronomer, but my understanding is that in Euclidian geometry, two points can be connected to form a line at all times. So, technically, the sun and the center of the galaxy are always "aligned" from this standpoint. And, if we're dealing with this as the sun and the center of the galaxy aligning from the perspective of Earth, it seems like it should happen more often. Since the Earth revolves around the Sun, and the Sun revolves around the center of the galaxy, there have to be plenty of times (once a year, at least) where these three points (Earth, Sun, and Center of Galaxy) line up.

What am I missing? I'm not worried about a cosmic catastrophe, but this seems to me like it should be a fairly common occurrence, and not justification for losing your mind about the apocalypse.

posted by jasondbarr to science & nature (12 comments total)

My lay person's understanding is that in addition to orbiting the galactic center, our solar system rises above and dips below the galactic plane (like a sine wave). Supposably we pass through the galactic plane in 2012. IANYAstronomer.
posted by grateful at 7:32 AM on February 10

In two dimensional space you'd be right, but in three dimensional space... not so much. This is why there one or two eclipses a year instead of one every month: the moon does make one complete orbit of the earth once per month, but given the movement of all three bodies in space--sun, moon, earth--the three form a straight line rather infrequently. Take a look at this diagram. There are only two possible times in each orbit of the moon for there to be an eclipse, but given that the sun's progression across the sky takes a year, there will only be eclipses on rare occasions.

Think of it as two waves of different frequency. Every so often there will come a time when the two are in phase for a beat or two, but this only lasts for a moment and then they're out of phase again. Depending on the frequencies, this can happen every few seconds or, as is the case with the position of the earth, sun, and center of the galaxy, every 26,000 years.
posted by valkyryn at 7:39 AM on February 10

the end of this particular "long cycle" in the Mayan calendar occurs on a winter solstice, and the fact that our sun will align with the center of the Milky Way galaxy on the same day. Apparently, this type of alignment occurs only once every 26,000 years.

You're right that the Sun crosses the Milky Way band twice a year. The 26,000 years thing is just when this crossing also occurs on a winter solstice.

More info, plus a debunking here.
posted by vacapinta at 7:44 AM on February 10

To be clear, there is nothing magical about 26,000 years except that thats how long it takes for the precession of the equinoxes.
posted by vacapinta at 7:51 AM on February 10

The putative cause of catastrophe here on Earth is that an alignment of the Earth, Sun, and the center of the galaxy will cause some sort of gravitational convergence that rips us apart. I heard the same thing in the late 80s when there was an alignment of five or six planets.

The fundamental misunderstanding here is how gravity works over distance, which is to say a misunderstanding of the inverse square law that says, in this case, that the force of gravity diminishes with the square of the distance. If a mass exerts 1 unit of gravitational force on you at 1 unit of distance, then at 2 units of distance its force is only 1/4 as strong; at 3 units of distance it's 1/9th, at 10 units of distance, 1/100th, etc.

I heard a physicist one time express this in a really good way: someone standing three feet away from you exerts a stronger gravitational pull on you than the Sun does. The alignment of planets or galactic cores happens at such great distances that there's no perceptible effect at all.
posted by fatbird at 9:17 AM on February 10

It's bullshit science reporting. The center of the galaxy we call "The Milky Way" happens to be aligned roughly with the plane of our solar system so that it lies beyond the constellation of stars we call Sagittarius (the milky way in the night sky is the plane of the galaxy). The hub of the galaxy is obscured by interstellar dust, so nobody from a vantage point on Earth could say it has an exact center (unless the Mayans had x-ray telescopes that revealed a super massive black hole). Also, the galactic pane is a cloud of stars and gas. It's not something the Earth has dipped into or risen out of in any sort of crisply definable way within the span of humanity's consciousness.

You can see Sagittarius (and the center of the galaxy) on a summer night because the Sun is on the other side of the sky, behind the Earth. In winter, the Earth has moved around to the other side of the solar system and the Sun is in front of Sagittarius and thus, "aligned with" the center of the galaxy. It happens every year. Mayan calendars had more interest in how cycles of the bright planets and moon coincided with each other.
posted by bonobothegreat at 9:41 AM on February 10 [1 favorite]

"The Milky Way" happens to be aligned roughly with the plane of our solar system

I mean't to say that the Milky Way intersects with the plane of our solar system roughly in the area of Sagittarius...
posted by bonobothegreat at 9:45 AM on February 10

..crap, I said it basically right in the first place. The Earth orbits in a plane and the galaxy rotates on another plane. The orbit of the Earth carries it around the Sun and the Sun appears to pass in front of a band of stats that compose certain constellations. We call this band of constellations the Zodiac. The Milky Way (the place of the Galaxy, a far more distant ribbon of glowing gas, dust and stars) crosses over the plane of our solar system and by coincidence, the galactic center lies within the Zodiac in the direction beyond Sagittarius. The sun passes in front of the galactic center when it passes in front of the the constellation Sagittarius. In the course of human history, the positions of the planets and our view of the galaxy can be considered as "fixed" and unmoving. The only things that change position are the Sun, Moon and planets.
posted by bonobothegreat at 10:09 AM on February 10

Oh God, I really have to start going over my previews more thoroughly.

...the positions of the stars and our view of the galaxy can be considered as "fixed" and unmoving. The only things that change position are the Sun, Moon and planets.

posted by bonobothegreat at 10:12 AM on February 10

You might find this answer to a similar question on Google Answers helpful.
posted by blue_beetle at 11:59 AM on February 10

Depending on the frequencies, this can happen every few seconds or, as is the case with the position of the earth, sun, and center of the galaxy, every 26,000 years.

Sorry, but this is completely wrong. The OP can mark as best answer whatever they wish, but having erroneous information out there compels me to respond.

The Sun takes 250 Million years to orbit the Galactic center. In 25,000 years the Sun has hardly moved at all in this orbit. So, there are only two objects involved here: Sun. Earth. So, the lunar orbit analogy, which involves 3 objects, is completely off.

The apparent position of the Sun changes because of a third variable, the Earth's precession - its rotation axis is actually slowly changing.
posted by vacapinta at 2:17 PM on February 10

vacapinta is correct.
posted by dsword at 4:43 AM on February 11