Thursday, February 26, 2009

What's special about Dec 21st 2012?

So what’s so special about December 21st 2012?

Because of the gradual change of Earth’s rotational axis, on that date the Sun rise will be nearly aligned to our galaxy’s center. But why does the Earth axis gradually change to cause this alignment?

When objects spin, their rotational axes can be quite stable. A child’s top that spins on its point doesn’t fall over for a long time, even though the same top would immediately fall if it weren’t spinning. This is because spinning bodies have angular momentum which is conserved–the body will spin forever unless a torque is exerted on it by another body. This is what makes bicycles stable. We all know it is almost impossible to balance on a stationary bicycle. But, it’s easy to keep your balance on a moving bike because the angular momentum of the rotating tires and wheels provides stability.

The Earth spins on its axis once each day, making its axis stable (almost). Earth’s axis is titled by 23.5 degrees from a line perpendicular to the plane of Earth’s orbit. As the Earth orbits the Sun, its axis continues to point in the same direction (almost). That’s why we have seasons—why winter in the north is colder than summer. Because Earth isn’t a perfect sphere (it bulges at the equator due to its spin), the Sun and Moon exert a torque on Earth by pulling on that bulge. This torque causes Earth’s axis to precess, to slowly turn around a circle. A laser pointing straight up at the North Pole would sweep a circle across the heavens with one full turn every 26,000 years. It takes 71 years for the axis to turn by just one degree. That’s not much, but enough that ancient astronomers, including the Greeks and Mayans, detected this precession and measured its rate. As the axis precesses, the date of winter solstice slowly changes, as does the solstice Sun’s apparent position in the sky.

The Mayans believed that on (December 21st 2012 to you) at winter solstice the Sun would align with the intersection of the central plane of the galaxy, the plane of Earth’s orbit, and the galactic center. With all that going for it, they expected all hell to break loose—this would be the End Date. Considering they didn’t have telescopes, the Mayans were great astronomers. But they were off a little. We were closer to this magic alignment in 1997 (off by only 1/60th of a degree) than we will be in 2012 (off by 1/5th degree), and we will be moving away from that perfect alignment for the next 13,000 years.

Since we survived the near perfect alignment in 1997, we should be OK (at least as far as this Mayan prophesy) for nearly 26,000 years.

But, can the alignment of the stars really effect us? More later.

Thursday, February 19, 2009

Is the world coming to an end?

At a recent talk, I was asked if the world would come to an end on December 21st 2012, as the ancient Mayan calendar predicted. I did some digging and found some things I hope you’ll find interesting.

In Mayan:
A day is 1 kin;
20 kin (20 days) are 1 uinal;
18 uinal (360 days) are 1 tun;
20 tun (19.7 years) are 1 kactun;
20 kactun (394.3 years) are 1 backtun; and
13 bactun (5125.3 years) are 1 epoch.

Their calendar apparently started on August 11th, 3114 B.C. (Some say it started two days later – why quibble?) and it stopped at the end of one epoch (13 bactuns).

Just for fun, I wrote a program to compute Mayan dates from our dates (Gregorian calendar dates). Today, February 19th 2009, is Mayan standard time (12 bactun, 19 kactun, 15 tun, 1 uinal and 14 kin). Incidentally, counting all the leap years, that’s 733,456 days after January 1st of our year 0.

If anyone wants to know the Mayan for their favorite date, just let me know.

The Mayan End Date is, which corresponds to December 21st 2012 on our calendar. This is the day of the winter solstice – the shortest day of the year in the northern hemisphere. It turns out an unusual astronomical event will occur on that date. More on that later.