Carol Whitlow, of Ithaca, New York, asks :-

I overheard some people talking about the galactic equator and planetary alignment on Dec 21, 2012. What is expected to happen then?

Patrick Bowman, who teaches astronomy at Massey University (Albany), responded.

Regarding the planetary alignment, there will be an approximate visual alignment in the sky of Mercury, Venus, the Sun, and Mars.Unfortunately, with the Sun in the middle, we will see only Mercury and Venus in the morning, and Mars in the evening.

The alignment with the galactic equator will take a little more explanation. At sunset and sunrise it is possible to see which constellation the Sun is "in", i.e., which stars are behind it when viewed from Earth. At the time of writing (23rd of March) the Sun is in Pisces. Over the course of the year, the Sun will pass through each of the twelve constellations of the Zodiac. The Sun also goes through a much longer cycle, caused by a wobble in the Earth's rotation (axial precession). An observer in 2000 years' time will see, on the same day of the year (23rd of March), the Sun in Aquarius: it's moved back one constellation. Over thousands of years, the Sun will move backwards through the Zodiac until, after about 26,000 years, it's back in Pisces.

On the date you asked about, 21/12/2012, the Sun will be in Sagittarius. That constellation intersects the Milky Way. If you draw a line longways through the middle of the Milky Way, that defines the "galactic equator", so the Sun will be on or near the galactic equator (as viewed from Earth) on that date. Note that the galactic equator is not a precisely defined thing, and the rate of motion of the Sun through the Zodiac is very slow, so it's difficult to say exactly when it is on the line: this year, last year, next year?