Why doesn't Chinese New Year fall on New Year's Day?

In 2014, the Chinese New Year falls on Jan. 31, a full month after the western calendar celebrated New Year's Day on Jan. 1.

Not only do the New Year celebrations not match up this year, they never will: Chinese custom dictates that the winter solstice falls in the 11th month, so the lunar new year begins on the second new moon after that, falling between Jan. 21 and Feb. 20.

But the calendar mismatch goes deeper than that. It's all about astronomy.

The Chinese calendar, like the Hebrew and Muslim calendars, is a lunar calendar. Lunar calendars, as the name implies, are based on the orbits of the moon around the Earth. It actually only takes the moon 27.32 days to lap the Earth, but lunar months, measured from new moon to new moon, last 29.5 days. The extra two days creep in because the Earth isn't holding still in space, but is continuing to move around the sun, and the phase of the moon depends on the relative positions of the Earth, moon, and sun.

The calendar hanging on your wall, usually called a Gregorian or Western calendar, is a solar calendar, based on the 365.24 days it takes the Earth to orbit the sun. The fraction of a day beyond 365 days is the reason for leap years. 

Even with leap days accounted for, there's no way to fit lunar months (29.5 days) evenly into solar years (365 days), so the lunar and solar calendars have never fit together well. You can shave off that extra half-day by alternating months at 29 and 30 days, but after 12 moon-months you'll still have 10 or 11 days left over each solar year.

A true lunar calendar, such as the Muslim Hijri calendar, will slip backward those 10 to 11 days every solar year, so a lunar calendar will cycle back to the same position in a solar year every 33 years. That's why the month of Ramadan, when observant Muslims fast between dawn and dusk, will straddle the longest day of the year in 2015 and 2016, while back in 1999 and 2000, Ramadan fell during the shortest days of the year – as it will again in 2032 and 2033. 

To avoid the 10-day time-slip, most calendar systems shoehorn in an extra month every two or three years. These lunisolar calendars, including the Hebrew and Chinese systems, harmonize the lunar and solar years well enough that the Hebrew New Year (Rosh Hashanah) always falls in September or October, and the Chinese New Year is always celebrated in January or February. 

In 2014, the second new moon after the winter solstice falls on Jan. 31. Happy New Year!