By John Major
The Japanese scatter dried beans to drive evil from the house. The Chinese eat long-life foods. The Balinese observe a day of silence. Thais splash water on one another. People in many parts of Asia, as well as elsewhere in the world, love to celebrate a holiday marking the start of a new year – a festival of putting aside the problems and disappointments of the past, of finding new hope, of beginning anew. But when should the new year be celebrated? There is no obvious answer to that question. Days, months and years all flow continuously; there is no scientific way of identifying a particular day that marks a “new year.”
The day, the months, the seasons, and the year – the major divisions of the calendar – are determined by the physical properties of the solar system, especially the earth, moon, and sun. But not only is there no “start” to these markers of time, they also do not fit together very well.
A day is one revolution of the Earth on its own axis: twenty-four hours.
- A lunar month is one orbit of the Moon around the Earth, about 29 1/2 days, usually measured from new moon (unilluminated moon) to new moon, or from full moon (fully illuminated moon) to full moon. A lunar year (twelve lunar months) therefore is about 354 days long;
- A solar year is one orbit of the Earth around the sun, about 365 1/4 days;
- There are four seasons, each a bit more than 91 days long. The axis of the Earth’s rotation is tilted (like a spinning top leaning to one side), so that on any given day the hemisphere (northern or southern) tilted toward the sun receives more light, the hemisphere tilted away receives less. The summer solstice marks the longest day of the year, the winter solstice marks the longest night; the spring and autumn equinoxes occur when days and nights are of equal length worldwide. In some cultures (e.g. the U.S.), the solstices and equinoxes are considered to start the seasons; in other cultures (e.g. China) they are regarded as the mid-points of the seasons. Seasons tend to be culturally important in the higher latitudes, where the changing length of days and nights is dramatically apparent, but not near the equator, where days and nights are always equal.
Therefore there are three basic types of calendar:
- The solar calendar, in which solstices and equinoxes fall on about the same date every year and a leap-day is added every four years. Civil months are generally 30 or 31 days long and are unrelated to lunar months. The modern Western (Common Era) calendar is a solar calendar.
- The lunar calendar, a repeating sequence of twelve lunar months. It tracks the phases of the moon but shifts (by about eleven days a year) annually relative to the solar year; months do not correlate with the four seasons. The Islamic calendar is a lunar calendar.
- The soli-lunar calendar, in which extra lunar months (“leap months”) are added from time to time (on average, about 7 extra months per 19 years) to keep the solar and lunar years approximately synchronized. The Chinese and Jewish calendars are soli-lunar calendars.