DR. T.V. PADMA
How different is the lunar calendar?
The Gregorian and Hindu calendar years are roughly the time the earth takes to go once around the sun. This is useful for farmers, because it helps keep track of seasons. Why doesn't Deepavalli always arrive on the same Gregorian date? According to the Gregorian calendar, it falls every year in the autumn months of either October or November. But the specific date depends on when the new moon occurs between those two months.
Hindu calendars are pretty complicated. They are lunisolar, which means that they care about the moon's orbit around the earth and the earth's orbit around the moon (the apparent movement of the sun). Each month in the Hindu calendar begins with the first day or a lunar cycle. But, the lunar month doesn't divide nicely into the solar year, and if we base a calendar on just lunar months, the months won't recur in the same season - they'll drift, as we've seen with the Islamic calendar. To fix that, we can make some adjustments. For instance, we can add a month every few years - which is done in some Hindu calendars.There's another problem, a solar year is not an exact number of days. That means a whole lot of calculations and corrections and small changes in the Gregorian calendar date of Hindu festivals, from one year to the next.None of these calendars is perfect, because none of them accounts precisely for the movements of the earth and the moon and the sun. That leads to astronomical complications - quite literally! But though each calendar has its advantages and disadvantages, all of them help us tell when it's time to wish someone "Happy Holidays"!