Keeping Time with the Skies

The bright portion of the Moon decreases from a full circle to a half circle in about a week.

The bright portion continues to shrink for another week until it is no longer visible. This two-week period is called the waning period of the Moon. The day when the Moon appears as a full bright circle is called the full Moon day (or Purnima), and the day when it is not visible is called the new Moon day (or Amavasya).

After the new Moon, its bright side grows to a half circle in about a week and to a full circle (full Moon) in another week. The period when the bright part of the Moon increases is called the waxing period.

In India, the waning period of the Moon is generally called the Krishna Paksha, while the waxing period is called the Shukla Paksha. The Moon goes through a waning period followed by a waxing period in a cyclical manner. The cycle from one full Moon to the next takes about a month.

Locating the Moon

On a full Moon day, the Moon is nearly opposite the Sun - when the Sun rises in the East, the Moon is almost setting in the West. On subsequent mornings at sunrise, as its bright part continues to decrease, the Moon appears to move closer in the sky to the Sun. When the bright part of the Moon decreases to a half circle shape, the Moon is overhead at Sunrise. A few days later, the crescent Moon appears even closer to the Sun.

Knowing the phase of the Moon and whether it is waxing or waning can thus help us find out where and when to look for the Moon on any given day. A waxing Moon is easiest to spot at sunset, and a waning Moon at sunrise. Because of these shifts, the Moon always rises and sets at different times than the Sun.

Calendars

When viewed from the Earth, the Sun appears to rise in the eastward direction, set in the westward direction every day, and rise again the next day. This apparent periodic motion of the Sun seen by us is primarily due to the rotation of the Earth around its own axis. This natural cycle of the Sun due to the rotation of the Earth, is the foundation of the day, a unit to measure time.

The average time that the Sun takes to go from its highest position in the sky on one day to the highest position in the sky the next day, is 24 hours, and is called the mean solar day. The highest position of the Sun in the sky can be found by measuring the length of the shadows cast by an object during the day. The shadow is shortest when the Sun is at the highest point in the sky.

The phases of the Moon give us another natural cycle with a duration that is longer than a day. The Moon takes about 29.5 days (nearly a month) to cycle through all its phases. The cycle of the phases of the Moon is the basis for a month, another unit to measure time.

The next larger unit to measure time is related to the natural cycle of seasons. The Earth revolves around the Sun and takes nearly 365 and a quarter days to complete one revolution around the Sun. The Earth undergoes one cycle of seasons during this time, which can be used to define a solar year.

Lunar Calendars

In ancient times, people had noticed that during one cycle of seasons, one can fit nearly 12 cycles of the phases of the moon, that is, 12 lunar months. This is how lunar calendars came into being, with the day as the shortest unit, a month of nearly 29.5 days, and a lunar year consisting of 12 lunar months. The phases of the Moon gave an easy and a perfectly sound way to track the passage of time.

However, in a lunar calendar the seasons do not remain synchronised to the same lunar months in successive lunar years. The reason is that the seasons repeat in approximately 365 days while the lunar year is 354 days long.

Solar Calendars

It was important to know the arrival of seasons for agricultural purposes. This need for a year to synchronise with seasons led to the creation of solar calendars. The Gregorian calendar, widely used today, is a solar calendar. The months in solar calendars are adjusted to add up to 365 days. That is why in Gregorian calendars, some months have 30 days, others 31, and February has only 28 days.

On top of the 365 days, the Earth takes nearly an extra quarter of a day to complete one revolution around the Sun. These extra hours add up to approximately one day every four years. To adjust for this, solar calendars add an extra day every four years using the concept of a leap year. In the Gregorian calendar, if a year is divisible by four, then an extra leap day is added. So in a leap year, February has 29 days, which keeps the calendar well synchronised with the seasons.

Luni-solar Calendars

There is another kind of calendar which primarily uses the Moon’s phases for counting days and months but also make adjustments to stay in sync with the cycle of seasons.

The 12 lunar months add up to 354 days and thus fall short by nearly 11 days compared to the solar year. Thus every 2-3 years, the accumulated difference becomes close to a full month. Therefore, every few years, an extra month (called Adhika Maasa or intercalary month) is added to the year in some calendars. This keeps the solar year and the lunar cycle in step. Such calendars are called luni-solar calendars. They combine elements from both the solar and the lunar calendars and are used in many parts of India.

Festivals

Many Indian festivals are tied to the phases of the Moon and hence are based on either lunar or luni-solar calendars.

For example, Diwali falls on the new Moon of the month of Kartika, Holi on the full Moon of Phalguna, Buddha Purnima on the full Moon of Vaisakha, Eid-ul-Fitr is celebrated after sighting the crescent Moon at the end of the month of Ramazan, while Dussehra is celebrated on the tenth day in the month of Ashwina. Hence, they occur on different dates in the Gregorian calendar in successive years.

For festivals based on luni-solar calendars, the Gregorian calendar dates can shift, but this shift is typically less than a month. This is because the luni-solar calendars add the intercalary month every few years which correct for the difference between the lunar and the solar year. In contrast, purely lunar calendars do not account for this difference. Any festival celebrated according to the phases of the Moon, such as Eid-ul-fitr, therefore can occur in different months of the Gregorian calendar year after year.