Radio serial on astronomy
Episode 7: Story of the calendar
[Note for script writer: This brief has been prepared keeping the average non-specialist viewer in
mind. The attempt has been to develop the subject in a logical sequence that could bring out the
excitement of the process of making new discoveries and relating them to the gradual understanding
of the nature of the Universe. It is not designed to be a popular account of the development of
astronomy as a scientific discipline and its impact on our understanding of the cosmos. Since the
treatment is not rigorous, many details are not included. The script has to be developed along the
outline suggested using simple colloquial language as far as possible. The dialogues and
conversations need to be lively without any suggestion of pedagogy.]
Points to be covered:
- Calendars are a method of keeping time
- Calendars are based on solar motion, lunar motion, or a combination of both (luni-solar)
- The evolution of the Gregorian calendar based on solar motion relative to Earth
- Calendars used in other parts of the country
Outline of content:
We all use calendars in our day-to-day life to plan our work schedule, or plan a vacation, or look up
festival dates. But what is a calendar and how is it made? The first calendars were almost all based on
the cycles of the Moon. Often calendars were created locally, city by city, based on eye witness
observation of the phases of the Moon. Each complete cycle became a month. Months had varying
numbers of days because the phases of the Moon do not line up exactly with the day/night cycle of the
Earth. Much effort was placed in putting the proper number of days in a month as religious
celebrations needed to be held at specific times.
We can say, all calendars began with people recording time by using natural cycles such as the
rotation of the Earth around its axis, which we call a 'day'; the changing phases of the Moon from
New Moon to New Moon, which we call a lunar 'month'; and the revolution of the Earth around the
Sun, which we call a 'year.' We take a day to be 24 hours long, but actually the Earth spins once
around its axis in 23 hours 56 minutes and 4 seconds, which is the actual length of a day. Similarly the
length of a lunar month; that is, the time from New Moon to New Moon, is 29.53 days. The year is
365 days 5 hours 48 minutes 46 seconds long.
Various peoples in different civilisations have attempted to organise these cycles into calendars to
keep track of time and to be able to predict future events of importance, such as a festival or season.
But it was not an easy task. The main problem is that these natural cycles do not divide evenly. So a
month measured by the Moon does not equal an even number of days, and a solar year is not equal to
a certain number of Moon cycles or lunar months.
One way out was to go by only the lunar months, as is the case with the Islamic Hejira calendar. But
the problem with such a lunar calendar is that the months do not keep pace with the seasons and as a
result dates of festivals, etc., keep changing. For example, the festival of Id ul Fitr may fall during
winter in one year but in summer in another unlike the festival of Dussera, which always falls in
autumn.
1
Radio serial on astronomy
To avoid this problem it was decided to have lunar months with alternating combinations of 29 and 30
days, with special rules to adjust occasionally with a solar year by adding leap months – adhika
maasa (intercalary month) – such as the Hindu, Chinese and Jewish calendar. Such a calendar is
known as a luni-solar calendar. If the lunar cycles are totally abandoned and only the solar year is
taken into account – such as the Ancient Egyptian calendar of 12 same-sized months – it is called a
solar calendar.
Over the centuries as civilisation grew to a global society, the need to regulate and synchronise civil
functions became increasingly crucial. Our current western calendar began as the Julian calendar of
ancient Rome, which was introduced by Julius Caesar in 45 BC. The names of months are roughly
consistent with the Julian months in many western languages. The weeks and names of days are also
similarly related. But the Julian calendar did not quite fit the solar year either, because the Earth does
not orbit the Sun in a whole number of days. The calendar was reformed several times more, most
notably by Pope Gregory XIII, adding an extra day in leap years to make up the extra time in the new
Gregorian calendar.
The Hindu calendar used in ancient times has undergone many changes in the process of
regionalisation, and today there are several regional Indian calendars, as well as an Indian national
calendar. Most of these calendars are inherited from a system first enunciated in Vedanga Jyotisha of
Lagadha, a later adjunct to the Vedas, standardised in the Surya Siddhanta and subsequently reformed
by astronomers such as Aryabhata, Varahamihira, and Bhaskara.
There are essentially two types of traditional Indian calendars. One is the solar calendar based on the
transit of the Sun and the other is the lunar calendar based on the transit of the Moon. The solar
calendar is based on the Sun’s entry into various signs. For example the Indian solar calendar starts on
April 14-15 each year. This signifies the Sun’s ‘entry’ into Mesha rasi (Aries) and is celebrated as the
New Year in Assam, Bengal, Orissa, Manipur, Nepal, Kerala, Punjab, Tamil Nadu and Tripura. The
first month of the year is called Baisakh in Bengali/Punjabi, ‘Chitterai’ in Tamil, and ‘Medam’ in
Malayalam.
The lunar calendar is the most widely followed calendar in India. It is based on the Moon’s transit.
There are two types of lunar calendars. One starts with the month of Kartika and counted from
poornima or the Full Moon. The other starts with the month Chaitra and counted from amavasya or
the New Moon. The names of the 12 months are Chaitra, Visakha, Jyestha, Ashadha, Shravana,
Bhadrapada, Ashwija, Kartika, Margasira, Pushya, Magha and Phalguna, which are name after the
respective nakshatras near which the Full Moon is seen.
The lunar months are counted by tithis or the phases of the Moon. Every 12 degree difference
between the Sun and the Moon forms a tithi. For example, when the Moon is 12 degrees away from
Sun after new Moon the tithi is pratipada, or the first lunar day. If the Moon is 24 degrees away the
tithi is dwitia, or the second lunar day, and so on. The lunar tithi or day is taken from sunrise, i.e., the
tithi prevailing at the time of sunrise is the day’s tithi. The period from New Moon to New Moon
completes a lunar month.
As the motion of the Moon is not uniform, a tithi may be longer than a day or shorter than a day. A
lunar fortnight, or paksha, can vary from 13 to 15 days because of this. As the Moon takes about
29.53 days to complete one round of the Earth, the 12 lunar months amount to 354 days
2
Radio serial on astronomy
approximately. As it is less by 11 days as compared to the solar year, every year 11 days are added to
one of the months, which adds up to one lunar month in three years. This is called adhika masa.
The official Indian calendar, called the Indian National Calendar, was introduced by the Government
of India from 1957 as per the Calendar Reform Committee in which the year starts with the month of
Chaitra, 1st of Chaitra corresponding to 22nd March. It follows the Saka era.
3