11.4 Calendars
A calendar is just a way of packaging those sky cycles — days, months and years — into a usable count. Different choices of which cycle to follow gave rise to different calendars.
Lunar calendars
Following the Moon Ancient people noticed that about 12 cycles of the Moon’s phases fit into one cycle of seasons. So a lunar calendar uses the day as its smallest unit, a month of ~29.5 days, and a lunar year of 12 lunar months ≈ 354 days. Simple and reliable for tracking time — but there’s a catch.
The Drift Problem A lunar year (354 days) is about 11 days shorter than the season year (365 days). So the seasons slowly drift through the lunar months — a festival fixed to a lunar month gradually slides earlier and earlier relative to the seasons.
Solar calendars and the leap year
Following the Seasons Farmers needed the calendar to match the seasons, so solar calendars were created, with months adjusted to total 365 days. The Gregorian calendar we use today is solar — which is why months have 30 or 31 days, and February only 28.
Why We Need Leap Years The Earth actually takes about 365¼ days per orbit. That extra quarter-day adds up to roughly one whole day every four years. To fix it, solar calendars add a leap day every fourth year (February gets a 29th day). In the Gregorian calendar, a year divisible by 4 is a leap year — except century years (1700, 1800, 1900) which are skipped, to keep the calendar perfectly in step with the seasons.
Check any year below:
Our Scientific Heritage: Uttarayan & Dakshinayan For thousands of years, people in India tracked the year by watching the Sun’s rising point shift along the horizon. The Sun rises a little north of East in summer and south of East in winter; the turning points are the solstices (around June 21 and December 21). The Sun’s northward journey (Dec→Jun) is Uttarayan, its southward journey (Jun→Dec) Dakshinayan — recorded in the Taittirīya Saṁhitā and the Surya Siddhanta. Careful sky-watching let them fix the year’s length at ~365 days, without knowing the Earth orbits the Sun.
Luni-solar calendars
The Best of Both A luni-solar calendar counts days and months by the Moon’s phases but keeps in step with the seasons. Since 12 lunar months fall ~11 days short of the solar year, every 2–3 years an extra month — Adhika Maasa (intercalary month) — is added to catch up. Many Indian calendars work this way, with months named Chaitra, Vaisakha, Jyeshtha… A month may start the day after the new Moon (Amant) or after the full Moon (Purnimant).
The Indian National Calendar India’s National Calendar (a solar calendar of 365 days, based on the Shaka Era) is used alongside the Gregorian for official purposes. Its year begins on 22 March, the day after the spring equinox; months have 30 or 31 days. It was adopted on 21 March 1956 on the advice of the Calendar Reform Committee (1952), chaired by the astrophysicist Meghnad Saha (of the famous Saha equation).