The Story of Calendars

Natural Units of Time

Our timekeeping is strictly based on astronomy:

The Problem of Synchronization

Nature’s cycles don’t fit perfectly together. This led to different types of calendars.

1. Solar Calendars (e.g., Gregorian Calendar)

• Basis: The revolution of Earth around the Sun (Seasons).
• Length: ~365.25 days.
• The Issue: We can’t have a quarter of a day on a calendar.
• The Solution (Leap Year): We count 365 days for 3 years. In the 4th year, we add the remaining quarters ($0.25 \times 4 = 1$ day) to February, making it 29 days.
  • Rule: A year is a leap year if divisible by 4. (Century years like 1900 must be divisible by 400).

2. Lunar Calendars (e.g., Islamic Calendar)

• Basis: The phases of the Moon.
• Length: $12 \text{ months} \times 29.5 \text{ days} = 354 \text{ days}$.
• The Issue: A lunar year is 11 days shorter than a solar year ($365 - 354 = 11$).
• Result: Festivals drift backward through the seasons. If a festival is in summer this year, in a few years, it will be in winter.

3. Luni-Solar Calendars (e.g., Indian National Calendar)

• Basis: Uses Lunar months but adjusts to the Solar year to keep seasons in place.
• The Solution (Adhika Maasa): Every 2-3 years, the accumulated difference (11 days + 11 days + …) forms roughly one extra month. This extra month, called Adhika Maasa (Intercalary Month), is added to the calendar.
• This keeps festivals like Holi or Diwali in their respective seasons, though the exact date shifts slightly.

Comparison Table