Astronomy in India | Class 11 Knowledge Traditions Practices of India Notes

Astronomical Practices in India

The Indian calendar, often called the Hindu calendar, is a lunisolar calendar that integrates both lunar and solar cycles. Unlike purely lunar or solar calendars, the Hindu calendar uses the lunar month to fix dates of festivals and auspicious days, while the solar month regulates daily life activities.

The lunar month is based on the moon's orbit around the Earth. The sidereal period of the moon (relative to stars) is 27.3 days, while the synodic period (relative to the sun, as observed from Earth) is 29.5 days. The lunar month is divided into two halves of approximately 15 days each: the dark half (Kṛṣṇapakṣa), starting with the full moon as day one (ekam), and the bright half (Śuklapakṣa), starting with the new moon as day one. Different regions in India have varying practices regarding whether the month starts on the new moon or full moon day.

Solar months are defined by the sun's apparent motion through the zodiacal constellations (Rāśis). The ecliptic is the path of the Earth around the sun, and a belt of about 8 degrees on either side of the ecliptic is called the Zodiac or Rāśi Cakra, containing 12 constellations. The sun takes about one month to pass through each Rāśi. The day the sun enters a new Rāśi is called Saṅkrānti; for example, Makarā Saṅkrānti is when the sun enters Capricorn (Makarā Rāśi). The solar month length varies between 29 and 32 days due to the unequal sizes of constellations and the sun's non-uniform orbital speed.

Nakṣatras are prominent stars or star groups used to track the moon's position. There are 27 or 28 Nakṣatras, each associated with a day of the lunar orbit. The lunar month is named after the Nakṣatra seen on the full moon day.

Because a lunar year is about 11 days shorter than a solar year, the Hindu calendar reconciles this difference by adding an extra lunar month (adhikamāsa) approximately every three years. This adjustment keeps festivals aligned with the seasons. The rules for adding this extra month are complex but ensure that lunar and solar years remain synchronized.

Tithi, or lunar date, is another important concept. It is defined by the angular separation between the sun and the moon, with each tithi representing a 12-degree separation. There are 30 tithis in a lunar month, but their duration varies between 19 and 26 hours due to elliptical orbits and orbital perturbations. This variability causes tithis to sometimes change during a single day, leading to different interpretations about which tithi governs a particular day.

Thus, the Indian calendar system is a sophisticated integration of lunar and solar cycles, astronomical observations, and cultural practices, enabling the precise timing of festivals, rituals, and daily activities.

📊 Diagram: See figure_7: The constellations of the zodiac with months and dates indicated; table_1: Calendar month names in Hindu calendars.

🧪 Activity: Answer the following questions by observing any Hindu calendar of the year 2019: 1. What was the time of sunrise and sunset on January 15, 2019? 2. In year 2019, find the pakṣa and tithi on your birthday. 3. On which date does the Caitra month start in 2019?

🔗 Connection: This section's explanation of the Indian calendar leads to the next section on the astronomical explanation of eclipses, which are important celestial events tracked in the calendar.

Table on page 4 (12×2)

Table on page 17 (7×2)