Page 155 - Innovator's Science-8

Know Your Scientist
Meghnad Saha (1893-1956)
Meghnad Saha was a pioneering astrophysicist of India who studied stars and their temperatures and
developed a mathematical equation, famously known as the Saha equation. The Saha Institute of Nuclear
Physics, in Kolkata, is named after him. He was also the chairperson of the Calendar Reform Committee.

Grasp-Up

Solar day: The me taken by the Sun to return to the same highest point in the sky - about 24 hours.
Equinox: The me when day and night are of equal length, occurring around March 21 and September 23.
Sols ce: The me when the Sun reaches its farthest point north or south of the equator (around June 21 and
December 21).
Tropical year: The me between two successive spring equinoxes; about 365 days.
Sidereal year: The me taken by the Earth to complete one revolu on around the Sun with respect to fixed
stars.
Leap year: A year having one extra day (366 days) to keep the calendar aligned with the seasons.
Solar calendar: A calendar based on the apparent mo on of the Sun.
Lunar calendar: A calendar based on the Moon's phases.

Points to Reflect

• People have long observed the sky to track time and the changing seasons.
• The apparent movement of the Sun helped early observers mark solstices and equinoxes.
• In India, ancient texts such as the Taittiriya Samhita and Surya Siddhanta mention the Sun's northward and
southward journeys, Uttarayan and Dakshinayan.Calendars were created to match human activities with
natural cycles.

• Solar calendars base their year on the Earth's revolution around the Sun.
• The Gregorian calendar, used worldwide, has 365 days, with one extra day added every four years to make a
leap year.
• To correct small variations, some leap years are skipped every 100 years and restored every 400 years.
• The tropical year relates to seasons, while the sidereal year is measured by the position of stars.
• The lunar calendar is based on the Moon's cycles of about 354 days.
• Luni-solar calendars adjust lunar months to stay in step with the solar year.

• The Indian National Calendar is a solar calendar adopted by the Government of India for official use along
with the Gregorian calendar.
• It begins with the month of Chaitra and follows the Shaka Era that started in 78 CE.
• The Moon is Earth's natural satellite, while many artificial satellites are launched by nations for
communication, navigation, and research.
• ISRO (Indian Space Research Organisation) has developed and launched several important satellites,
including Cartosat, AstroSat, and the Chandrayaan missions.
• Space activities also create space debris, which can endanger active satellites; countries work together to
manage it safely.
• The Earth's rotation makes the Sun appear to rise and set daily, giving us the 24-hour day or mean solar day.

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