Keeping Time with the Skies

About this chapter

Keeping Time with the Skies is Chapter 11 of Class 8 Curiosity. Meera, in Ahmedabad on Makar Sankranti, sees the Moon by day and wonders why it is not a full circle. The chapter shows the Moon shines by reflecting sunlight; as it revolves around the Earth, the changing illuminated portion gives the phases — new Moon (Amavasya), waxing (Shukla Paksha), full Moon (Purnima), waning (Krishna Paksha), gibbous and crescent. Phases are NOT due to Earth's shadow; that shadow causes a lunar eclipse on full Moon (solar on new Moon), not every month because the Moon's orbit is tilted. Cycles fix time units — rotation = 24-hour mean solar day, Moon's cycle = 29.5-day month, revolution = 365-day-6-hour year. Three calendars are compared — lunar (354 days), solar (Gregorian; leap year every 4 with 100/400-year corrections; tropical vs sidereal year), luni-solar (Adhika Maasa; 12 Indian months Chaitra–Phalguna; Amant vs Purnimant). Indian astronomy — Uttarayan, Dakshinayan, Taittiriya Samhita 6.5.3, Surya Siddhanta, Indian National Calendar (Shaka era, 22 March), 1952 Calendar Reform Committee under Meghnad Saha, and ISRO satellites (Cartosat, AstroSat, Chandrayaan, Aditya L1, Mangalyaan) with Vikram Sarabhai as Father of the Indian Space programme. The four tests cover this at exam depth.