Chapter 11 — Keeping Time with the Skies

Chapter 11, 'Keeping Time with the Skies', is about the phases of the Moon and why they occur, how these natural sky cycles led to the development of lunar, solar, and luni-solar calendars, and the role and purpose of artificial satellites launched into Earth's orbit.

Phases of the Moon are caused by the changing fraction of the Moon's illuminated half that faces Earth as the Moon revolves around Earth. The Moon reflects sunlight; only the half facing the Sun is lit. As the Moon orbits, we see different portions of that lit half, producing shapes from new Moon to crescent, gibbous, and full Moon.

No — that is a common misconception. Earth's shadow on the Moon causes a lunar eclipse, not the Moon's phases. Phases occur because of the relative change in the positions of the Sun, Moon, and Earth as the Moon revolves around Earth.

The waning period (Krishna Paksha in India) is the roughly two-week stretch when the bright portion of the Moon decreases from a full circle to invisibility (new Moon/Amavasya). The waxing period (Shukla Paksha) is the two-week stretch when the bright portion grows back to a full circle (full Moon/Purnima). The full cycle takes about 29.5 days.

A mean solar day is the average time (24 hours) for the Sun to go from its highest point in the sky on one day to the highest point the next day. It can be measured by tracking the shadow of a vertical stick — the shadow is shortest when the Sun is at its highest position.

A lunar calendar is based on the Moon's cycle of about 29.5 days per month, giving a lunar year of 354 days (12 lunar months). A solar calendar is based on the cycle of seasons, adjusted to total 365 days in a year (with an extra leap day every four years). Because the lunar year is about 11 days shorter than the solar year, seasons drift through lunar calendar months over time.

A luni-solar calendar primarily follows the Moon's phases for counting days and months but also makes adjustments to stay in sync with the seasonal (solar) cycle. Because 12 lunar months fall short of the solar year by nearly 11 days, an extra intercalary month called Adhika Maasa is added every 2–3 years when the accumulated difference becomes close to a full month. Many calendars used in India are luni-solar.

The Indian National Calendar is a solar calendar adopted by the Government of India, with 365 days in a year. The year begins on 22 March (the day after the spring equinox). Its months have either 30 or 31 days, and their names are taken from traditional Indian calendars. In leap years a day is added to Chaitra (the first month), and the year begins on 21 March. It was adopted from 21 March 1956 (1 Chaitra 1878 Saka), based on the recommendation of the Calendar Reform Committee chaired by Meghnad Saha.

These festivals are tied to the phases of the Moon and are based on lunar or luni-solar calendars. Diwali falls on the new Moon of Kartika, Holi on the full Moon of Phalguna, and Eid-ul-Fitr is celebrated after sighting the crescent Moon at the end of Ramazan. Because the lunar year is shorter than the Gregorian year, these festivals shift in the Gregorian calendar each year. Festivals following purely lunar calendars (like Eid) can shift by about 11 days each year and may occur in different Gregorian months, while luni-solar festival dates shift less (usually under a month) because of the intercalary month adjustment.

Artificial satellites are man-made objects launched into orbit around Earth. They are used for communication, navigation, weather monitoring, disaster management, and scientific research. Most orbit about 800 km above Earth's surface and take roughly 100 minutes to complete one orbit. ISRO (the Indian Space Research Organisation) has launched many such satellites, including the Cartosat series and AstroSat.

As the Earth completes one full rotation in 24 hours, the Moon has moved ahead in its own orbit around Earth. Earth therefore needs to rotate a little more for the Moon to return to nearly the same position in the sky, making the Moon rise approximately 50 minutes later each successive day. This also means moonrise can sometimes happen in the afternoon (around 2:00–4:00 p.m.), making the Moon visible in daylight.

Uttarayan is the Sun's apparent northward movement from December to June, while Dakshinayan is its apparent southward movement from June to December. In summer the Sun rises a little northward of East, and in winter a little southward of East. These extremes happen on the solstices, around June 21 and December 21. This cycle is closely linked to the changing seasons and is recorded in ancient Indian texts including the Taittiriya Samhita.

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