Keeping Time with the Skies – Class 8 Science Chapter 11 Notes & NCERT PDF

Probe and Ponder

Introduction

It is Makar Sankranti, and Meera is in Ahmedabad for the Patang Mahotsav (International Kite Festival). Among the kites she spots the Moon shining in daytime, and it is not a full circle. She knows the Moon is spherical and shines by reflecting sunlight — so why isn't the whole Moon visible every night? It can't be an eclipse, because eclipses are rare and brief. This chapter explains the phases of the Moon, how natural cycles gave us calendars, how festivals follow the sky, and why we launch artificial satellites.

Key Concepts

11.1 How does the Moon's appearance change?

Activity 11.1 asks you to watch the Moon for a month, starting at sunrise the day after a full Moon (look in the west), shading its bright part each day. After about 15 days you can no longer see it at sunrise, so for the next 15 days you observe at sunset.

Phases of the Moon:

The changing shapes of the bright portion of the Moon, as seen from the Earth, are called the phases of the Moon.

Locating the Moon: on a full Moon day the Moon is nearly opposite the Sun — as the Sun rises in the East, the Moon is setting in the West. As it wanes it appears closer to the Sun each morning; at half phase it is overhead at sunrise. A waxing Moon is easiest to spot at sunset, a waning Moon at sunrise. The Moon rises about 50 minutes later each day; sometimes moonrise is in the afternoon (around 2–4 p.m.), so the Moon can be seen in daylight.

Why the phases happen

A step further: Phases are not caused by Earth's shadow. Earth's shadow on the Moon causes a lunar eclipse. Lunar eclipses happen only on full Moon days and solar eclipses only on new Moon days — but not every month, because of the small tilt of the Moon's orbit.

11.2 How did calendars come into existence?

Three natural cycles give three units of time:

Lunar calendars: about 12 lunar months fit in one cycle of seasons. A lunar year has 354 days, so seasons slowly drift away from the same months.

Solar calendars: made to match the seasons for agriculture. The Gregorian calendar is solar; months add up to 365 days (30, 31, and February 28). The extra quarter day adds up to about one day every four years, so a leap year has 29 days in February. Leap years are skipped in century years like 1700, 1800, 1900, but added back every 400 years (1600, 2000).

Our scientific heritage: the Sun rises a little north of East in summer and south of East in winter; the extremes are the solstices (around June 21 and December 21). The Sun's apparent northward movement from December to June is Uttarayan; the southward movement from June to December is Dakshinayan. The Taittirīya Saṁhitā (6.5.3) says: "Thus the Sun moves southwards for six months and northwards for six months." The Surya Siddhanta noted that Capricorn (Makar) was behind the Sun around the winter solstice in ancient times.

Luni-solar calendars: 12 lunar months (354 days) fall short of the solar year by nearly 11 days; every 2–3 years an extra month — Adhika Maasa (intercalary month) — is added. Used in many parts of India. Month names: Chaitra, Vaisakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashwin, Kartika, Margashirsha (Agrahayan), Pausha, Magha, Phalguna. Months ending on new Moon → Amant; ending on full Moon → Purnimant.

The Indian National Calendar: a solar calendar of 365 days. The year begins on 22 March, the day after the spring equinox. Months have 30 or 31 days — in a regular year the second to sixth months have 31 days, the rest 30. In leap years a day is added to Chaitra and the year begins on 21 March. The Calendar Reform Committee (1952), chaired by astrophysicist Meghnad Saha, recommended it; it was adopted from 21 March 1956 CE (1 Chaitra 1878 Saka).

11.3 Festivals and the sky

11.4 Artificial satellites

Activities in the Chapter

Key Terms

Real-life Connections

Common Misconceptions

Keep the Curiosity Alive (exercise hints)