CBSE Class 8 Annual Assessment

Chapter 11: Keeping Time with the Skies

Summary

This chapter explains the phases of the Moon and how astronomical cycles became the basis of calendars. The Moon does not give its own light but shines by reflecting sunlight; only the half facing the Sun is illuminated, and from Earth we see different fractions of this illuminated half, producing the phases from new Moon to full Moon and back. The full cycle, from one full Moon to the next, takes about a month, with the waxing (Shukla Paksha) and waning (Krishna Paksha) periods. A ball-and-lamp activity models how the visible illuminated portion changes with the Moons position. The Moon rises about fifty minutes later each day because it moves along its orbit while the Earth rotates. The chapter then traces how natural cycles define units of time: the rotation of the Earth gives the day, the cycle of the Moons phases gives the month, and the Earths revolution around the Sun gives the year. Lunar calendars follow the Moon, solar calendars (like the Gregorian) follow the seasons and use leap years, and luni-solar calendars combine both by adding an intercalary month (Adhika Maasa). The Indian National Calendar, a solar calendar starting on 22 March, is described, along with how festivals are tied to lunar, luni-solar, or solar cycles. Finally, it introduces artificial satellites and ISRO missions that aid communication, navigation, weather monitoring, and research.

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