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.
Phases of the MoonWhy the Moon rises later each dayDay, month, and year as time unitsLunar, solar, and luni-solar calendarsArtificial satellites and ISRO missions
Key terms
Phases of the Moon
The changing shapes of the bright portion of the Moon seen from Earth as it revolves around the Earth.
Waxing and waning
The waxing period is when the bright part of the Moon increases; the waning period is when it decreases.
Mean solar day
The average time of 24 hours the Sun takes to return to its highest position in the sky on successive days.
Lunar calendar
A calendar based on the cycle of the Moons phases, with a year of about 354 days.
Leap year
A year with an extra day added to a solar calendar to keep it synchronised with the seasons.
Artificial satellite
A human-made object launched to orbit the Earth for communication, navigation, weather monitoring, or research.
Extra questions & answers
Probe and ponder
Keep the curiosity alive
Discover, design, and debate
Try one from this chapter
Why does the Moon shine?
Flashcards & interactive practice
Practice quiz · Keeping Time with the Skies
Score on this chapter, climb the leaderboard, and get an AI diagnosis of your mistakes.
Dual AI-verified questions Real exam pattern 1 free quiz every month, then ₹9 per quiz — or a monthly plan
#1
Keeping Time with the Skies
Start here
Science 10 Qs · ~10 min
Instant score, answer review and weak-topic diagnosis
6
2
