What this chapter is about
This chapter helps you understand the three bodies that shape our daily life: the Earth we live on, the Moon that lights our nights, and the Sun that gives us day and warmth. You will learn how their movements cause day and night, seasons, and the changing phases of the Moon.
At Class 7, you are ready to connect what you see in the sky with simple science. Why does the Sun seem to move across the sky? Why does the Moon look different each week? Why do we have summer and winter? These questions have clear answers once you know how Earth spins and travels around the Sun, and how the Moon circles Earth.
After studying this chapter, you should be able to explain eclipses, draw the positions of the Earth, Moon and Sun during different phases, and understand why places on Earth have different seasons at the same time.
Key ideas
- The Earth rotates (spins) on its axis once every 24 hours; this rotation causes day and night.
- The Earth revolves (orbits) around the Sun once every 365¼ days; this revolution, along with the tilt of Earth's axis, causes seasons.
- The Moon revolves around the Earth once in about 27 to 28 days and does not produce its own light; it reflects sunlight.
- We see different phases of the Moon (new moon, crescent, half, gibbous, full moon) because the lit part visible from Earth changes during its orbit.
- A solar eclipse happens when the Moon comes between the Sun and Earth, blocking sunlight for a short time.
- A lunar eclipse happens when the Earth comes between the Sun and Moon, and Earth's shadow falls on the Moon.
- The Earth's axis is tilted at about 23.5 degrees; this tilt causes the Northern and Southern Hemispheres to receive different amounts of sunlight at different times of the year.
Formulas and facts to remember
1. Rotation period of Earth: 24 hours (one day-night cycle). 2. Revolution period of Earth: About 365¼ days (one year). 3. Revolution period of Moon around Earth: About 27–28 days. 4. Axis tilt of Earth: About 23.5 degrees from the vertical to its orbit. 5. Moon's light source: The Moon has no light of its own; it reflects sunlight. 6. Solar eclipse condition: Sun → Moon → Earth (Moon's shadow on Earth). 7. Lunar eclipse condition: Sun → Earth → Moon (Earth's shadow on Moon). 8. Seasons depend on: Earth's tilt and its position in its orbit around the Sun.
Worked examples
Example 1: Day and night
Imagine you place a torch in a dark room and a globe on a table. The torch is the Sun. When you spin the globe slowly, only one half is lit at a time.
- The side facing the torch has day.
- The side away from the torch has night.
- As you turn the globe, places move from day to night and back. This is how Earth's rotation gives us day and night every 24 hours.
Example 2: Moon phases
Ravi noticed that on Monday the Moon was a thin crescent, and two weeks later it was a bright full circle.
- On Monday, only a small part of the lit side of the Moon faced Earth, so it looked like a crescent.
- Two weeks later, Earth was between the Sun and Moon, so the entire lit side of the Moon faced us. This is a full moon.
- In another two weeks, the Moon will be new (almost invisible) because the lit side faces away from Earth.
Example 3: Why June is summer in Delhi but winter in Sydney
In June, Earth's Northern Hemisphere tilts toward the Sun.
- Delhi, in the north, gets more direct sunlight and longer days, so it is summer.
- Sydney, in the Southern Hemisphere, tilts away from the Sun, gets less direct sunlight and shorter days, so it is winter.
- Six months later, the positions reverse.
Common mistakes
- Thinking the Moon glows by itself → The Moon only reflects the Sun's light.
- Believing seasons happen because Earth moves closer to or farther from the Sun → Seasons happen because of Earth's tilted axis, not distance.
- Mixing up rotation and revolution → Rotation is spinning in place (causes day-night); revolution is orbiting another body (causes year and seasons).
- Expecting a lunar eclipse every full moon → The Moon's orbit is slightly tilted, so its shadow often misses Earth; eclipses happen only when alignments are exact.
- Thinking the Moon takes 30 days to orbit Earth → It takes about 27–28 days; the calendar month is slightly longer.
Quick revision
- Earth rotates once in 24 hours, giving day and night.
- Earth revolves around the Sun in about 365¼ days, giving us a year.
- The Moon revolves around Earth in about 27–28 days and shows phases because we see different portions of its sunlit side.
- Earth's 23.5-degree tilt causes seasons, not distance from the Sun.
- Solar eclipse: Moon blocks the Sun; lunar eclipse: Earth's shadow covers the Moon.