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Tamil Nadu (TNBSE)Class 7 Science★ 6-10 marks in the TN Class 7 Term 3 exam through book-back MCQs, true/false and short answers

Universe and Space

Samacheer Kalvi (TN State Board) Class 7 Science — Term 3 Chapter 2, Universe and Space. The geocentric and heliocentric models, the telescope, galaxies and constellations, the Moon's revolution, and India's space agency ISRO, with the actual book-back questions, common mistakes and quick revision.

⏱ 15 min readEasy difficulty✓ Free · NCERT-aligned

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Key points, glossary and the questions most likely to show up.

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By the end of this chapter you'll be able to…

1Differentiate the geocentric and heliocentric models
2Recall who invented the telescope
3Describe galaxies and constellations
4State the Moon's period of revolution
5Explain the vision of ISRO

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Why this chapter matters

Universe and Space explains our place in the cosmos and India's space achievements. The geocentric/heliocentric models, galaxies and the Moon's motion are directly tested book-back content in the TN Class 7 Term 3 exam.

Before you start — revise these

A 5-minute refresher here will save you 30 minutes of confusion below.

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Class 6 — The Earth and the Sky

Sun, Moon and stars.

Universe and Space — Class 7 Science (Samacheer Kalvi)

TN State Board (Samacheer Kalvi) Class 7 Science, Term 3 — Chapter 2. Stars, galaxies and space exploration.

1. About this chapter

This chapter covers the geocentric and heliocentric models, the telescope, galaxies and constellations, the Moon's motion, and ISRO.

2. Models of the universe

The geocentric theory (held by ancient people) said the Earth is at the centre and the Sun and planets revolve around it.
The heliocentric theory states that the Sun is at the centre and the planets revolve around it. This is the correct model.

3. Telescopes, galaxies and constellations

The telescope (used to view distant objects) was invented by Hans Lippershey.
A galaxy is a huge collection of stars, gas and dust. A spiral galaxy contains young and hot stars. Our Solar System lies in a barred spiral galaxy (the Milky Way). All galaxies appear to be moving away from us.
Constellations are recognisable star patterns that are an optical appearance, not real objects grouped together.

4. The Moon and ISRO

The Moon takes about 27 days to complete one revolution around the Earth. So if it is near the star Karthikai (Krittika) today, after 27 days it will again be near the star Asvini.
All planets except Mercury and Venus have moons.
ISRO (Indian Space Research Organisation) aims to harness space technology for national development while pursuing space science and planetary exploration (e.g. Mars missions).

5. Worked examples

Example 1. How long does the Moon take to revolve around the Earth? About 27 days.

Example 2. Who invented the telescope? Hans Lippershey.

Example 3. Which model is correct, geocentric or heliocentric? The heliocentric model (Sun at the centre).

6. Book-back questions (Samacheer Kalvi)

I. Choose the correct answer

The Moon takes about ____ days to revolve around the Earth — (a) 365 / (b) 27. Ans: (b) 27.
The telescope was invented by — (a) Hans Lippershey / (b) Newton. Ans: (a) Hans Lippershey.
The galaxy containing young and hot stars is the — (a) spiral galaxy / (b) elliptical galaxy. Ans: (a) spiral galaxy.

II. True or False 4. All galaxies appear to move away from us. — True. 5. The Solar System lies in a barred spiral galaxy. — True. 6. Constellations are an optical appearance and not real objects grouped together. — True. 7. All planets except Mercury have moons. — False (Mercury and Venus have no moons).

III. Answer briefly 8. Differentiate the geocentric and heliocentric theories. — Geocentric: Earth at the centre; heliocentric: Sun at the centre (the correct view). 9. What is the vision of ISRO? — To harness space technology for national development and pursue space science and planetary exploration.

7. Common mistakes

Mistake: Saying the geocentric model is correct. Fix: The heliocentric model (Sun-centred) is correct.
Mistake: Thinking constellations are real groups of nearby stars. Fix: Constellations are an optical pattern; their stars are at very different distances.
Mistake: Saying only Mercury has no moon. Fix: Both Mercury and Venus have no moons.

8. Quick revision

Term 3 · Ch 2 · universe and space.
Geocentric (Earth centre) vs heliocentric (Sun centre, correct); telescope by Hans Lippershey.
Spiral galaxy = young/hot stars; Solar System in a barred spiral galaxy; galaxies move away; constellations are optical patterns.
Moon revolves in ~27 days; Mercury and Venus have no moons; ISRO harnesses space technology for national development.

Key formulas & results

Everything you need to memorise, in one card. Screenshot this for revision.

Models

geocentric (Earth centre) vs heliocentric (Sun centre, correct)

Heliocentric is right.

Telescope

invented by Hans Lippershey

Views distant objects.

Galaxies

spiral galaxy = young & hot stars; Solar System in a barred spiral

Galaxies move away.

Moon / ISRO

Moon revolves in ~27 days; ISRO for national development

Mercury & Venus have no moons.

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Common mistakes & fixes

These are the exact errors that cost students marks in board exams. Read them once, save yourself the trouble.

WATCH OUT

✗ Saying the geocentric model is correct

✓ The heliocentric model (Sun-centred) is correct.

WATCH OUT

✗ Thinking constellations are real groups of nearby stars

✓ Constellations are an optical pattern; their stars are at very different distances.

WATCH OUT

✗ Saying only Mercury has no moon

✓ Both Mercury and Venus have no moons.

NCERT exercises (with solutions)

Every NCERT exercise from this chapter — what it covers and how many questions to expect.

Choose the correct answer

Book-back MCQs on the Moon's period, the telescope and spiral galaxies.

Questions

True or False

Galaxies, the Milky Way, constellations and moons.

Questions

Answer briefly

Geocentric vs heliocentric and ISRO's vision.

Questions

Practice problems

Try each one yourself before tapping "Show solution". Active recall > rereading.

Q1EASY· MCQ

The Moon takes about ____ days to revolve around the Earth.

▶ Show solution

27 days.

Q2EASY· MCQ

The telescope was invented by ____.

▶ Show solution

Hans Lippershey.

Q3EASY· True/False

True or False: All planets except Mercury have moons.

▶ Show solution

False — both Mercury and Venus have no moons.

Q4EASY· True/False

True or False: Constellations are an optical appearance and not real groups of stars.

▶ Show solution

True.

Q5MEDIUM· Answer briefly

Differentiate the geocentric and heliocentric theories.

▶ Show solution

The geocentric theory places the Earth at the centre with the Sun and planets revolving around it, while the heliocentric theory (the correct one) places the Sun at the centre with the planets revolving around it.

Q6EASY· Answer briefly

What is the vision of ISRO?

▶ Show solution

To harness space technology for national development while pursuing space science research and planetary exploration.

5-minute revision

The whole chapter, distilled. Read this the night before the exam.

•Term 3 Chapter 2 of Samacheer Kalvi Class 7 Science.
•Geocentric theory: Earth at the centre; heliocentric theory: Sun at the centre (correct).
•The telescope was invented by Hans Lippershey.
•A spiral galaxy has young, hot stars; the Solar System lies in a barred spiral galaxy; galaxies move away.
•Constellations are optical star patterns, not real groups.
•The Moon revolves in about 27 days; Mercury and Venus have no moons; ISRO harnesses space technology for national development.

Tamil Nadu (TNBSE) marks blueprint

Where the marks come from in this chapter — so you can plan your prep.

Typical chapter weightage: 6-10 marks across book-back MCQ, true/false and short answers

Question type	Marks each	Typical count	What it tests
MCQ / True-False	1	5-7	Models, galaxies, the Moon
Short Answer	2	1-2	Geocentric vs heliocentric, ISRO

Prep strategy

Separate the two models clearly
Pair the telescope with Hans Lippershey
Remember the Moon's 27-day period
Note Mercury and Venus have no moons

Where this shows up in the real world

This chapter isn't just an exam topic — it lives in the world around you.

Astronomy

Telescopes and models help us study the universe.

Space technology

ISRO's satellites aid communication and weather forecasting.

Navigation

Stars and constellations have long guided travellers.

Exam strategy

Battle-tested tips from teachers and toppers for this chapter.

Contrast geocentric and heliocentric models
Quote Hans Lippershey for the telescope
State the Moon's 27-day revolution
Recall ISRO's vision

Going beyond the textbook

For olympiad aspirants and curious learners — topics that build on this chapter.

Explain why distant galaxies appear to move away from us.
Research one ISRO mission and its purpose.

Where else this chapter is tested

CBSE board isn't the only one — other exams test this chapter too.

TN Class 7 Term 3 ExamHigh

NMMS / Foundation ScienceMedium

School unit testsHigh

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Questions students ask

The real ones — pulled from the Q&A community and tutor sessions.

A galaxy is a real, vast collection of stars, gas and dust bound by gravity, while a constellation is just an apparent pattern of stars in the sky whose stars may actually lie at very different distances.

Careful observations showed that the planets, including the Earth, revolve around the Sun; the Sun-centred (heliocentric) model explains their motions correctly, unlike the older Earth-centred geocentric model.

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