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Explain Solar
System Concepts
& Cosmic Structures

Understand how the Sun, planets, moons, and orbits fit together — and how scientists measure the vast distances of space.

GED Science

What Is the Solar System?

The solar system is the Sun and everything bound to it by gravity: 8 planets, their moons, dwarf planets, asteroids, and comets. The Sun holds about 99.8% of all the system's mass.

Key Terms to Know

Star
A giant ball of hot gas producing light and heat by nuclear fusion. The Sun is a star.
Planet
A large body orbiting a star, rounded by its own gravity, that has cleared its orbital path.
Moon
A natural satellite that orbits a planet, such as Earth's Moon.

Order of the Planets

Sun Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune

Why It Matters

Understanding the solar system helps explain everyday events: day and night, the seasons, the phases of the Moon, and tides. These all come from rotation, orbits, and gravity.

The Rule: Gravity Holds It Together

Newton's law of universal gravitation states that every two objects attract each other. The force depends on their masses and the distance between them:

How Orbits Work

Sun Planet Velocity Gravity

Rotation vs. Revolution

Rotation
Spinning on its own axis. Earth rotates once every 24 hours — this gives us day and night.
Revolution
Orbiting around another body. Earth revolves around the Sun once every 365.25 days — one year.

Scaling Up: Cosmic Structures

1
Solar System: The Sun and its orbiting planets, moons, and debris.
2
Galaxy: Billions of stars bound by gravity. We live in the Milky Way.
3
Universe: All galaxies, matter, energy, and space — everything that exists.

Measuring Space Distances

Space is so vast that miles or kilometers become impractical. Scientists use larger units like the light-year — the distance light travels in one year.

How to Approach a Question

1
Identify the objects involved (Sun, planet, moon) and their roles.
2
Decide the cause: is it rotation, revolution, gravity, or distance?
3
Match the effect (day/night, seasons, orbit, phases) to the correct cause.
4
Check the scale: are we talking solar system, galaxy, or universe?

Example 1: Day and Night

Question: What causes one side of Earth to be in daylight while the other is dark?

Example 2: Gravity & Distance

Question: If the distance between the Sun and a planet doubled, how would the gravitational force change?

Example 3: Light-Years

Question: A star is 4 light-years away. What does that actually tell us?

Common Mistakes

✕ Seasons = Distance
Seasons come from Earth's tilt, not from being closer to the Sun.
✕ Sun "Moves"
The Sun appears to move because Earth rotates, not the other way around.
✕ Light-Year = Time
A light-year measures distance, not how long something takes.

Cause and Effect Summary

Cause Result
Day / NightEarth's rotation24-hour cycle
SeasonsEarth's axial tiltOne year cycle
Moon phasesMoon orbiting Earth~29-day cycle
Stable orbitsGravity + motionCurved path

Question 1 of 5

What force keeps the planets in orbit around the Sun?

Question 2 of 5

Which motion of Earth causes day and night?

Question 3 of 5

In Newton's law \( F = G \dfrac{m_1 m_2}{r^2} \), what happens to the force if the distance \( r \) is doubled?

Question 4 of 5

A light-year is a unit that measures what?

Question 5 of 5

Which list correctly orders structures from smallest to largest?