Gravity in the solar system and galaxies

Learn it by playing

Answer these questions to earn energy, then fish and explore. No account needed.

For teachers: ready-to-use lesson slides, revision notes, diagrams for Gravity in the solar system and galaxies (NGSS Middle School Science, Earth and Space Science) — use them in your lesson, or run the topic as an interactive class activity your students play as a live game.

Lesson notes

Gravity and Orbits

**Gravity** is the force that attracts objects with mass toward each other.
An **orbit** is the curved path an object takes around a more massive body due to gravity.
Planets orbit stars, moons orbit planets, and artificial satellites orbit Earth or other bodies.
Gravity provides the centripetal force that keeps objects in orbit, balancing their forward motion.

Kepler's Laws of Planetary Motion

**First law**: Planets move in **elliptical** orbits with the Sun at one focus (not the center).
**Second law**: A planet's orbital speed changes — it moves faster when closer to the Sun and slower when farther away.
**Third law**: The square of a planet's orbital period (T²) is proportional to the cube of its average distance from the Sun (a³).
For example, Jupiter (5.2 AU, 11.86 years) and Venus (0.723 AU, 0.615 years) both satisfy a³/T² ≈ 1.

Newton's Law of Universal Gravitation

Newton showed that gravity follows an **inverse-square law**: force decreases with the square of distance.
The gravitational force between two objects depends on their masses and the distance between them.
Newton's version of Kepler's third law: T² ∝ a³ / M, where M is the combined mass of the two bodies.
For a planet orbiting a star, the center of mass is very close to the star's center because the star is much more massive.

Gravity in the Solar System

The **Sun's gravity** holds the planets, asteroids, and comets in orbit around it.
Each planet's **gravity** holds its moons in orbit.
Gravity also causes **tides** on Earth due to the Moon's pull.
Without gravity, objects would fly off in straight lines instead of orbiting.

Gravity in Galaxies

**Galaxies** like the Milky Way are held together by gravity.
Stars, gas, and dust orbit the **galactic center**, which contains a supermassive black hole.
Gravity also causes galaxies to cluster together in **galaxy groups and clusters**.
Dark matter, an invisible substance, contributes additional gravity that affects galactic motion.

Elliptical Orbits and Conic Sections

Newton proved that orbits under gravity are **conic sections** (ellipses, parabolas, or hyperbolas).
Bound orbits (planets, moons) are **ellipses**; unbound trajectories (some comets) are parabolas or hyperbolas.
The **eccentricity** of an ellipse describes how stretched it is — Earth's orbit is nearly circular (eccentricity 0.017).
Kepler's first law corrected the earlier belief that orbits were perfect circles.

Historical Models of the Solar System

Ancient Greeks (Eudoxus, Aristotle) used **celestial spheres** with planets attached to rotating shells.
Ptolemy added **epicycles** (small circles on larger circles) to explain planetary motion.
Copernicus placed the **Sun at the center** (heliocentric model), simplifying the system.
Kepler's laws, based on Tycho Brahe's precise observations, replaced the epicycle model with elliptical orbits.

Perturbations and Predictions

**Perturbations** are small gravitational tugs from other bodies that cause deviations from perfect Keplerian orbits.
In 1846, Le Verrier used perturbations in **Uranus's orbit** to predict the existence and position of **Neptune**.
Newtonian mechanics successfully explains most orbital motions, but **general relativity** is needed for extreme gravity (e.g., near black holes).

Diagram of an elliptical orbit: the Sun at one focus, planet moving faster near the Sun (perihelion) and slower at aphelion.

Conceptual analogy: gravity's inverse-square law is similar to how electric force decreases with distance in an atom.

Elliptical orbit with Sun at one focus, showing semi-major axis a.

Kepler's second law: a planet sweeps equal areas in equal times, moving faster when closer to the Sun.

Slides

Step through every slide for this topic — plus flashcards and revision notes — with a free account.

Practice questions

Free preview — 8 of 40 questions. Sign up to see them all.

1.What force keeps planets in orbit around the Sun?
Easy
- AGravity
- BMagnetism
- CFriction
- DAir resistance
2.Which of the following best describes the shape of planetary orbits?
Easy
- APerfect circles
- BEllipses
- CSquares
- DTriangles
3.What is the name of our home galaxy?
Easy
- AAndromeda Galaxy
- BMilky Way Galaxy
- CSombrero Galaxy
- DWhirlpool Galaxy
4.According to Kepler's third law, if a planet is farther from the Sun, its orbital period is:
Medium
- AShorter
- BLonger
- CThe same
- DUnrelated
5.An object that orbits a planet is called a:
Medium
- AStar
- BMoon
- CComet
- DAsteroid
6.Which scientist first formulated the three laws of planetary motion?
Medium
- AIsaac Newton
- BGalileo Galilei
- CJohannes Kepler
- DNicolaus Copernicus
7.The force of gravity between two objects depends on their masses and the:
Hard
- ASum of their radii
- BDistance between them squared
- CProduct of their volumes
- DColor of the objects
8.If the distance between two planets is doubled, the gravitational force between them becomes:
Hard
- AHalf
- BDouble
- COne quarter
- DFour times

Revision notes

## Gravity in the Solar System and Galaxies ### Key Concepts - **Gravity** is the force that keeps planets orbiting the Sun and moons orbiting planets. It also holds galaxies together. - **Orbits** are curved paths objects follow due to gravity. Most orbits are elliptical (oval-shaped), not perfect circles. - **Kepler's Laws**: 1. Planets orbit the Sun in ellipses, with the Sun at one focus. 2. A planet moves faster when closer to the Sun and slower when farther away. 3. The square of a planet's orbital period (T²) is proportional to the cube of its average distance from the Sun (a³). For any two planets, \(\frac{a^3}{T^2}\) is constant. - **Newton's Law of Gravitation**: Gravity is an attractive force between all objects with mass. The force depends on mass and distance. For a planet orbiting a star, \(T^2 \propto \frac{a^3}{M}\), where M is the mass of the star. - **Center of Mass**: Two objects actually orbit their common center of mass. If one is much more massive (e.g., Sun vs. Earth), the center is inside the larger object. ### Examples in the Solar System - Earth orbits the Sun once every 365.25 days at an average distance of 1 AU (astronomical unit). - Jupiter is about 5.2 AU from the Sun and takes ~11.86 years to orbit. Check Kepler's third law: \(5.2^3 / 11.86^2 \approx 1.0\). - The Moon orbits Earth due to Earth's gravity. ### Galaxies - The Milky Way is a spiral galaxy held together by gravity. Stars, gas, and dust orbit the galactic center, which contains a supermassive black hole. - Gravity causes galaxies to cluster and interact over cosmic time. ### Historical Models - Early models (Ptolemy) used complex circles (epicycles) to explain planet motions. - Copernicus placed the Sun at the center, simplifying the model. - Kepler used Tycho Brahe's data to discover elliptical orbits. - Newton explained orbits using gravity and calculus. ### Key Formula (from Newton) - \(T^2 \propto \frac{a^3}{M}\) where T = orbital period, a = semi-major axis (average distance), M = mass of central object. ### Study Tips - Remember: gravity is the "glue" of the universe. It determines orbits, from moons to galaxies. - Practice using Kepler's third law: if you know distance, find period, or vice versa. - Understand that all orbits are ellipses; circles are a special case.

Orbit: The curved path of an object around a star, planet, or moon, caused by gravity.
Gravity: A force that attracts objects with mass toward each other; it keeps planets in orbit and holds galaxies together.
Kepler's Laws: Three laws describing planetary motion: (1) planets move in ellipses with the Sun at one focus; (2) planets sweep equal areas in equal times; (3) the square of the orbital period is proportional to the cube of the average distance from the Sun.
Ellipse: An oval shape; the path of a planet around the Sun is an ellipse.
Orbital Period: The time an object takes to complete one full orbit around another object.
Astronomical Unit (AU): The average distance from Earth to the Sun, about 150 million kilometers; used to measure distances in the solar system.
Center of Mass: The point around which two orbiting objects both revolve; for a planet and a much larger star, it lies inside the star.
Milky Way: The spiral galaxy that contains our solar system; held together by gravity.
Inverse-Square Law: A law stating that the force of gravity between two objects decreases with the square of the distance between them.

Flashcards

What is an orbit?

An orbit is the curved path of an object around another object due to gravity, such as a planet around a star or a moon around a planet.

What shape are most orbits in the solar system?

Most orbits are elliptical (oval-shaped), not perfect circles, with the larger object at one focus of the ellipse.

What force keeps planets in orbit around the Sun?

Gravity is the force that keeps planets in orbit around the Sun, pulling them toward the Sun while their forward motion keeps them from falling in.

What is Kepler's first law of planetary motion?

Kepler's first law states that planets orbit the Sun in elliptical paths, with the Sun at one focus of the ellipse.

What is Kepler's second law of planetary motion?

Kepler's second law says that a planet moves faster when it is closer to the Sun and slower when it is farther away, sweeping equal areas in equal times.

What is Kepler's third law of planetary motion?

Kepler's third law states that the square of a planet's orbital period is proportional to the cube of its average distance from the Sun (T² ∝ a³).

How does distance from the Sun affect a planet's orbital period?

Planets farther from the Sun have longer orbital periods; for example, Jupiter (5.2 AU) takes about 11.86 years, while Venus (0.723 AU) takes about 0.615 years.

What is an astronomical unit (AU)?

An astronomical unit (AU) is the average distance from Earth to the Sun, about 150 million kilometers, used to measure distances within the solar system.

What did Isaac Newton contribute to our understanding of orbits?

Newton showed that gravity follows an inverse-square law and that Kepler's laws can be derived from his law of universal gravitation.

What is the inverse-square law of gravity?

The inverse-square law states that the gravitational force between two objects decreases with the square of the distance between them.

What is the role of gravity in the solar system?

Gravity holds the solar system together, causing planets to orbit the Sun, moons to orbit planets, and keeping all objects in their paths.

How does gravity affect galaxies?

Gravity holds galaxies together, causing stars, gas, and dust to orbit the galactic center, and it governs the motions of galaxies in clusters.

What is the Milky Way?

The Milky Way is the galaxy that contains our solar system; it is a large spiral galaxy with billions of stars, all held together by gravity.

What is the difference between revolution and rotation?

Revolution is the orbit of one object around another (e.g., Earth around Sun), while rotation is the spinning of an object on its own axis.

What did Johannes Kepler discover about planetary orbits?

Kepler discovered that planets orbit in ellipses (not circles), that their speed varies with distance from the Sun, and that orbital period relates to distance.

What is a natural satellite?

A natural satellite is a moon or other celestial body that orbits a planet or larger object, such as Earth's Moon or Jupiter's moons.

What is an artificial satellite?

An artificial satellite is a human-made object placed into orbit around Earth or another celestial body for communication, observation, or other purposes.

How does the mass of a star affect the orbits of its planets?

A more massive star exerts a stronger gravitational pull, affecting the orbital speeds and periods of its planets; the combined mass of the system influences orbit size and period.

What is the center of mass in an orbiting system?

The center of mass is the point around which two objects orbit; for a planet and a star, it is usually inside the star due to the star's much larger mass.

What are Lagrangian points?

Lagrangian points are positions in space where the gravitational forces of two large bodies (e.g., Sun and Earth) balance, allowing smaller objects to stay in place.

How did the prediction of Neptune demonstrate Newton's laws?

In 1846, Urbain Le Verrier used Newton's laws to predict Neptune's position from perturbations in Uranus's orbit, confirming the accuracy of Newtonian gravity.

What is a perturbation in orbital mechanics?

A perturbation is a small change in an orbit caused by the gravitational pull of other bodies, such as the effect of Jupiter on an asteroid's path.

What is the difference between a geocentric and heliocentric model?

A geocentric model places Earth at the center of the universe, while a heliocentric model places the Sun at the center; the heliocentric model is correct.

Why do planets not fall into the Sun?

Planets have forward momentum (inertia) that balances the Sun's gravity, causing them to fall around the Sun in a curved path rather than straight into it.

Unlock all 40 questions, flashcards & more

Create a free account to see every question, the slides, flashcards and revision notes for this topic.

Past papers

Past-paper practice for this topic is coming soon.

Coming soon