Kepler’s Laws

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Gravity Noun

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[grav-i-tee]

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Gravity

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The fundamental force of attraction that exists between any two objects with mass, dependent on mass and distance.

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Example: Gravity is a force of attraction between two objects with mass, such as the Earth and Moon, pulling them toward each other.

Front

Orbit Noun

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[or-bit]

Back

Orbit

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The curved, repeating path an object in space takes around another, maintained by gravity and tangential velocity.

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Example: This diagram shows Earth's elliptical orbit around the Sun, a key part of Kepler's Laws, highlighting the closest (perihelion) and farthest (aphelion) points.

Front

Ellipse Noun

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[ih-lips]

Back

Ellipse

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An oval shape that has two central points, called foci, which describes the path of a planet's orbit.

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Example: A planet's path around the sun is an ellipse, an oval shape, with the sun located at one of the two focus points.

Front

Foci Noun

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[foh-sahy]

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Foci

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The two fixed points inside an ellipse that are used in its formal definition; the Sun is at one.

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Example: This diagram shows an ellipse with its two foci (F and G). For any point (P) on the ellipse, the total distance from both foci (FP + GP) is always the same.

Front

Perihelion Noun

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[per-i-hee-lee-uhn]

Back

Perihelion

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The point in a planet's elliptical orbit where it is closest to the Sun and moves at its fastest.

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Example: This diagram shows Earth's elliptical orbit, highlighting 'Perihelion' as the point where our planet is closest to the Sun, including the distance.

Front

Aphelion Noun

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[ap-hee-lee-uhn]

Back

Aphelion

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The point in a planet's elliptical orbit where it is farthest from the Sun and moves at its slowest.

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Example: This diagram shows Earth's elliptical orbit around the Sun, illustrating aphelion as the point where Earth is farthest from the Sun.

Front

Orbital Period Noun

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[or-bi-tuhl peer-ee-uhd]

Back

Orbital Period

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The amount of time it takes for a celestial body to complete one full orbit around another body.

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Example: This diagram shows that objects in larger orbits, like the Moon, have a longer orbital period (T) than objects in smaller orbits, like a satellite.

Front

Centripetal Force Noun

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[sen-trip-i-tl fors]

Back

Centripetal Force

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A force that causes an object to follow a curved path; gravity provides this force for planetary orbits.

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Example: An object moving in a circle has a velocity (blue arrow) tangent to the path, but a constant inward force (red arrow) pulls it toward the center.

Front

Tangential Velocity Noun

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[tan-jen-shuhl vuh-los-i-tee]

Back

Tangential Velocity

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The velocity of an object moving in a curved path that is directed tangent to that path.

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Example: If an object in circular motion is released, it travels in a straight line tangent to the circle (path 2), which is its tangential velocity.

Front

Semi-major Axis Noun

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[sem-ee mey-jer ak-sis]

Back

Semi-major Axis

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One half of the longest diameter of an ellipse, which represents a planet's average distance from the Sun.

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Example: This diagram shows an ellipse, the shape of a planet's orbit. The semi-major axis, labeled 'a', is the distance from the center to the farthest edge.