Kepler's Laws

Front

Ellipse Noun

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[ih-lips]

Back

Ellipse

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An oval shape that describes the path of an orbiting object, defined by a specific mathematical equation.

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Example: This diagram shows Earth's path around the Sun is an ellipse, an oval shape, which demonstrates Kepler's First Law of planetary motion.

Front

Focus Noun

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[foh-kus]

Back

Focus

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One of two fixed points inside an ellipse; the object being orbited is located at one focus.

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Example: An ellipse has two special points called foci (plural of focus); a planet's orbit is an ellipse with the Sun located at one of these foci.

Front

Semimajor axis Noun

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[sem-ee-mey-jer ak-sis]

Back

Semimajor axis

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Half of the longest diameter of an ellipse, representing the average distance of an orbiting body from its primary.

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

Front

Semiminor axis Noun

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[sem-ee-mahy-ner ak-sis]

Back

Semiminor axis

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Half of the shortest diameter of an ellipse, a line segment perpendicular to the semimajor axis.

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Example: This diagram shows an ellipse, like a planet's orbit. The semi-minor axis is the shorter line from the center to the edge, defining the orbit's width.

Front

Eccentricity Noun

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[ek-sen-tris-i-tee]

Back

Eccentricity

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A measure of how much an orbit's elliptical shape deviates from a perfect circle, ranging from 0 to 1.

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Example: This diagram shows that Earth's orbit is an ellipse (an oval), not a perfect circle, with the Sun at one focus, demonstrating orbital eccentricity.

Front

Perihelion Noun

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[per-i-hee-lee-uhn]

Back

Perihelion

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The point in an object's orbit around the Sun where it is closest to the Sun.

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

Front

Aphelion Noun

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[ap-hee-lee-uhn]

Back

Aphelion

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The point in an object's orbit around the Sun where it is farthest from the Sun.

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

Front

Orbital Period Noun

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[awr-bi-tl peer-ee-uhd]

Back

Orbital Period

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The amount of time it takes for an object to complete one full orbit around another object.

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Example: This diagram shows the complete path, or orbit, an object like the Moon takes around another object like the Earth. The orbital period is the time it takes to finish one full loop.

Front

Astronomical Unit Noun

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[as-truh-nom-i-kuhl yoo-nit]

Back

Astronomical Unit

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A unit of distance equal to the average distance from Earth to the Sun, about 149.6 million kilometers.

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Example: This diagram shows that one Astronomical Unit (AU) is the average distance between the Sun and the Earth, a key measurement for planetary orbits.

Front

Oort Cloud Noun

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[awrt kloud]

Back

Oort Cloud

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A vast, theoretical spherical shell of icy bodies surrounding the solar system at its most distant edges.

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Example: This diagram shows the Oort Cloud as a vast, spherical shell of icy objects surrounding the Sun, the planetary region, and the Kuiper Belt.