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Ave. Distance (R, au) – average distance from the Sun, in astronomical units (1 au = Earth–Sun distance). German mathematician and astronomer, formulated the three Laws of Planetary Motion between 1609 and 1619. His work transformed astronomy by showing that planets move in elliptical orbits, not perfect circles.KEPLER’SKEPLER’SKEPLER’SKEPLER’SJ o h a n n e s K e plerBefore Kepler, two major models dominated astronomy: Geocentric Model (Ptolemaic): Earth was placed at the center, with planets moving in complex circular paths and epicycles. Heliocentric Model (Copernican): Proposed by Nicolaus Copernicus in 1543, this model placed the Sun near the center. However, it still assumed planets moved in perfect circles. Kepler’s breakthrough: He replaced the idea of perfect circular orbits with elliptical orbits, making planetary motion more accurately match observations. Before Kepler, two major models dominated astronomy: Geocentric Model (Ptolemaic): Earth was placed at the center, with planets moving in complex circular paths and epicycles. Heliocentric Model (Copernican): Proposed by Nicolaus Copernicus in 1543, this model placed the Sun near the center. However, it still assumed planets moved in perfect circles. Kepler’s breakthrough: He replaced the idea of perfect circular orbits with elliptical orbits, making planetary motion more accurately match observations.Planet – the planet in that row. Period (T, yr) – time to orbit the Sun once. Earth = 1 year. T²/R³ – Kepler's 3rd Law ratio (period² ÷ distance³). It's about 1 for every planet, proving the law holds.J o h a n n e s K e pler
The farther a planet isThe farther a planet is from the Sun, the longer it takesfrom the Sun, the longer it takes to complete a year. This isn't justto complete a year. This isn't just because the planet has a larger circle tobecause the planet has a larger circle to travel; it is also because the Sun'stravel; it is also because the Sun's gravity is weaker at ggravity is weaker at greater distances,reater distances, causcausing outer planets to move muching outer planets to move much slower along their paths.slower along their paths. •the square of a planet’s orbital period is•the square of a planet’s orbital period is directly proportional to the cube of thedirectly proportional to the cube of the semi-major axis of its orbit.semi-major axis of its orbit. The farther a planet is from the Sun, the longer it takes to complete a year. This isn't just because the planet has a larger circle to travel; it is also because the Sun's gravity is weaker at greater distances, causing outer planets to move much slower along their paths. •the square of a planet’s orbital period is directly proportional to the cube of the semi-major axis of its orbit. States that if you draw a line fromStates that if you draw a line from the Sun to a planet, that line will sweep out the exactthe Sun to a planet, that line will sweep out the exact same slice of space (area) in 30 days, no mattersame slice of space (area) in 30 days, no matter where the planet is in its orbit.where the planet is in its orbit. Imagine cutting slices of a pieImagine cutting slices of a pie Perihelion: The planet is close to the Sun, making thePerihelion: The planet is close to the Sun, making the pie slice very short. To get the exact same amount ofpie slice very short. To get the exact same amount of pie area in those same 30 days, the slice has to bepie area in those same 30 days, the slice has to be much wider. The planet has to sprint along its path tomuch wider. The planet has to sprint along its path to cover that wide distance in time.cover that wide distance in time. Aphelion: The planet is far from the Sun, making theAphelion: The planet is far from the Sun, making the pie slice very long. Because the slice is so long, itpie slice very long. Because the slice is so long, it doesn't need to be very wide to contain a lot of pie.doesn't need to be very wide to contain a lot of pie. The planet can travel slowly along its path and stillThe planet can travel slowly along its path and still cover the required area.cover the required area. States that if you draw a line from the Sun to a planet, that line will sweep out the exact same slice of space (area) in 30 days, no matter where the planet is in its orbit. Imagine cutting slices of a pie Perihelion: The planet is close to the Sun, making the pie slice very short. To get the exact same amount of pie area in those same 30 days, the slice has to be much wider. The planet has to sprint along its path to cover that wide distance in time. Aphelion: The planet is far from the Sun, making the pie slice very long. Because the slice is so long, it doesn't need to be very wide to contain a lot of pie. The planet can travel slowly along its path and still cover the required area. Unlike the uniformUnlike the uniform circular orbits proposed bycircular orbits proposed by early astronomers, Kepler statedearly astronomers, Kepler stated that every planet moves in an elliptical orbitthat every planet moves in an elliptical orbit with the sun locatedwith the sun located at one focus of the planet's elliptical pathat one focus of the planet's elliptical path •Perihelion: The p•Perihelion: The point where the planet isoint where the planet is closest to the Sun.closest to the Sun. •Aphelion: The point where the planet is farthest•Aphelion: The point where the planet is farthest from the Sunfrom the Sun •Eccentricity (e): A measurement describing•Eccentricity (e): A measurement describing how stretched out an ellipse is compared to ahow stretched out an ellipse is compared to a perfect circleperfect circle Unlike the uniform circular orbits proposed by early astronomers, Kepler stated that every planet moves in an elliptical orbit with the sun located at one focus of the planet's elliptical path •Perihelion: The point where the planet is closest to the Sun. •Aphelion: The point where the planet is farthest from the Sun •Eccentricity (e): A measurement describing how stretched out an ellipse is compared to a perfect circleMercury has the most eccentric orbit of any planet in ourMercury has the most eccentric orbit of any planet in our solar system. Its aphelion distance from the Sun is 69.8solar system. Its aphelion distance from the Sun is 69.8 million km, and its perihelion distance is 46.0 million km.million km, and its perihelion distance is 46.0 million km. Determine the eccentricity of Mercury's orbit and interpretDetermine the eccentricity of Mercury's orbit and interpret what this value suggests about the shape of