Summary
Highlights
Kepler's first law states that planets orbit the Sun in elliptical paths, with the Sun located at one of the ellipse's two foci. An ellipse is a 'squashed' circle, and its foci define its shape. The sum of the distances from any point on the ellipse to the two foci is constant. If the foci are very close, the ellipse is nearly circular; a single focus means a perfect circle. Real orbits are never perfectly circular.
Eccentricity measures how flattened or 'squashed' an ellipse is compared to a circle. It's calculated using an equation involving the semi-major (long) and semi-minor (short) axes of the ellipse. An eccentricity of zero means a perfect circle. As eccentricity increases, the ellipse becomes flatter. An eccentricity greater than or equal to one indicates a parabolic or hyperbolic path, not an elliptical orbit. For example, Oumuamua had an eccentricity of 1.2, suggesting it wasn't from our solar system, while Earth's orbit has a low eccentricity of 0.0167.
Kepler's third law states that the square of a planet's orbital period is directly proportional to the cube of its mean distance from the Sun. This means that for every planet, the ratio of the square of its period to the cube of its mean distance is a constant value.
Kepler's second law explains that a planet moves more slowly when it's further from the Sun. This is due to the conservation of angular momentum. Angular momentum (mass × distance × velocity) remains constant. So, as the distance from the Sun increases, the planet's velocity must decrease, causing it to slow down.
The second law indicates that Earth moves fastest when it is closest to the Sun (perihelion), which occurs in early January at about 92 million miles. Conversely, it moves slowest when it is furthest from the Sun (aphelion) in early July, at approximately 95 million miles. Despite the 3 million mile difference, Earth's orbit is so vast that it is considered nearly circular.