An Etymological Dictionary of Astronomy and Astrophysics
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An equation that enables the position of a body in an elliptical orbit to be calculated at any given time from its orbital elements. It relates the → mean anomaly of the body to its → eccentric anomaly.

Keplerian, adj. of → Kepler; → equation.

Planets move in elliptical paths, with the Sun at one focus of the ellipse.

→ Kepler; → first; → law.

1) The planets move about the Sun in ellipses, at one focus of which the Sun is situated.
2) The → radius vector joining each planet with the Sun describes equal areas in equal times.
3) The ratio of the square of the planet's period of revolution to the cube of the planet's mean distance from the Sun is the same for all planets.

→ Kepler; → law.

A line joining a planet to the Sun sweeps out equal areas in equal intervals of time.

→ Kepler; → second; → law.

A → supernova in → Ophiuchus, first observed on 1604 October 9, and described by Johannes Kepler in his book De stella nova (1606). It reached a maximum → apparent magnitude of -3 in late October. The star remained visible for almost a year. The → light curve is that of a → Type Ia supernova. The → supernova remnant consists of a few filaments and brighter knots at a distance of about 30,000 → light-years. It is the radio source 3C 358. Also known as SN 1604 and Kepler's supernova.

→ Kepler; → star.

The ratio between the square of a planet's → orbital period (P) to the cube of the mean distance from the Sun (a) is the same for all planets: P²∝ a³. More accurately, P² = (4π²a³) / [G(M₁ + M₂)], where M₁ and M₂ are the masses of the two orbiting objects in → solar masses and G is the → gravitational constant. In our solar system M₁ = 1. The → semi-major axis size (a is expressed in → astronomical units and the period (P) is measured in years.

→ Kepler; → third; → law.