semipääakselia

Semipääakselia, often translated as semi-major axis, is a fundamental concept in describing elliptical orbits. It represents half the length of the longest diameter of an ellipse, which passes through both foci and the center of the ellipse. The term is most commonly encountered in astronomy, where it is used to characterize the size and shape of the orbits of planets, moons, comets, and other celestial bodies around a primary mass.

In the context of an ellipse, the semipmajor axis is denoted by the letter 'a'. It is a crucial parameter because it, along with the eccentricity of the ellipse, defines the entire geometry of the orbit. A larger semipmajor axis indicates a larger orbit. For a circular orbit, which is a special case of an ellipse where the two foci coincide at the center, the semipmajor axis is equal to the radius of the circle.

The semipmajor axis is also directly related to the orbital period of an object through Kepler's Third Law of Planetary Motion. This law states that the square of the orbital period is proportional to the cube of the semipmajor axis. Therefore, by knowing the semipmajor axis, astronomers can calculate how long it takes for an object to complete one orbit.