The semi-major axis is a key measurement in the study of ellipses, particularly in astronomy. It refers to half the length of the longest diameter of an ellipse, which is the distance from the center of the ellipse to its farthest edge. In the context of orbits, such as those of planets around the Sun, the semi-major axis helps determine the size of the orbit. In orbital mechanics, the semi-major axis is crucial for calculating the orbital period of celestial bodies. According to Kepler's laws of planetary motion, the square of the orbital period is proportional to the cube of the semi-major axis. This relationship allows scientists to predict the motion of planets and other objects in space.