A Planetary Ephemeris is a table or data set that provides the positions of celestial bodies, such as planets, moons, and asteroids, at specific times. It is essential for astronomers and navigators to predict the locations of these objects in the sky, aiding in observations and space missions. These calculations are based on the laws of celestial mechanics, which describe how objects move under the influence of gravity. The ephemeris can include information about the orbits of planets like Earth, Mars, and Jupiter, helping scientists understand their movements and interactions within the Solar System.