An ephemeris is a table or chart that shows where celestial objects like planets, moons, or comets will be in the sky at specific times. Astronomers use ephemerides (the plural form) to predict when and where to look for these objects.

Think of an ephemeris like a train schedule, but for objects in space. Just as a train schedule tells you when a train will arrive at each station, an ephemeris tells you where Mars or Jupiter will appear in the night sky on any given date. Before space exploration, sailors used ephemerides to navigate by the stars and planets, calculating their position on Earth by comparing what they saw overhead with what the tables predicted.

Today, astronomers rely on ephemerides to point telescopes at distant objects, and space agencies use them to plan missions. When NASA sends a spacecraft to Mars, engineers consult ephemerides to know exactly where Mars will be months or years in the future, so they can plot the right course.