Apsis

An apsis is a point in an object's orbit where it gets really, really close or really, really far away from whatever it's orbiting around. Imagine you're riding on a swing set in the park. As you swing back and forth, there are two points where you get really high up in the air - those are the apsides of your swing. Similarly, objects orbiting the Sun or the Moon have apsides, where they get closest or farthest from their central body. The closest point is called the perapsis and the farthest point is called the apoapsis. These points are important because they can affect the speed and trajectory of the object's orbit.