What is a retrograde orbit?

In a retrograde orbit, the orbiting body travels in the direction opposite to the primary body’s rotation. This is unusual in planetary systems because moons and planets usually form from rotating disks that favor the same direction. Some irregular moons of giant planets, such as Neptune’s Triton, have retrograde orbits, likely because they were captured rather than formed in place. Retrograde motion can also be seen in artificial satellites and in the apparent backward motion of planets when viewed from Earth, though that is a different phenomenon.