Why does Mars seem to move backward? Retrograde motion, explained

Transcript

Why does Mars seem to move backward? Watch Mars against the stars for a few months, and it usually drifts slowly east. But about every two years, something strange happens. It slows, stops, and slides back west for two or three months, then turns around and carries on. Ancient astronomers built elaborate machinery of circles on circles to explain it. The real answer is simpler. Mars isn't reversing. We're overtaking it. Earth orbits closer to the Sun, so it moves faster, about thirty kilometers a second to Mars's twenty-four, on a shorter lap. Most of the time, the line from Earth to Mars sweeps forward. But as we catch up and pass, that line swings backward against the distant stars, like a slower car on the highway that seems to slide back as you overtake it. Now drag the date toward February nineteenth, twenty twenty-seven. That's opposition, when Earth passes between Mars and the Sun. Mars is up all night, at its brightest, right in the middle of its backward loop. Try it yourself at Explainer Tools dot com.