A time-lapse recording of the moons Europa and Io orbiting the Solar System’s giant planet Jupiter has been widely viewed on Reddit over the past few years, largely for its spectacular display. The time-lapse video offers such a good picture that many reviewers claim it can’t be true.
While it’s good practice to be skeptical about images shared online, especially on the internet where fake images are proliferating, we have to say that the amazing short video you’ll find below is 100 percent real.
Like many other spectacular images of the planets, this time-lapse was created by Kevin M.Gill, NASA-JPL engineer and expert processor of planetary images. Gill used photographs taken by the space probe as it passed by Jupiter in 2001 before Cassini went to Saturn.
The video shows Io and Europa orbiting the planet. Perhaps the most important of the reasons why people are skeptical of the clip is that Io, closer to Jupiter, seems to rotate more slowly than the farther Europa. But there is a simple explanation for this.
Gill says on Twitter, “The images depict motion visible from a spacecraft moving faster than the orbit of satellites. That is, Io would be moving faster than Europa if it were in a stationary perspective.”
Europa appears to orbit faster than Io because it is seen as the Cassini spacecraft sees it as it moves to the left. Europa seems to be moving faster because it is closer. This is actually a fairly common misconception: Objects appear to move more slowly as they move away because they appear smaller and therefore take longer to pass our view.
Imagine you are on a highway and look at cars going in the opposite direction. When you see an airplane behind the cars going in the same direction as the vehicles, from your point of view it seems like the cars are going faster than the planes, but in reality you can be sure that the plane is faster…
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