A timelapse of Jupiter's moons Europa and Io orbiting the gas giant has been viewed widely on Reddit over the last few days, largely because of how plain awesome it is (in the true sense of the word, not the "well that was an awesome sandwich" way that the word has been used in recent years).
The timelapse is so good, in fact, that many commenters have suggested it can't be real.
While it's good to be skeptical about images shared online — particularly when fake images get circulated online as real for clicks — we are happy to confirm that the gorgeous short video is 100 percent legit.
Like so many other gorgeous views of the planets, the timelapse was made by Kevin M.Gill, a NASA-JPL engineer and expert processor of planetary images. He used images taken by the Cassini space probe on a flyby of Jupiter in 2001 (in order to gain a gravitational assist, while also imaging the gas giant and making other observations) before it headed on to Saturn.
The video shows Io and Europa orbiting their planet. One of the reasons people have been skeptical of the clip is that Io, which is closer to Jupiter, appears to be orbiting slower than Europa, which is further away. However, there's a simple explanation for this.
"The motion isn't wholly accurate as I made it to look prettier than it was correct," Gill explained on Twitter. "It's meant to portray the motion visible from a spacecraft that's moving at a velocity faster than the moons are orbiting. So, from a stationary perspective, Io would move faster than Europa."
Europa looks to be orbiting faster than Io because you are seeing it from the point of view of a spacecraft, such as Cassini, that is moving to the left. Since Europa is closer, it appears to be moving faster. This is a common illusion: things appear to move slower when they are further away because they seem to be smaller and so take longer to cross our vision.
Think of when you are on a highway, and look over at the cars going in the opposite direction. Behind the cars you see an airplane, also going in the same direction as the vehicles. From your perspective, the cars look like they are going faster than the aircraft, but I assure you, they are not.