A timelapse of Jupiter's moons Europa and Io orbiting the gas giant has gone viral on the internet lately, owing to its absolute awesomeness. Many users have stated that the timelapse isn't real because it's so good.
While it's important to be suspicious of photographs posted online, especially when fraudulent images are distributed as real for clicks, we're delighted to report that the lovely little video is 100 percent real. Snopes did the fact check as well, you an see that here.
This amazing videos was actually made by Kevin M.Gill. M.Gill is a NASA-JPL engineer and expert processor of planetary images. He made this video by using many images taken by the Cassini space probe on a flyby of Jupiter back in 2001. (Yes none of these images or part of the video is computer generated).
Io and Europa are seen circling Jupiter in the footage. One of the reasons for the clip's skepticism is that Io, which is closer to Jupiter, looks to orbit slower than Europa, which is farther away. There is, however, a straightforward reason for this.
People seemed to like the Europa/Io/Titan gifs, so as an experiment I went and made a short video of them. The Voyage of the Moons.
— Kevin M. Gill (@kevinmgill) October 22, 2018
Image data via @CassiniSaturn https://t.co/8SLjERSRWc pic.twitter.com/kvznw9ck6J
Kevin M.Gill explains: "The motion isn't wholly accurate as I made it to look prettier than it was correct. 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 seems to be circling faster than Io because you are viewing it from the perspective of a spacecraft travelling to the left, such as Cassini. Europa looks to be travelling quicker now that it is closer. This is a frequent illusion: objects appear to move slower when they are farther away because they appear to be smaller and hence take longer to pass through our eyes.
Consider being on a freeway and looking across at the automobiles travelling in the opposite direction. Behind the autos, you can see an aircraft flying in the same direction as the cars. The automobiles appear to be travelling quicker than the aeroplane from your vantage point, but they are not.
You can check out more of his work here.