NASA’s Solar Dynamics Observatory (SDO) recently captured a series of images taken on March 6, 2019, which have since been turned into a video. The clip shows the moon, as it appears to cross the face of the sun before doubling backward, almost as if it had changed direction.
As Gizmodo reports, as bizarre as NASA’s images of the moon may appear to be, what they are really showing is an optical illusion that astronomers actually understand quite well.
Because NASA’s SDO is currently in orbit around the Earth, it is actually quite common for this satellite to witness the moon making a lunar transit. However, what was so strange about the most recent transit (which was captured on camera) is that the moon appeared to be traveling normally across the sun’s face before it suddenly — and without warning — swung back and moved in the opposite direction.
But as strange as these images and video may look, astronomers have explained that the moon’s appearance can be attributed to retrograde motion. According to NASA, an optical illusion — like the one witnessed with the moon at the beginning of March — normally occurs when “a celestial object appears to move backwards because of the way that different objects move at different speeds at different points in their orbits.”
Watch the Moon Yo-Yo Across the Sun in This Bizarre Video https://t.co/qjAHuC5GtE
— SPACE.com (@SPACEdotcom) March 9, 2019
Astronomer Christopher Crockett also discussed what is actually the very normal phenomenon of retrograde motion, when he described how this action could be tested when passing cars on the road.
When quickly overtaking a slower car, for a brief moment this car may look as if it traveling backwards, rather than moving forward as it actually is. However, once the car has been fully overtaken and passed, it will once again appear to be moving forward behind you. Crockett explained that a similar phenomenon occurs when planets pass by each other.
“The same thing happens as Earth passes the slower-moving outer planets. When we pass Jupiter or Mars or Saturn, for example, these more outward planets in orbit—which move more slowly than Earth in orbit—appear to reverse course in our sky for a couple of months.”
However, with the SDO, astronomers have noted that the moon’s apparent trajectory is related to orbit size rather than speed. Because the SDO is in orbit around the Earth — and much closer to us than it is to the moon — every once in a while, the SDO and the moon align.
Because of this optical illusion, rather than the moon actually moving backwards against the face of the sun, what is really being demonstrated is the SDO doubling back as it orbits the Earth.
So while the NASA’s SDO images — which were captured in March over a period of four hours — may have eerily shown what looks like the moon reversing and doubling back over the face of the sun, this was merely an optical illusion, and a rather common one, it seems.