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NASA Satellite Captures Optical Illusion Where the moon is visible to go backwards

The Moon seems to change the direction of this moon transit captured by the SDO on March 6, 2019. GIF: NASA / Goddard / SDO / Gizmodo Earlier this week, NASA's Solar Dynamics Observatory (SDO) captured a strange series of images where the moon seems to be villain, crosses the sun as usual but then briefly pauses to change direction. No, it is not a sign of cosmological armageddon but an optical illusion known to astronomers. SDO is currently in orbit around the world, and it is not uncommon for the NASA satellite to capture a lunar transit. However, the one observed on March 6, 201 9 was unusual, as the moon seemed to move from left to right across the surface of the sun (normally), but then it stopped for a while and reverse (definitely not normal). It is certainly not the kind of behavior you can expect to see from the moon – an object that is locked in an unchanging, one-way orbit around the Earth. But there is a perfect rational explanation for this event: retrograde motion. This optical illusion happens when "a celestial object seems to move backwards because of the way different objects move at different speeds at different points in their paths," according to NASA. Writing in EarthSky last year, astronomer Christopher Crockett described retrograde movement this way: You can test it yourself, the next time you pass a car on the highway. As you approach a slower car, you are clearly moving in…

The Moon seems to change the direction of this moon transit captured by the SDO on March 6, 2019. GIF: NASA / Goddard / SDO / Gizmodo

Earlier this week, NASA’s Solar Dynamics Observatory (SDO) captured a strange series of images where the moon seems to be villain, crosses the sun as usual but then briefly pauses to change direction. No, it is not a sign of cosmological armageddon but an optical illusion known to astronomers.

SDO is currently in orbit around the world, and it is not uncommon for the NASA satellite to capture a lunar transit. However, the one observed on March 6, 201

9 was unusual, as the moon seemed to move from left to right across the surface of the sun (normally), but then it stopped for a while and reverse (definitely not normal). It is certainly not the kind of behavior you can expect to see from the moon – an object that is locked in an unchanging, one-way orbit around the Earth.

But there is a perfect rational explanation for this event: retrograde motion. This optical illusion happens when “a celestial object seems to move backwards because of the way different objects move at different speeds at different points in their paths,” according to NASA. Writing in EarthSky last year, astronomer Christopher Crockett described retrograde movement this way:

You can test it yourself, the next time you pass a car on the highway. As you approach a slower car, you are clearly moving in the same direction as you are. When you pull along the side and send it, the car still seems to go back in an instant. Then the car emerges as it continues forward.

The same thing happens when the earth passes the slower outer planets. When we pass Jupiter or Mars or Saturn, for example, these more outgoing orbits – moving more slowly than the Earth in orbit – seem to turn the course into our skies for a few months.

In this case, the event lasted for a few minutes, rather than a few months, due to SDO’s proximity to the moon. So, instead of the Moon’s changing direction, it is the changed viewpoint of the SDO because it revolves around the earth that caused the visual effect. An animated GIF provided by NASA illustrates this beautifully.

The frames used to produce the moon transit video were captured as the satellite entered the Moon’s shadow transported by the vertical black line – with the sun as the background. At that time, the SDO moved about 1.9 miles per second, as opposed to the moon’s 0.6 miles per second, allowing the satellite to catch up with the moon’s shadow.

As an interesting side blow, retrograde motion stir up early astronomers who believed that the earth was in the middle of the universe. To explain the faults of the planets that were likely to circulate around the earth, these astronomers designed all kinds of explanations, including the idea that the planets rotated around a point of motion in their orbit, in addition to rotating around the earth. It was not until the assumption of heliocentrism – the acknowledgment that the planets rotated around the sun – that the apparent back and forth movement of the outer planets was finally understood.

[NASA]

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