On March 6, 2019 the Moon started to transit the sun, but it doubled back and retraced its steps. Well that is what it looked like from NASA’s Solar Dynamics Observatory (SDO) that is in orbit around Earth.

The SDO does see regular lunar transits when the Moon passes in front of its view of the sun. The unusual apparent behavior of the moon in the transit is a phenomenon similar to retrograde motion: When a celestial object appears to move backwards because of the way that other objects move at different speeds at different points in their orbits.

For this transit, the first part the Moon moves left to right, it appears to be “reverse” motion. The SDO overtakes the Moon, moving at about 1.9 miles per second perpendicular to the Sun-Earth line compared to the Moon’s 0.6 miles per second, making the Moon appear to move in the opposite direction you would see if you were standing still on Earth.

The second part of the transit. The Moon looks to stop and then rewind. This happens as the SDO is entering the dusk part of its orbit and starts to move away from the moon. This is nearly parallel to the shadow that Moon is casting. This is the point that the Moon once again is moving faster than the SDO. This makes the spacecraft seeing it move in the opposite direction, the same direction that a stationary observer on Earth would see.

This is not the first time that the SDO has seen the Moon move in two directions in a lunar transit. The Moon just happened to stay in the SDO’s sight when it began the dusk part of the orbit, which lead to the freeze frame effect this time.

This lunar transit lasted about four hours, from 5 p.m. to 9:07 p.m. EST. At one point the Moon covered 82 percent of the Sun. The Moon’s edge appears sharp because the Moon has no atmosphere.

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