SDO Sees the Moon Transit the Sun. Twice.

On 9 and 10 September, 2018, the Solar Dynamics Observatory, SDO, saw two lunar transits as the Moon passed in front of the Sun. A transit happens when one celestial body passes between another and an observer. This first lunar transit lasted one hour, from 2030 to 2130 UTC and covered 92 percent of the Sun. The second transit happened several hours later, 0152 until 0241 UTC and only obscured 34 percent of the Sun at its peak.

From SDO’s perspective, the Moon seems to move in one direction and then double back. It appears to do so because the spacecraft’s orbit catches up and passes the Moon during the first transit.

Image Credits: NASA

Dark Side of the Moon

Whatever your point of view, the dark side of the Moon is the one facing away from the Sun. Yesterday, beginning at 13:31 UTC, the Moon moved between the Solar Dynamics Observatory and the Sun. The satellite had a view of a partial solar eclipse from space. These a lunar transit occur two or three times each year. Yesterday’s lasted two and a half hours, the longest thus far. When the next one will occur will be determined after planned adjustments in SDO‘s orbit.

Notice how crisp the horizon is on the Moon. The Moon has no atmosphere around it to refract the light from the Sun.

The movie shows the Sun seeming to wobble quite a bit. The fine guidance systems on the SDO instruments need to see the whole Sun in order keep the images centered, but the Moon blocks too much light during the transit. Once the transit is finished, the fine guidance systems come back online to provide steady images of the Sun.


Video Credit: NASA