The September 9-10 Double Lunar Transits

SDO will experience a double lunar transit on September 9 and 10, 2018. Here is a movie from the Flight Operations Team that shows SDO, the Sun, and the Moon during the transits. During the first transit, from 2030-2130 UT (4:30-5:30 pm ET) on September 9, the Moon will cover up to 92% of SDO's view of the Sun. The second transit lasts from 0152-0241 UT on September 10 (or 9:52-10:41 pm ET on September 9 for those of us on the East coast of the USA.) Only 34% of the Sun will be covered by the Moon.

The Moon moves from left to right during the first transit and right to left for the second. The first transit is caused by SDO overtaking the Moon as SDO moves in the afternoon part of its orbit. (SDO orbits over the Mountain Time Zone of the USA.) SDO's velocity of about 3 km/s is faster than the Moon's of 1 km/s and SDO overtakes and moves past the Moon-Sun line. The second transit happens after SDO has moved into the evening part of the orbit and is now moving mostly away from the Moon in SDO's orbit around the Earth. The Moon's velocity takes it past the Sun and the shadow appears to move from right to left.

During the total solar eclipse last year the Moon's shadow moved from the West coast of the US towards the East. This is because the speed of the rotation of the Earth (less than 0.5 km/s) is slower than the speed of the Moon. That means the motion of the Moon overtakes the motion of the Earth. The shadow follows the Moon and moves from West to East, like the second transit.

Although you can see the Moon throughout the movie SDO's instruments cannot see the Moon when it is not covering the Sun. The little white flash seen in the Moon is the word "Moon" being written by the software and then quickly covered. The boxes drawn around and on the Sun help the FOT run the spacecraft. The time is displayed in the lower left corner of the movie. The first seven numbers are the year (2018) and the day of year (252 and 253). The six numbers after the period are the hour, minutes, and second of UTC (2 numbers each).

When I first saw this movie I thought we were going to talk about retrograde motion. Other planets, especially Mars, move in retrograde as the Earth moves past them with our faster orbital velocity. But it isn't just that. The first transit is like retrograde motion as SDO passes by the Moon-Sun line with its faster velocity and the Moon appears to move backwards. But the second transit happens because SDO is moving mostly away from and a little in the opposite direction of the Moon.

This double transit shows how complicated the motions of objects can appear even as they move along simple orbits.

Enjoy!