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


This video takes us around the Moon and shows how it is illuminated not only by the brilliant light of the Sun but also by light reflected from the Earth. The trip starts on the side facing away from Earth where part of the surface is brightly illuminated by the Sun but the rest is totally dark. Moving around the Moon, the Earth rises, and its reflected bluish light illuminates the Moon’s surface. This dull glow is the earthshine. (You can clearly see it from Earth when the Moon appears as a crescent in the evening or morning sky.) When the Sun emerges from behind the Moon, the brilliant crescent is seen, but the earthshine is still faintly visible.

Video Credit: ESO

A Hole on the Sun

coronalholeThe Solar Dynamics Observatory took these images of a large coronal hole on the Sun last week. Coronal holes are the source of a high-speed wind of solar particles that streams off the Sun some three times faster than the normal solar wind. It’s not clear what causes coronal holes, but they correlate to areas on the Sun where magnetic fields flow away from the surface without looping back as they do elsewhere.

Image Credit: NASA

Lines of Force

magnetic_field_lines_2When we were in school, many of us saw that demonstration about a magnet’s lines of force using iron filings on a piece of paper covering a magnet. This illustration lays a depiction of the Sun’s magnetic fields over an image taken by the Solar Dynamics Observatory. Note how the magnetic fields are densest near the bright spots visible on the Sun’s surface. They are magnetically strong active regions, and many of the field lines link one active region with another.

Image Credit: NASA