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NASA’s SDO Catches a Double Photobomb

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Vanessa Thomas

The headshot image of Sarah Frazier

Sarah Frazier

Sep 14, 2015
Article
On Sept. 13, 2015, NASA’s Solar Dynamics Observatory captured images of Earth and the Moon as they crossed the Sun’s bright face. Just before the Moon entered the shot, Earth blocked SDO’s view completely. By the time SDO had a clear view again, the Moon was just completing its journey across the Sun’s face.
Credits: NASA Goddard’s Scientific Visualization Studio/SDO
Download this video in HD formats from NASA Goddard’s Scientific Visualization Studio

On Sept. 13, 2015, as NASA’s Solar Dynamics Observatory, or SDO, kept up its constant watch on the Sun, its view was photobombed not once, but twice. Just as the Moon came into SDO’s field of view on a path to cross the Sun, Earth entered the picture, blocking SDO’s view completely. When SDO’s view of the Sun emerged from Earth’s shadow, the Moon was just completing its journey across the Sun’s face.

This animation shows the relative movement of Earth and the Moon as they both crossed SDO’s field of view on Sept. 13, 2015. Just as the Moon came into SDO’s field of view on a path to cross the Sun, Earth entered the picture, blocking SDO’s view completely. When SDO’s orbit finally emerged from behind Earth, the Moon was just completing its journey across the Sun’s face.
Credits: NASA/SDO

Though SDO sees dozens of Earth eclipses and several lunar transits each year, this is the first time ever that the two have coincided. This alignment of the Sun, Moon, and Earth also resulted in a partial solar eclipse on Sept. 13, visible only from parts of Africa and Antarctica.

SDO’s orbit usually gives us unobstructed views of the Sun, but Earth’s revolution around the Sun means that SDO’s orbit passes behind Earth twice each year, for two to three weeks at a time. During these phases, Earth blocks SDO’s view of the Sun for anywhere from a few minutes to over an hour once each day. 

The Sun appears as an incomplete crescent, with the top and left parts of the Sun obscured. The Moon covers about a third of the Sun on the left and creates a sharp boundary along its edge. The top part of the Sun is missing, but the edge of the missing section appears fuzzy.
NASA’s Solar Dynamics Observatory captured this image of Earth and the Moon transiting the Sun together on Sept. 13, 2015. The edge of Earth, visible near the top of the frame, appears fuzzy because Earth’s atmosphere blocks different amounts of light at different altitudes. On the left, the Moon’s edge is perfectly crisp, because it has no atmosphere. This image was taken in an extreme ultraviolet wavelength of 171 angstroms. Though this light is invisible to our eyes, it is typically colorized in gold.
Credits: NASA/SDO

You may notice that Earth’s outline looks fuzzy, while the Moon’s is crystal-clear. This is because—while the planet itself completely blocks the Sun’s light—Earth’s atmosphere is an incomplete barrier, blocking different amounts of light at different altitudes. On the other hand, the Moon has no atmosphere, so during the transit we can clearly see the crisp edges of the Moon’s horizon.

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Sarah Frazier
NASA’s Goddard Space Flight Center, Greenbelt, Md.