NASA’s SDO (Solar Dynamics Observatory) spacecraft captured this image of Earth and the Moon transiting the Sun together on September 13, 2015.
This image was taken by NASA’s Solar Dynamics Observatory at 3:12 a.m. EDT on September 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 extreme UV wavelengths of 171 angstroms. Though this light is invisible to our eyes, it is typically colorized in gold. Image credit: NASA / SDO.
Launched in 2010, SDO provides ultra high-definition imagery of the Sun in 13 different wavelengths, utilizing two imaging instruments.
Each wavelength is based on one or two types of ions – though slightly longer and shorter wavelengths produced by other ions are also part of the picture. Each wavelength was chosen to highlight a particular part of the Sun’s atmosphere, from the solar surface to the upper reaches of the Sun’s corona.
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 September 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, our planet blocks SDO’s view of the Sun for anywhere from a few minutes to over an hour once each day.
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.
However, the Moon has no atmosphere, so during the transit we can see the crisp edges of the Moon’s horizon.
