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The Difference Between Earth and Moon, in Two Simple Images

Out There iconOut There
By Corey S Powell
Mar 12, 2013 11:52 PMNov 20, 2019 3:41 AM


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NASA's Solar Dynamics Observatory has been monitoring the sun in detail since its launch in February 2010. Its primary goal is to understand solar variability and improve our ability to forecast it, a capability that has the potential to avert serious blackouts, satellite damage, and interruptions to GPS and radio communications. As a side benefit, though, the observatory produces some great scientific art--like this lovely pair of solar eclipse images.

NASA’s Solar Dynamics Observatory observed the moon partially eclipsing the sun on March 11 at 8:00 a.m. EDT. (Credit: NASA/SDO) The first shows the sun blotted out by the moon. Note the size of the moon, defined by its curved outline, and the crisp edge. That sharpness is what you see when an airless body passes in front of the sun. All you get is the hard shadow of light blocked by a big spherical ball of rock. On this scale, all the craters and mountains on the moon are insignificant compared to the moon's overall tidy roundness. The Solar Dynamics Observatory circles 23,000 miles from the Earth (a distance at which it orbits our planet exactly once a day) and about 215,000 miles from the moon in this image. The moon therefore looks roughly the same size as the sun. In reality, it is 400 times smaller, but also more than 400 times closer.  Now look at the second one, of the sun eclipsed by Earth. The curvature is much gentler, for two reasons. First, the Solar Dynamics Observatory is just much closer to Earth than it is to the moon. Second, Earth is nearly four times the moon's size: 7,913 miles versus 2,160 miles in diameter. But the more dramatic difference is that the outline of Earth is decidedly blurry and soft. What you are seeing is the effect of atmosphere: clouds, dust, and above all refraction. Air acts as a lens, bending light. That is what makes stars twinkle at night. That is what makes the setting sun look ruddy and distorted. Here, the refractive properties of the atmosphere--which get more intense closer to the surface, where the air is denser--makes Earth's edge very soft. In a sense, the sunlight is twinkling through the atmosphere from the perspective of the Solar Dynamics Observatory.

Earth passed in front of the sun as seen by NASA’s Solar Dynamics Observatory on March 11 at 2:20 a.m. EDT. The color here is artificial; the image was taken in the extreme ultraviolet band. (Credit: NASA/SDO) Essentially all of the biology on our planet is sustained by that little bit of blur. The oxygen we breathe, the cycling of water from the ocean to rainfall and back, and the protective ozone layer all lie within that thin layer. The difference between blurry and crisp is the difference between a living and a dead world. Incidentally, you'll also notice that the sun looks a lot different in these images than it does when you look up at the sky. The Solar Dynamics Observatory has three instruments. One measures magnetism. One measures the sun's ultraviolet output. And one--the Atmospheric Imaging Assembly, which produced the views here--snaps pictures of the sun mostly in ultraviolet rays, which are not sensible to the human eye. Ultraviolet images capture details of solar activity that do not show up in visible light. The images are then colorized to make them easier to understand and, yes, to make them prettier. Together, the observatory's three instruments gather 150 million bits of information every second, making it the most data-intensive spacecraft NASA has ever created. Follow me on Twitter: @coreyspowell

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