Why the rising Sun looks flat

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Tony Rice
, NASA Ambassador

Meteorologist Zach Maloch captured this morning's gorgeous sunrise from cameras atop the tower atop the WRAL studios on Western Boulevard.  The image shows off the Raleigh skyline but also demonstrates a lot of what our atmosphere does to light as it passes through.

Near the horizon, the Sun looks a bit squashed. The rising full Moon looks the same way, for the same reason.

When you look straight up, you are looking through one air-mass, about 300 miles of air. At the horizon, we are looking through more than 38 air masses.  All that air bends the light, especially at near the horizon where the atmosphere is at its densest. 

Our atmosphere bends light from the Sun (and Moon) at the horizon as light passes through it.

This flattens the image by about 20%, creating a flattened Sun. Actually the light is bent so much, the Sun is actually physically below the horizon, but about half a degree. Sailors have been aware that, when conditions are right, they might see a ship which is geometrically below the horizon. 

The effect is greater as the temperature drops and/or air pressure increases.

The United States Naval Observatory's tables for sunrise and sunset takes atmospheric refraction using 34 arcminutes (about half a degree) as an average. Again the actual amount of refraction varies by the atmospheric conditions at the time (temperature, pressure) between us and the horizon.

The changing color of the Sun

The Sun viewed in many wavelengths by NASA's Solar Dynamics Observatory (Credit: NASA/SDO)

This morning's yellow-orange sunrise was created as short-wavelength colors like green and blue are scattered out by the atmosphere while longer wavelengths on the red end of the spectrum make it through to our eyes.

The sun actually emits energy at all wavelengths from radio to gamma ray. Scientist use instruments aboard spacecraft like NASA's Solar Dynamic Observatory (SDO) to study different of the Sun's atmosphere producing artificially colored images like the ones above. 

What color is the Sun really?

When astronauts see the Sun from above the atmosphere, all those wavelengths in the visible part of the spectrum are combined together and our brains tell us its white. Instruments aboard SDO tell us the Sun emits a little more visible light around the 500 nm wavelength.

Which means you could say our sun is actually a shade of blue-green.


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