Astronomical Refraction

By Dan Briscoe

February 15, 2019

In the picture above the sky was darkened for artistic value and to remove the distracting hazy sunset.  Location: Ocean Shores, WA. September 22, 2018 at 7:12:11 PM

We have all seen refraction when a stick is placed in the water at an angle and the stick appears to bend at the point where it enters the water.  Rainbows are another example of refraction where light is separated and refracted at different angles depending on its wavelength. We see refraction in many forms.

Astronomical Refraction, also known as Atmospheric Refraction, is when a celestial body (the sun, stars, moon, or planets), appears to an observer on earth to be in a slightly different position than its actual geometric position. The actual position of the sun when this picture was taken was below the horizon, or nearly so. When a celestial body is low on the horizon, the light from it has to travel through the Earth’s atmosphere, which causes it to be refracted or bent. Because of the low position in the sky, the light must pass through much more of the Earth’s atmosphere than when the object is higher in the sky.

Astronomical Refraction is greatest at the equator. On a typical day, the setting sun is visible 2 minutes after it has geometrically passed below the horizon. Humidity, temperature, air pressure, and air pollution affect the refraction properties of the atmosphere.

The picture above was taken 2 minutes and 5 seconds before the upper lobe of the sun disappeared below the horizon.  The picture below was taken just as the lower lobe touched the horizon at 7:11:11 PM PDT, September 22, 2018, exactly one minute before the top photo was taken.  As the sun passes further below the horizon, the silhouettes of the clouds across the face of the sun began to get larger. They didn’t appear prior to the sun setting, but may have been lost in the haze. Cloud silhouettes appear larger as the sun sets; the clouds in this image are actually beyond the horizon and their image is being refracted along with the light of the sun.

Location, Ocean Shores, WA.  Lower lobe touched the horizon at 7:11:11 PM PDT, September 22, 2018. Refracted cloud silhouettes starting to appear across the face of the sun.

Navigators have long used sextants in celestial to measure the altitude of a celestial body relative to calculate their position on the surface of the earth.  . A sextant is a very precise optical device used to measureing degrees, minutes (60th of a degree), seconds (60th of a minute), and fractions of a seconds.  along with Geographical position can also be calculated using very precise time clocks and astronomical almanacs. geographical position can be calculated.  Yes, e Even with modern day navigation systems like GPS, celestial navigation is still used as a backup by ocean going sailors, but due. Due to refraction, celestial sights (measurements with a sextant) are not considered reliable or used  for measuring the position of when celestial bodies when they are low on the horizon.

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