Saturday, November 17, 2012

Sunset from 35,000 feet!

Part of the Nightly Observing Program at Kitt Peak National Observatory has us leading the group up to a sunset vista.  Even on partly cloudy evenings there is usually something of interest to see.  While flying back from Hawaii recently at 35,000 feet on what seemed a flight that was twice as long as the trip west, we again saw a very pretty sunset - but with a twist.  While we've seen many sunsets while flying, we are always flying east or west, so we've never seen the sun directly!  But we can still see many of the effects of the sunset, this being no exception.

On the long flight, with them showing a movie we'd already watched on the outbound trip, I was doing some reading (Alan Alda essays!), mostly playing a zombie when I noticed an interesting glow from outside the plane.  The sun was still up, bathing the wing and engine cowling in a yellowish glow.  My camera was at my feet, so I wrestled it out and started taking pictures. 
The lighting changed pretty quickly - I could imagine the golden disk of the sun sitting on the hazy edge of the horizon.  As it sinks it gets redder as the light path travels a longer path through the atmosphere.  This photo, taken 5 minutes after the first one above, shows the reddening effect.  You can also see something new - at the left-side horizon there is a darker line above the horizon - the shadow of the earth!  From our elevation, we saw a different perspective than the setting sun, which is projecting the edge of the earth's surface into the atmosphere.
3 minutes later, and from our vantage point, it appears that the sun has set - no glow at all directly from the sun's disk.  But look at the shadow at left again - as the sun sinks, the shadow rises higher into the sky!  It is so very apparent because the shadow is projected into the lower part of the horizon where there is lots of haze and moisture to back scatter the light to our observing location.  As the shadow rises, it quickly rises into the higher atmosphere where there is less to scatter the light, and it soon fades away into the darkening sky.  You can also see the "Belt of Venus", the golden-orange glow atop the earth's shadow, caused by the reddened sun  - the edge of the shadow being caused by that same low sun that caused the color shift above.  We see these effects from the Kitt Peak sunsets at 7,000 feet, but a careful observer can likely observe them from nearly anywhere.  But they do seem sharper and better defined from atop the atmosphere where we were!

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