Institute of Astronomy

Flying near the edge of darkness

Published on 20/01/2014 
Question: 

On a flight to Greenland on the 18th  January 2014, we were approx 30 mins from Kangerlussauq, arriving at 9.40 local time.

When we looked out of the right hand window of the aircraft the sky above was blue - daylight - underneath that there was a layer of darkness where we could see the moon and a star and underneath that the 'land' which was a cloud layer.

Out of the left hand widows the sun was rising and looked normal.

Is that possible due to flying close to the 'night and day line' on the earth?

This sort of effect is possible when flying close to the terminator (the line between day and night).  At the time you were over Greenland and saw this effect I presume you were probably at cruising altitude 10-12km up.  Line of sight and the curvature of the Earth means that from that height you could see the Sun, although on the ground below the Sun would not yet have risen. Similarly in the sky on the 'nightward' side of the aircraft the Sun would have been shining high up in the atmosphere, but not nearer the ground, and being high up yourselves you could see the effect more clearly.  On the ground this is why the sky toward the horizon where the Sun has either recently set or is soon going to rise appears lighter.

Page last updated: 20 January 2014 at 10:44