It was a hazy evening at Pune. At the time of take off, the Sun was not shining as bright as on a clear day. With the scheduled sunset in Pune at 6.16 pm, the Sun was already reddish due to the haze. After 5-10 minutes of take off, the haze had gone and there it was, a crystal clear blue sky with the Sun shining as bright as at 11 am on the ground!
I was eagerly waiting for the sunset to happen, which finally occurred at about 6.27 pm. But, even five minutes before that, it was very bright. The Sun disc turned a little bit orange in colour and went into the horizon within 2 minutes. As seen from the Earth’s surface, five minutes before sunset, the Sun will not be that bright, and it will be little larger in size and red in colour. The Sun disc also takes about 3-4 minutes to go down completely into the horizon, as observed from the Earth’s surface.
On the one side, it was twilight and a little dark right below on the ground and on the other side, beautiful sunset was occurring far away in the horizon. After the sunset, I was also fortunate to sight planet Budha (Mercury), which one can rarely sight from the Earth’s surface.
For Pune (73° 57° N, 18° 29° E), the scheduled sunset on Jan 12, 2008 was at 6.16 pm IST. Similarly, for Bangalore (77° 40° N, 12° 54° E), the scheduled sunset was at 6.10 pm IST. But at a latitude/longitude somewhere in-between, how can I see the sunset at 6.27 pm? To understand this, I checked with the captain of the flight about the possible latitude/longitude/altitude of the aircraft at 6.27 pm IST. The altitude was at about 10,660 m.
The captain asked me to take the in-between value of the latitude and longitude of Pune and Bangalore as a very good approximation. So, at 6.27 pm IST, the aircraft can be estimated at about (75° N, 15° E), somewhere over the Hubli-Gadag region, with an altitude of around 10 km.
Later on, I did some trigonometric calculations. The calculations revealed that from an altitude of about 10 km, and with the Earth radius of about 6300 km, one will be able to see the horizon of a place about 3° West in longitude (72° N, 15° E). I verified that for this latitude and longitude, the sunset will do occur at about 6.27 pm IST.
The fast setting of the Sun disc can also be explained due to very less atmospheric refraction and other effects at 10 km altitude. In terms of km, every degree of longitudinal movement at 15°N Latitude is about 100 km. So, I was observing the glorious sunset occurring at a place about 300 km West, perhaps somewhere 100 km off the Goa coast.
If you are flying at an altitude of about 10 km (35,000-40,000 ft) over the Indian region during sunrise or sunset, you are able to observe the sunrise or sunset occurring at a place about 300 km away! “Its an early sunrise and a late sunset.”
Now I understand the fact “the horizon expands at higher altitudes from the surface of the earth” in a better way.
