Is it possible to distinguish sunrise from sunset from an image (i.e. with no knowledge of the direction in the sky or time of day, and in a time frame with no discernible solar motion)?
Keith Macpherson
Clevedon, Somerset, UK
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I sympathise with the questioner. I was an airline pilot for 30 years and always observed that, seen from above, the divide in the atmosphere between day and night 鈥� known as the 鈥搘as more distinct at sunset than at sunrise. Why that should be, I don鈥檛 know, unless it was purely in my mind.
Spencer Weart
Hastings-on-Hudson, New York, US
In a large sample of photos, sunsets will, on average, be redder than sunrises because, on average, the air holds more moisture after the warmth of the day.
Garry Trethewey
Arkaroola, South Australia
In semi-arid South Australia, mornings are generally whiter, perhaps due to higher relative humidity causing a slight mist, alongside less wind overnight.
Evenings are generally redder, with afternoon winds putting more dust into the air.
Richard Bradford
Chesterfield, Derbyshire, UK
Yes, there are visible differences between a sunrise and a sunset.
At dusk, the air above the ground is warmer and rises with small but noticeable variations in its refractive index. These variations bend the light and distort the edges of regular shapes. This is most noticeable in distant objects and is beloved by some makers of moving images.
In addition, activity during the day tends to stir up dust and release more pollutants. The result of this is to make sunsets warmer, since blue light is more easily scattered. It isn鈥檛 for nothing that we talk of the 鈥渃old light of dawn鈥�.
The differences mentioned here will be most apparent when there is sunlight and at least some blue sky; a completely cloudy sky tends to be grey whatever the time of day.
Eric Kvaalen
Les Essarts-le-Roi, France
Under certain circumstances, no. If you are just south of the Arctic circle on the winter solstice, the sun rises and then immediately sets. There can鈥檛 be a difference between these two events.
David Muir
Edinburgh, UK
The axial tilt of the sun is 7.25 degrees and that of Earth is 23.44 degrees, very roughly in the same direction. So, if you can identify the sun鈥檚 north pole, you can get an approximation of the direction of Earth鈥檚, which would be to the right or left as you face the rising or setting sun.
If geographical north is to the left, you are facing east and a rising sun; if geographical north is on your right, you are facing west and a setting sun.
But how to discern the solar north pole? You need a current sunspot map, readily available online, and a method of detecting sunspots, such as projecting the sun鈥檚 image through a telescope or binoculars onto a white screen: a paper plate might work for this. Whatever you do, don鈥檛 look directly at the sun.
John Welford
Barlestone, Leicestershire, UK
Have a look at J. M. W. Turner鈥檚 magnificent painting The Fighting Temeraire, in which the ship of that name is shown being towed up the river Thames on its final journey to a scrapyard.
The right-hand side of the painting depicts a splendid sunset 鈥� what else could it be? But the direction is all wrong because that would have to be the east, given what is depicted on the left-hand side. So, is that a sunrise? I would defy anyone to claim that is so. Instead, Turner is making a symbolic point 鈥� this is the sunset of a great ship.
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