It will be 74 years tomorrow since Charles Lindbergh took off from Long Island in The Spirit of St Louis to make the first non-stop solo fight across the North Atlantic.
He was not the first to fly across that stretch of water, but his major achievement in May 1927 was that he made the journey on his own - and he flew all the way to Paris from New York.
He went solo simply to eliminate the extra weight that bringing the customary navigator would entail. He planned his flight all the way to Paris because he wanted to be the prizewinner of $25,000, which was on offer for the first person to make a non-stop flight between the two capitals.
Some 30 hours after take-off, when he estimated that he must be getting close to Europe, Lindbergh descended several hundred feet to get below a layer of cloud. He described the experience later in a book:
"The plane's shadow rushes in to meet me as I nose down closer to the waves. I last saw it centred in the rainbow, high up in the morning clouds. Such a small shadow, skipping from crest to crest, but losing itself in the troughs, seemingly fearful it won't catch up before I reach the surface."
Now when Lindbergh refers to the shadow that he last saw centred in the rainbow, he was not indulging his undoubted romantic streak, nor was he imagining a faint silhouette beneath some illusory multicoloured arc. He was referring to a glory.
A glory can be seen by any airline passenger whenever an aercraft flies in sunlight above a layer of relatively low cloud, a cloud composed of water droplets. The phenomenon appears as a series of concentric coloured rings centred, target-fashion, around the shadow of the aircraft projected on the cloud below.
The glory is not in any sense caused by the plane's shadow. The glory and the shadow are two distinct phenomena, but it so happens that both occur in the same place.
The glory is caused by a complex optical process called diffraction, where the water droplets of the cloud interfere with the direct progress of the tiny waves of sunlight. The light is split into its constituent colours - the familiar colours of the spectrum.
Glories vary considerably in size, depending on the radius of the cloud droplets. Indeed, as an aircraft proceeds along its track, the glory may sometimes be seen to expand and contract, as it reacts to the changing composition of the cloud-layer below. The smaller the water droplets in the cloud, the larger the diameter of the glory.