Even in wetter climates, looming is normal at night, so that objects normally beyond the horizon in the daytime will routinely become visible at night if artificial light is available near them. This effect was used extensively in the 19th-Century Survey of India to extend baselines (in some cases, beyond 20 miles [32 km]).
At sea, or over lakes and rivers, the great thermal inertia of water allows inferior mirages to persist after sunset. Once again, quite distant land may be seen miraged, particularly hills or mountains well above the water.
Typical car headlamps produce about 1000 lumens — comparable to the total light output of a 60-W incandescent light bulb — but of course most of this is concentrated into a fairly narrow beam by the reflector and lens optics. The output beam of a car headlight covers roughly 0.1 steradian, so the concentration by the optics is on the order of a thousand. That means a car's headlights can be seen about 30 times as far away as an ordinary light bulb of the same power. If they're pointing right toward you, and the air is clear, you should have no trouble seeing car headlights a couple of hundred kilometers away on a dark night.
Furthermore, once the light is trapped in a duct, its irradiance falls off only inversely with the distance (apart from extinction), instead of with the usual inverse-square law; of course radiance is preserved, but the solid angle subtended by the magnified and multiple images of the source increases with distance to make up the difference. So it isn't hard to see artificial lights at distances exceeding 100 km, if the air is very clear.
A similar observation had been made by Joseph Huddart in 1797. He noticed that a lighthouse lamp appeared brighter when magnified by the inferior mirage.
At ranges of many kilometers, the image of a small light source is blurred and enlarged by turbulence in the intervening atmosphere. This blurring is only a few seconds of arc at distances of a few km, but can become a minute of arc or more at great ranges — large enough to give the light a perceptible angular diameter, even to the naked eye.
People easily become disoriented in the dark, and imagine that a perfectly stationary light is “moving” or even “jumping” around. I've had students tell me they saw stars moving in the sky, during constellation-teaching sessions with undergraduates. (These were identifiable “fixed” stars, not artificial satellites.)
The distorted or multiple image of a distant miraged light is likely to be unidentifiable. Observers unfamiliar with such phenomena often attribute supernatural properties to the “mysterious” images.
In areas where these phenomena are common, they have received local names.
There's an interesting experiment done in 2004 by the Dallas chapter of the Society of Physics Students, indicating that the lights are just miraged automobile headlights. A followup to this by Michael Hall in the June, 2006, issue of Texas Monthly agrees with them, and basically debunks the claims of pre-automotive observations.
And I've already referred to Ed Darack's 2008 article in Weatherwise on the Marfa Lights.
Wikipedia's article on “Will-o'-the-wisp” lists many more such phenomena.
In particular, there are sites on the Web that claim automobile headlights can't be seen beyond distances as short as four miles. Rubbish! Let me refer them to NOAA's website that tells some of the history of surveying — http://celebrating200years.noaa.gov/magazine/signals/welcome.html — in particular, the part that discusses the lights used in surveying by the U.S. Coast and Geodetic Survey:
Around 1900, bicycle acetylene lamps were used by the C&GS for the survey along the 98th meridian and were found useful for surveying distances up to about 34 miles. Larger acetylene automobile headlights were also eventually used successfully. Two of these automobile lamps stacked one above the other were observed at a distance of 133.9 miles at Pilot Peak, Nevada.
The development of the electric automobile headlight resulted in an excellent signal lamp. A bulb with a special filament was made at the request of the C&GS. In 1920, the light from two of these headlights was visible at a distance of 152.9 miles to an unaided eye (no telescope). By the mid-1920s, the electric signal lamp was the standard. …
… The signals described above allowed accurate triangulation methods to be used to survey the nation.
For readers accustomed to international units, 34 miles is about 55 km; 133.9 miles is over 215 km; and 152.9 miles is just over 246 km.
Modern headlights are considerably brighter than those used in 1920 — not to mention the older acetylene lights, which are compared with the electric ones in
E. G. Fischeras shown above the line in the table below:
“The new triangulation signal lamp of the U. S. Coast and Geodetic Survey”
Scientific American 114, pp. 454, 456 (April 29, 1916)
at 100 feet
|Luminous intensity (cd)|
|Ordinary headlamp||10.8 W||50 000||54|
|Special triangulation lamp||15 W||250 000||270|
|Regular sealed-beam||35 W||—||2 500|
|Halogen sealed-beam||35 W||—||27 000|
|Brightest legal headlight||—||—||150 000|
I've added modern automobile headlights below the horizontal lines in the table. Even ordinary low-beam lights are an order of magnitude brighter than the special lamps used by the Survey after 1920 — those special lamps that were visible to the naked eye at nearly 250 km.
Of course, when the source of the lights is obvious, there's nothing mysterious about them. For example, look at the mirages over Lake Ontario shown on Kerry-Ann Lecky Hepburn's web pages. Notice how much easier it is to see the lights at night than the corresponding buildings in the hazy daytime pictures, at a range of 53 km. Put those lights twice as far away, and they'd still be easy to see at night; but nothing would be visible in the daytime.
When the air is cold, dry, and clear, nocturnal mirages should always be considered as a likely explanation for “unexplained” lights seen near the horizon.
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