Highly Flattened Moon.  The full moon setting near Marseilles, France on January 11, ’09. Images by Alain Origne of Laboratoire Astrophysique Marseille (website, other OPODs 1,2). Images ©Alain Origne, shown with permission.

As the moon sinks towards the distant hills (the red and white mast is 24 km away) it becomes impossibly flattened. Ordinary atmospheric refraction flattens the sun and moon when close to the horizon but its extent is usually less than 20% whereas that here was nearly 40%. There is a second unusual feature, the direction of flattening is tilted and aligned roughly with the hills. See later for the third odd feature.

A temperature inversion, put simply (but not quite accurately) - unusually cool air beneath warmer, is responsible for the abnormal refraction. Mirage expert Andrew Young adds, "you need a fairly large inversion in lapse rate to produce such strong flattening.  Also, as the flattening begins rather suddenly about a degree above the sea horizon, it would appear that the isopycnic [same density] surfaces are bent over the lower hill in the intermediate distance."

The tilted air layers are visible in Alain's images and especially in the contrast enhanced frame at left. Inversions trap dust and pollution thus making the lower cooler air visibly more opaque and darker.
Hillside tilted air layers are known to give green flashes and a phenomenon called tobogganing where setting stars or planets appear to slide down the hill silhouette.

Normal atmospheric refraction (no inversion) can produce equal flattening but you need a high airplane or spacecraft to see it. At high altitudes rays are refracted twice, on entry into the atmosphere and again on leaving - see these ISS images.

The 3rd oddity? The lower coastline is mirror imaged. The highly stratified cold air of the inversion flowed out over a low layer of sea-warmed air to produce an inferior mirage.


About - Submit Optics Picture of the Day Galleries Previous Next Today Subscribe to Features on RSS Feed