Peak of Eternal Light

A peak of eternal light (PEL) is a point on an astronomical body within the Solar System that is always in sunlight. Such a peak must have high altitude and be on a body with very small axial tilt. The existence of such peaks was first postulated by Beer and Mädler in 1837. The pair said of the lunar polar mountains, "...many of these peaks have (with the exception of eclipses caused by the Earth) eternal sunshine." These polar peaks were later mentioned by Camille Flammarion in 1879, who speculated that there may exist pics de lumière éternelle at the poles of the Moon. PELs would be advantageous for space exploration and colonization due to the ability of an electrical device located there to receive solar power regardless of the time of day or day of the year, and the relatively stable temperature range.

The ESA SMART-1, NASA Clementine and JAXA SELENE spacecrafts made maps of the lunar poles that have been used to identify sites receiving eternal light. The orbiting craft monitored the lighting at the poles and looked for seasonal variations as well as mapping the relief so that peaks of eternal light could be identified topographically.

Peaks of eternal light on the Moon would not be perfectly "eternal", since sunlight would still be cut off occasionally by the Earth's shadow during a lunar eclipse (which can last up to six hours) and by the shadows of other mountains and plateaus. The term "peak of eternal light" for the Moon is commonly used in the technical literature and newspaper articles as a popularization and is surprisingly applied even though the duration of illumination is not permanent, excluding the lunar eclipses. No peaks of eternal light have been positively identified on the moon, but many peaks have been detected that, via simulations based on imaging and laser and radar topography, appear to be illuminated for greater than 80% of a lunar year. Of course the concept of "eternity" in this case lasts only during the lifetime of the Sun (roughly 10 billion years) and will end when the Sun runs out of fuel and becomes a red giant, which will likely absorb and destroy the Moon, or else end when the Sun becomes a white dwarf and no longer produces enough light to directly illuminate the lunar surface.

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