Peak of eternal light
A peak of eternal light (PEL) is a point on a body within the Solar System which 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 saying with respect to 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.
On the Moon 
From 2005 to 2006 the ESA SMART-1 spacecraft made a systematic search of the lunar poles to more concisely identify sites receiving eternal light. The orbiting craft monitored the lighting at the poles and looked for seasonal variations, in order to attempt to confirm that they remained illuminated during their lunar winter. Confirmed sites may serve as targets for future landing craft that will utilize the steady lighting and temperature conditions for long-duration missions.
Peaks of eternal light on the Moon would not be perfectly "eternal", since sunlight would still be cut off occasionally by Earth's shadow during a lunar eclipse (which can last up to 6 hours). 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 lunar peaks with eternal light have been found and none seem likely based on the use of radar digital elevation model simulations.
Lunar north pole 
Based on images from the Clementine lunar mission, a team from Johns Hopkins University determined that four locations along the rim of the crater Peary are candidates for peaks of eternal light. This crater lies near the north pole of the Moon. Clementine's images were taken during the northern lunar hemisphere's summer season and were unable to confirm whether these four mountains were shaded at any point during their local winter season.
Lunar south pole 
The lunar south pole is situated in a huge depression (leading to 16 km altitude differences over the region). Careful analysis of imagery and topographic conditions on the lunar South Pole by teams from NASA and Europe revealed a small number of illuminated ridges within 15 km from the pole, each of them much like an island of no more than a few hundred meters across in an ocean of eternal darkness, where a lander could receive near-permanent lighting (~70–90% of time in lunar winter, likely 100% in lunar summer).
The Malapert Mountain region, on the rim of the crater Malapert 122 km from the lunar south pole on the Earth-facing side, may also have high levels of illumination, but available data are not sufficient to determine this. Considering its distance from the pole it can be calculated that its peak is unlikely to be permanently lit.
On Mercury 
The existence of peaks of eternal light on Mercury has also been theorized, but due to the lack of detailed mapping, no Mercurian peaks have been positively confirmed or ruled out as such. This may change when the data from the orbiter MESSENGER is fully analyzed. Such peaks would not even suffer the sporadic shadow of an eclipse, as Mercury has no moons.
- Beer, Wilhelm, Mädler, Johann Heinrich. Der Mond nach seinen kosmischen und individuellen Verhältissen oder allgemeine vergleichende Selenographie. Berlin, Simon Schropp and Co., 1837.
- Flammarion, Camille. Astronomie Populaire, description générale du ciel. Paris, 1879. 
- Kruijff, M., The Peaks of Eternal Light on the Lunar South Pole: How they were found and what they look like, 4th International Conference on Exploration and Utilization of the Moon (ICEUM4), ESA/ESTEC, SP-462, September 2000.
- Koschny, Detlef; Grieger, Björn. "Taking a SMART sidelong look at Peak of Eternal Light". Europlanet Research Infrastructure. Retrieved 2009-09-24.
- The Peak of Eternal Light video by the European Space Agency.