selenelion

selenelion
Robert Jay GaBany · CC BY-SA 3.0 · source
Name	selenelion
Caption	Sunrise and moonset during a selenelion
Phenomenon	Atmospheric refraction during an eclipse
Typical location	Worldwide
Visibility	Near lunar eclipse at sunrise or sunset

selenelion A selenelion is an atmospheric and observational phenomenon in which the rising or setting Sun and the eclipsed Moon appear above the horizon simultaneously, producing a visually striking conjunction observed at sunrise or sunset. It involves precise geometry and atmospheric refraction that allow observers in locations across Greenwich Observatory, Mauna Kea Observatories, Mount Wilson Observatory, Paranal Observatory, and similar sites to witness both bodies at once, and has been recorded by expeditions associated with Royal Astronomical Society, Smithsonian Institution, United States Naval Observatory, Observatoire de Paris, and private observatories during coordinated campaigns.

Definition and terminology

The term denotes the apparent co-presence of the Sun and eclipsed Moon at opposite horizons during a lunar eclipse event monitored by organizations such as International Astronomical Union, American Astronomical Society, Royal Astronomical Society, and European Southern Observatory teams. Astronomers from institutions including Harvard-Smithsonian Center for Astrophysics, Max Planck Institute for Astronomy, Jet Propulsion Laboratory, and California Institute of Technology employ the term when reporting observations to journals like Nature (journal), Science (journal), Monthly Notices of the Royal Astronomical Society, and The Astrophysical Journal. Historical nomenclature appears in logs from expeditions sponsored by Royal Society, British Museum, National Geographic Society, and Royal Astronomical Society of Canada.

Causes and astronomical mechanics

A selenelion arises from the interplay of celestial geometry involving the Earth, Moon, and Sun during a total lunar eclipse, where the eclipsed Moon lies near opposition to the Sun while both bodies are near the horizon as predicted by ephemerides produced by Jet Propulsion Laboratory, United States Naval Observatory, European Space Agency, and NASA research groups. The primary mechanism is atmospheric refraction documented in studies from University of Cambridge, University of Oxford, Princeton University, Massachusetts Institute of Technology, and University of Chicago which bend sunlight and moonlight along lines modeled by teams at National Oceanic and Atmospheric Administration, Met Office (United Kingdom), and Météo-France. Computational models developed at Los Alamos National Laboratory, Argonne National Laboratory, Lawrence Livermore National Laboratory, and CERN simulate ray tracing and optical path length differences that permit an observer in locations such as New York City, London, Tokyo, Sydney, and Cape Town to see both bodies despite geometric horizon constraints. Factors include atmospheric temperature gradients measured by NOAA, aerosol loading studied by NASA Goddard Space Flight Center, and local altitude effects noted at Mauna Kea Observatories, Atacama Large Millimeter Array, and Arecibo Observatory.

Visibility and observational conditions

Visibility depends on horizon clarity and timing, factors that have guided observing campaigns by Royal Astronomical Society, International Dark-Sky Association, National Park Service, Society for Popular Astronomy, and university astronomy clubs at venues like Griffith Observatory, Kitt Peak National Observatory, Yerkes Observatory, Lowell Observatory, and Green Bank Observatory. Optimal conditions occur during low aerosol periods measured by Copernicus Programme, low humidity monitored by European Centre for Medium-Range Weather Forecasts, and minimal cloud cover forecasted by Met Office (United Kingdom), National Weather Service, and Météo-France. Geographic latitude influences the local geometry, affecting observers in regions including Iceland, Chile, South Africa, India, Alaska, and New Zealand, while timing is calculated using ephemerides from Jet Propulsion Laboratory and United States Naval Observatory. Observers from amateur groups such as Royal Astronomical Society of Canada, Astronomical Society of the Pacific, British Astronomical Association, and American Association of Variable Star Observers often coordinate reports.

Historical observations and cultural interpretations

Records of concurrent Sun and eclipsed Moon sightings appear in chronologies compiled by Babylonian astronomy scholars, Ptolemy-era texts preserved in Alexandria, observations noted by Tycho Brahe, Johannes Hevelius, and logs from voyages of James Cook, Lewis and Clark Expedition, and HMS Beagle. Cultural interpretations feature in accounts from Ancient Greece, Ancient Rome, Medieval Europe, Mughal Empire, Qing Dynasty, and indigenous narratives from Hawaiian Islands, Polynesia, Mesoamerica, Inca Empire, and Aboriginal Australians. Treatises by scholars at University of Bologna, University of Padua, Sorbonne University, Charles University in Prague, and University of Vienna discuss atmospheric optics leading to folkloric and scientific discourse recorded by Royal Society minutes and published in journals like Philosophical Transactions of the Royal Society.

Notable recorded selenelions

Notable modern reports were made during eclipses observed by teams from NASA, European Space Agency, Royal Astronomical Society, Astronomical Society of the Pacific, and International Astronomical Union field parties in locations such as Hawaii, Atacama Desert, Canary Islands, Chile, and Namibia. Historical accounts include entries in the logs of James Cook during Pacific voyages, records associated with Tycho Brahe in Prague, and descriptions in seventeenth-century publications by Christiaan Huygens, Edmond Halley, and Isaac Newton correspondences archived at Royal Society. Recent documented events were photographed and analyzed by observatories like Mauna Kea Observatories, Paranal Observatory, La Silla Observatory, Kitt Peak National Observatory, and university teams at Harvard University and Caltech.

Safety and photography considerations

Observers are advised to follow guidelines from Royal Astronomical Society, American Astronomical Society, International Dark-Sky Association, National Park Service, and National Weather Service when planning sunrise or sunset observations, and to use solar filters recommended by NASA and European Space Agency when photographing near-sunrise phenomena. Photographers from organizations such as National Geographic Society, BBC Natural History Unit, Smithsonian Institution, and freelance professionals often use telephoto lenses and calibrated exposure bracketing protocols developed at Massachusetts Institute of Technology, Rochester Institute of Technology, Royal Photographic Society, and Institute of Photography labs; local regulations from Federal Aviation Administration, Civil Aviation Authority (United Kingdom), and park authorities must also be observed.

Category:Astronomical phenomena