611 articles
Natural satellite
A natural satellite, also called a planetary satellite or moon, is a celestial body that orbits a planet, dwarf planet, or other larger non-stellar body.
Navaratna (नवरत्न)
Navaratna (नवरत्न), also called Nabaratna, is an Indian jewelry arrangement of nine gemstones, each corresponding to one of the nine celestial bodies of Vedic astrology.
Nebular hypothesis
The nebular hypothesis holds that stars and planets form from the gravitational collapse of a rotating cloud of gas and dust; planets accrete from the protoplanetary disk.
Nemesis (hypothetical star)
Nemesis, or the Nemesis star, is a hypothetical dim companion of the Sun proposed in 1984 to explain a 26-million-year cycle in mass extinctions; infrared surveys have ruled it out.
NEOWISE
NEOWISE, the Near-Earth Object Wide-field Infrared Survey Explorer, was NASA's asteroid-hunting reactivation of the WISE infrared telescope, which detected over 3,000 near-Earth objects from 2013 to 2024.
Neptune
Neptune is the eighth and most distant planet of the solar system, an ice giant found in 1846 through mathematical prediction and flown past by Voyager 2 in 1989.
Nereid (moon)
Nereid, also known as Neptune II, is the third-largest moon of Neptune, discovered by Gerard Kuiper in 1949 and famous for one of the most eccentric orbits of any moon.
Neso
Neso (Neptune XIII, S/2002 N 4) is a small retrograde irregular moon of Neptune discovered in 2002 with the Blanco telescope. It orbits about 48.4 million km out with a 9,374-day period, eccentricity 0.495, and may be a fragment of a broken larger moon.
New Frontiers program
The New Frontiers program is a NASA planetary science program of medium-sized, PI-led spacecraft missions, initiated in 2003; its missions include New Horizons, Juno, and OSIRIS-REx.
New moon
The new moon is the lunar phase in which the Moon lies in the same direction as the Sun, so it rises and sets with the Sun and is invisible from Earth.
Nibiru cataclysm
The Nibiru cataclysm, also called the Nibiru collision, is a claimed doomsday scenario in which planet Nibiru destroys civilization; astronomers say no such object exists.
Nice model
The Nice model is a 2005 scenario in which the giant planets formed close to the Sun and migrated to their present orbits via planetesimal-driven gravitational scattering.
Numerical model of the Solar System
A numerical model of the Solar System is a computer integration of the gravitational equations of motion for the Sun, planets, and other bodies, used to produce ephemerides for spacecraft navigation.
Occultation
An occultation is the passage of one celestial body in front of a more distant one, hiding it from view, used to probe planetary atmospheres and discover rings.
Occultation observer networks
Occultation observer networks are organized groups of telescopes, often run by volunteers, spread along a predicted occultation path to measure asteroid and Kuiper Belt object shadows.
Occultation observing techniques
Occultation observing techniques are the methods used to record a star's light being blocked by the Moon or an asteroid, using high-resolution detectors and accurate GPS timing.
Occultation prediction
Occultation prediction is the computation of when and where a star's light will be blocked by the Moon, an asteroid, or another Solar System body.
Occultations by planets and satellites
An occultation by a planet or satellite occurs when the body passes in front of a star, briefly blocking its light; the dimming reveals atmospheres and rings.
Occultations in the history of astronomy
Occultations in the history of astronomy traces discoveries made when one celestial body hides another: six of eight known dense ring systems, Pluto's 1985 atmosphere discovery, and ancient Chinese records.
Occultations of stars by asteroids
Occultations of stars by asteroids occur when an asteroid briefly blocks a star's light. Observers along the shadow path measure asteroid sizes and shapes with kilometric precision.
Oligarchic growth
Oligarchic growth is a stage of planet formation, following runaway growth, in which similar-sized planetary embryos slowly accrete planetesimals from their own feeding zones while keeping nearly fixed orbital separations.
Olokun Vallis
Olokun Vallis is a 150 km sinuous channel (vallis) on Venus at 81.5°N, 269.0°E, named by the IAU in 1997 for the Bini sea and river deity Olokun; one of 72 named Venusian valles.
Orbit of the Moon
The orbit of the Moon is its slightly elliptical path around Earth, with a mean distance of 384,400 km, a 27.32-day sidereal period, and its plane tilted 5.1 degrees to the ecliptic.
Orbital period
The orbital period, or revolution period, is the time a body takes to complete one orbit around another, from Mercury's 88 days to satellites circling Earth in about 90 minutes.
Orbital resonance
An orbital resonance is a gravitational configuration where orbital periods of two or more bodies sit near a ratio of small integers, as with Pluto and Neptune.
Orbital speed
Orbital speed, or orbit velocity, is the speed at which a body travels along its orbit, increasing the closer it is to the central body.
Orgueil meteorite
The Orgueil meteorite is a carbonaceous chondrite that fell as an observed fall near Orgueil, France, in 1864, and its composition is the reference standard for the solar system.
Origin of the Moon
The origin of the Moon is explained by the giant impact hypothesis, the consensus since 1984: a Mars-sized body struck Earth about 4.5 billion years ago.
Orionids
The Orionids are an annual meteor shower produced when Earth crosses debris shed by Halley's Comet, active each October with a peak around October 21–22 and roughly 10 to 20 meteors per hour.
Pale Blue Dot
Pale Blue Dot: Voyager 1 photograph of Earth taken 14 February 1990 from 6 billion km, showing Earth as a 0.12-pixel crescent in scattered sunlight; part of the 60-frame Family Portrait, named after Carl Sagan's 1994 book.