611 articles
Grand Tack hypothesis
The Grand Tack hypothesis is a 2011 model of early Solar System evolution in which Jupiter migrated inward to about 1.5 AU, then reversed course and returned outward.
Gravitation of the Moon
The gravitation of the Moon, or lunar gravity, is the field generated by the Moon's mass, which accelerates surface objects at about one sixth of Earth's rate.
Gravitational instability model
The gravitational instability model, also called disk instability, is a planet-formation theory in which giant planets form rapidly from the self-gravity of a young star's gas disk.
Grazing lunar occultation
A grazing lunar occultation, also called a lunar graze, is an occultation where the Moon's edge passes in front of a star, making it disappear and reappear several times.
Great comet
A great comet is a comet that becomes exceptionally bright to the naked eye, such as Hale-Bopp in 1997, usually by passing close to the Sun or Earth.
Great conjunction
A great conjunction, or Jupiter Saturn conjunction, is the apparent close meeting of Jupiter and Saturn, occurring about every 20 years; the 2020 event was the closest since 1623.
Great Dark Spot
The Great Dark Spot (GDS-89) was an Earth-sized anticyclonic storm on Neptune, discovered by Voyager 2 during its 1989 flyby and named for Jupiter's Great Red Spot; it had vanished by 1994.
Great inequality
The great inequality (la grande inégalité) is a roughly 900-year oscillation in the mean longitudes of Jupiter and Saturn, caused by their near 5:2 orbital resonance.
Great Red Spot
The Great Red Spot (GRS) is a giant anticyclonic storm in Jupiter's southern hemisphere, the largest and longest-lived known vortex of any planet in the solar system.
Great White Spot
The Great White Spot (GWS) is a large, bright planet-encircling storm that erupts on Saturn roughly every 29.5 Earth years, with six recorded since 1876.
Hale cycle
The Hale cycle is the Sun's approximately 22-year magnetic cycle, named for George Ellery Hale, who discovered sunspot magnetic fields in 1908; its polarity reverses at each 11-year sunspot cycle.
Harrison Schmitt
Harrison Hagan "Jack" Schmitt is an American geologist, astronaut, and senator who flew on Apollo 17 in 1972 as the twelfth and last person to walk on the Moon.
Haumea
Haumea is a dwarf planet in the Kuiper Belt beyond Neptune, named for a Hawaiian goddess, with a fast 4-hour spin and two moons, Hi'iaka and Namaka.
HD 149026b
HD 149026b, officially named Smertrios, is a hot Saturn-mass exoplanet orbiting the metal-rich subgiant HD 149026 every 2.88 days. Discovered in 2005, it is the most metal-rich giant planet known.
Hecuba-gap asteroid
A Hecuba-gap asteroid is an asteroid in the 2:1 mean-motion resonance with Jupiter near 3.27 au, one of the largest Kirkwood gaps, home to a few hundred known asteroids.
Helene (moon)
Helene is a small Trojan moon of Saturn that shares Dione's orbit, staying 60 degrees ahead of it at a stable Lagrange point. It was discovered in 1980.
Helike (moon)
Helike, also designated Jupiter XLV and provisionally S/2003 J 6, is a small retrograde irregular moon of Jupiter in the Ananke group, discovered in 2003.
Heliopause
The heliopause is the outer boundary of the heliosphere, the sharp surface where the Sun's solar wind is stopped by the interstellar medium about 14 billion kilometers away.
Heliotail
The heliotail is the trailing region of the heliosphere, extending opposite the Sun's motion through the galaxy like a wake, first imaged by NASA's IBEX mission in 2013.
Herse
Herse (Jupiter L, S/2003 J 17) is a small retrograde irregular moon of Jupiter in the Carme group, discovered February 27, 2003 by Gladman, Kavelaars, Petit, and Allen; mean radius about 1 km, orbital period over 734 days.
Hiʻiaka (moon)
Hiʻiaka is the outer and larger of Haumea's two known moons, discovered in 2005 and named for the Hawaiian goddess of Hawaii island and hula dancers.
Hill sphere
The Hill sphere, also called the Roche sphere, is the region around a celestial body within which its gravity dominates the orbits of satellites against a more massive body.
Hill's lunar equations
Hill's lunar equations are the equations of motion introduced by George William Hill in his 1878 paper on the lunar theory, describing the Moon's motion around the Earth under solar perturbation.
Himalia
Himalia, Jupiter VI, is the fifth largest moon of Jupiter, discovered by Charles Perrine in 1904; a dark, irregular body about 85 km in radius.
Hippocamp (moon)
Hippocamp, also designated S/2004 N 1 and Neptune XIV, is the smallest known inner moon of Neptune, a roughly 34-kilometer-wide body discovered by Mark Showalter in 2013.
Historically significant lunar eclipses
Historically significant lunar eclipses are recorded eclipses that changed events, anchored calendars, or supplied scientific data, from omens before ancient battles to records that now measure changes in Earth's rotation.
History of Solar System formation and evolution hypotheses
The history of Solar System formation and evolution hypotheses traces models since the 17th century, from Descartes's vortices and Laplace's nebular hypothesis to modern solar nebula theory.
History of the Deep Space Network
The history of the Deep Space Network begins in 1958, when JPL built the first deep space antenna at Goldstone, California, and spans six decades of growth.
Hoba meteorite
The Hoba meteorite is the largest known intact meteorite on Earth, an iron of about 60 metric tons lying where it fell near Grootfontein in northern Namibia.
Hollow Moon
The Hollow Moon, or Spaceship Moon, is a fringe belief that the Moon is hollow or artificial, stemming from a 1970 Soviet article and the Moon's bell-like ringing.