611 articles
Samundra Vallis
Samundra Vallis is a 110-km-wide valley (vallis) on Venus at 24.1S, 347.1E, IAU-approved in 1994 and named after Samundra, an Indian river goddess, per the USGS gazetteer.
Sao (moon)
Sao, formally designated Neptune XI, is a small prograde irregular satellite of Neptune about 20 km across, discovered in 2002 and named for one of the Nereids, the sea nymphs of Greek mythology.
Saros (astronomy)
The saros, also called the saros cycle, is an eclipse cycle of 18 years, 11 days, and 8 hours, after which a very similar solar or lunar eclipse occurs.
Sati Vallis
Sati Vallis is a 225 km valley on Venus at 3.2°N, 334.4°E, in the V-31 Sif Mons quadrangle, named for an Egyptian river goddess and adopted by the IAU in 1994.
Saturn
Saturn is the sixth planet from the Sun and a gas giant, known for its rings, its density below that of water, and 274 confirmed moons as of 2025.
Saturn return
Saturn return is an astrological concept marking the moment Saturn returns to its birth position, near ages 27 to 31, read as a threshold of adulthood.
Saturn's fuzzy core
Saturn's fuzzy core is a diffuse central region where rock and ice mix gradually into the surrounding hydrogen and helium, extending to about 60 percent of Saturn's radius.
Saturn's hexagon
Saturn's hexagon is a persistent six-sided pattern in an eastward jet stream around Saturn's north pole, discovered in Voyager images from 1980 and 1981 and visible ever since.
Saykal Fossae
Saykal Fossae is a 300-km-long, narrow tectonic depression (fossa) on Venus at 73N, 139E, named by the IAU in 1997 for a Kyrgyz warrior maiden, one of 32 named Venusian fossae.
Secondary resonance
A secondary resonance is a commensurability between libration within a mean motion resonance and the apsidal precession period, a source of chaos in the asteroid and Kuiper belts.
Secular perturbation theory
Secular perturbation theory is the branch of celestial mechanics describing slow, long-term changes in planetary orbits by averaging away fast motion; its classical form is Laplace–Lagrange theory.
Secular resonance
A secular resonance is an orbital resonance caused not by matching orbital periods but by matching precession rates, which can drive large changes in a small body's eccentricity or inclination.
Semester (eclipse cycle)
The semester is an eclipse cycle of about 173.31 days, half an eclipse year, the time the Sun takes to travel from one lunar node to the other.
Setebos (moon)
Setebos is a small, dark irregular moon of Uranus, discovered in 1999, orbiting retrograde more than 11 million miles away and about 30 miles in diameter.
Sholpan Vallis
Sholpan Vallis, a 590 km valley on Venus at 63.9S 150E, named for the Kazakh and Karakalpak word for Venus, IAU-approved in 2003, one of 72 Venusian valles.
Shukra (शुक्र)
Shukra (शुक्र), also known as Shukracharya or Ushanas, is a Hindu deity identified with the planet Venus, preceptor of the asuras and one of the nine Navagrahas.
Sidereal year
The sidereal year is Earth's orbital period measured against the background stars, lasting 365.2564 days, about 20 minutes longer than the tropical year because of axial precession.
Sinann Vallis
Sinann Vallis is a 425-km-wide channel system (vallis) on Venus at 49S, 270E, approved by the IAU in 1994 and named for an Irish river goddess; it consists of disconnected lava-channel traces over 1,000 km long.
Sinope (moon)
Sinope is a small retrograde irregular moon of Jupiter, about 19 km in radius, discovered by Seth Barnes Nicholson at Lick Observatory in 1914 and named for a nymph who outwitted Zeus.
Skathi (moon)
Skathi (Saturn XXVII, formerly spelled Skadi) is a small irregular moon of Saturn about 8 kilometers across, discovered on September 23, 2000, and named for the Norse giantess Skadi.
SMASS classification
The SMASS classification, also called the Bus or Bus–Binzel taxonomy, classifies asteroids by visible-wavelength reflectance spectra, defining 26 classes grouped into S, C, and X complexes.
Solar Activity
Solar activity: magnetic phenomena on the Sun, including sunspots, solar flares, coronal mass ejections, solar wind, and solar energetic particles, varying over an 11-year solar cycle and driving space weather that affects satellites, GPS, radio, and power grids on Earth.
Solar birth cluster
The solar birth cluster is the star cluster where the Sun formed about 4.6 billion years ago, likely with 1,000 to 10,000 members, resembling Orion's Trapezium cluster.
Solar campfires
Solar campfires are small, transient extreme-ultraviolet brightenings in the quiet solar corona, first seen in 2020 by Solar Orbiter and studied as possible contributors to heating the Sun's million-degree corona.
Solar composition
Solar composition is the measured abundance of chemical elements in the Sun, determined from photospheric spectroscopy, CI chondrite meteorites, and Genesis solar wind samples; it serves as the reference for stellar chemistry.
Solar core
The solar core is the Sun's central region, where nuclear fusion of hydrogen into helium produces about 98% of the Sun's energy at roughly 15,000,000 °C.
Solar corona
The solar corona, also called the Sun's corona, is the outermost layer of the Sun's atmosphere, hotter than the surface below it at over 1 million degrees Celsius.
Solar cycle
The solar cycle, also known as the Schwabe or sunspot cycle, is the roughly 11-year rise and fall of the Sun's magnetic activity, tracked by sunspot numbers.
Solar cycle 25
Solar cycle 25 is the current 11-year cycle of solar activity, the 25th since record-keeping began in 1755, starting at solar minimum in December 2019.
Solar cycle prediction
Solar cycle prediction is the forecasting of the amplitude and timing of an upcoming or ongoing solar cycle, the Sun's roughly 11-year oscillation in magnetic activity.