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Solar luminosity
Solar luminosity is the total rate at which the Sun emits energy, about 3.828×10²⁶ watts; astronomers use it as a standard unit for expressing other stars' power output.
Solar magnetic field
The solar magnetic field is the magnetic field generated inside the Sun by a dynamo, producing sunspots and flares, with polarity reversing roughly every 11 years.
Solar prominence
A solar prominence is a large, bright structure of cool, dense plasma extending from the Sun's surface into the hot corona, held by magnetic fields; many erupt, driving space weather.
Solar sibling
A solar sibling is a star thought to have formed alongside the Sun in the same birth cluster from the same molecular cloud about 4.6 billion years ago.
Solar storm of 2015 March
The solar storm of March 2015, known as the St. Patrick's Day storm, was a severe G4 geomagnetic storm that struck Earth on 17 March 2015.
Solar System
The Solar System, also called our solar system, is the Sun and everything orbiting it: eight planets, hundreds of moons, and billions of smaller bodies.
Solar System Ambassadors
Solar System Ambassadors is a NASA volunteer program run by the Jet Propulsion Laboratory in which enthusiasts nationwide give free public talks about space missions.
Solar wind
The solar wind is a continuous supersonic outflow of charged particles from the Sun's corona, predicted by Eugene Parker in 1958 and first measured by spacecraft in 1959.
Solar wind switchbacks
Solar wind switchbacks are sudden, S-shaped reversals in the solar wind's magnetic field, first seen by Ulysses in the 1990s and found common near the Sun by Parker Solar Probe.
Solar wind termination shock
The solar wind termination shock is the boundary in the outer Solar System where the supersonic solar wind abruptly slows, crossed by Voyager 1 in 2004.
Spacewatch
Spacewatch is a University of Arizona research group, founded by Tom Gehrels and Robert McMillan in 1980, that observes asteroids and comets, especially near-Earth objects, from telescopes at Kitt Peak, Arizona.
Spin–orbit resonance
A spin–orbit resonance is a fixed ratio between a celestial body's rotation and its orbit; the Moon is synchronously locked 1:1, and Mercury is locked 3:2.
Sponde
Sponde (Jupiter XXXVI) is a small retrograde irregular moon of Jupiter in the Pasiphae group, discovered in 2001 at Mauna Kea and named for one of the Horae.
Stability of planetary ring systems
The stability of planetary ring systems depends on confinement by satellite resonances and shepherding moons, since dense rings otherwise spread and vanish within a few hundred million years.
Stability of the Solar System
The stability of the Solar System is the question of whether planetary orbits remain stable over long times. The motion is chaotic, yet catastrophic rearrangement is improbable.
Stellar encounters with the Solar System
Stellar encounters with the Solar System are close passages of other stars that gravitationally perturb Oort Cloud comets, occurring within 1 parsec about every 95,000 years.
Stephano (moon)
Stephano (Uranus XX, provisional designation S/1999 U2) is a small, dark retrograde irregular moon of Uranus about 32 kilometers wide, discovered in 1999 and named after a butler in Shakespeare's The Tempest.
Subglacial lake on Mars
The subglacial lake on Mars is a putative body of liquid water proposed in 2018 beneath the south polar ice cap, based on radar reflections that remain contested.
Subglacial water on Mars
Subglacial water on Mars is hypothesized liquid water beneath the planet's ice, notably a putative lake 1.5 km below the south polar cap reported in 2018.
Subsurface ocean of Enceladus
The subsurface ocean of Enceladus is a global body of liquid saltwater between the icy shell and rocky core of Enceladus, a moon of Saturn, confirmed by Cassini in stages from 2014 to 2017.
Summer solstice
The summer solstice, also called the estival or aestival solstice, is when Earth's axis tilts most toward the Sun, giving the Northern Hemisphere its longest day around June 21.
Sun
The Sun, also known as Star Sol, is the G2 yellow dwarf star at the center of the Solar System, about 4.57 billion years old.
Sun path
The sun path, or path of the Sun, is the apparent arc the Sun traces across the local sky, shifting through the year and depending on latitude and date.
Sunlight
Sunlight is the ultraviolet, visible, and infrared radiation the Sun emits, reaching Earth at about 1361 W/m² above the atmosphere and supplying nearly all the energy for Earth's climate.
Sunrise
Sunrise is the daily appearance of the sun on the eastern horizon caused by Earth's rotation, or the time when the sun's first part becomes visible at a given location.
Sunset
Sunset is the moment when the upper edge of the Sun's disk disappears below the horizon; atmospheric refraction means the Sun is already geometrically below it.
Supergranulation
Supergranulation is a cellular pattern of mostly horizontal flows on the Sun's surface, with cells about 30,000 km across that last one to two days.
Supermoon
A supermoon, or perigee moon, is a full Moon occurring at or near perigee, its closest point to Earth, appearing about 14% larger and 30% brighter than ordinary full Moons.
Sweden Solar System
The Sweden Solar System, the world's largest scale model of the Solar System, uses Stockholm's Globe arena as the Sun, with planets placed across 950 km of Sweden at 1:20 million scale.
Sycorax (moon)
Sycorax is the largest known irregular moon of Uranus, about 150 km across, discovered on 6 September 1997 at Palomar Observatory and named for Caliban's mother in Shakespeare's The Tempest.