Far Above the World
Far Above the World
The point at which a body orbiting the Earth's Sun is furthest from the Sun. Contrast perihelion.
The point at which an orbiting body is furthest from its primary. Contrast periapsis.
The point at which a body orbiting the Earth (such as the Moon or an artificial satellite) is furthest from the Earth. Contrast perigee.
An object that has been intentionally placed into orbit by humans, often around the Earth but also around other bodies within the Solar System. Contrast natural satellite.
A minor planet of the inner Solar System. They are primarily found orbiting the Sun between Jupiter and Mars, but can approach the Earth or occupy trojan orbits with Jupiter. Asteroids are somewhat arbitrarily distinguished from many different types of similar objects: small Solar System bodies primarily composed of dust and ice instead of mineral and rock are known as comets; bodies less than one meter in diameter are known as meteoroids; very large asteroids are sometimes called planetoids or planetesimals; and bodies similar to asteroids in size and composition but which lie beyond Jupiter are known as distant minor planets.
The circumstellar disc in the Solar System located roughly between the orbits of Mars and Jupiter that is occupied by numerous irregularly shaped small Solar System bodies ranging in size from dust particles to asteroids and minor planets. The asteroid belt is often called the main asteroid belt or main belt to distinguish it from other asteroid populations in other parts of the Solar System.
See orbital mechanics.
Also planetary geology.
A field that studies the geology of solidified bodies such as the planets and their moons, asteroids, comets, and meteorites.
Investigations are centered around the composition, structure, processes, and history of these objects.
Also celestial body.
A type of naturally occurring physical entity, association, or structure within the observable universe that is a single,
tightly bound, contiguous structure, such as a star, planet, moon, or asteroid. Though the terms astronomical "body" and
astronomical "object" are often used interchangeably, there are technical distinctions.
Also celestial object.
A type of naturally occurring physical entity, association, or structure that exists within the observable universe but
is a more complex, less cohesively bound structure than an astronomical body, consisting perhaps of multiple bodies or
even other objects with substructures, such as a planetary system, star cluster, nebula, or galaxy. Though the terms
astronomical "object" and astronomical "body" are often used interchangeably, there are technical distinctions.
A unit of length used primarily for measuring distances within the Solar System, or secondarily between the Earth and distant stars. Originally conceived as the semi-major axis of the Earth's orbit around the Sun, the astronomical unit is now more rigidly defined as exactly 149,597,870.7 kilometres (92,956,000 miles; 4.8481×10−6 parsecs; 1.5813×10−5 light-years).
The branch of astronomy that employs principles of physics and chemistry to determine the nature of astronomical objects and phenomena, examining properties such as luminosity, density, temperature, and chemical composition (rather than the positions or motions of objects in space, which is more specifically the emphasis of celestial mechanics).
An envelope of gases surrounding an astronomical body such as a planet and held in place by its gravity. This shell of gas has no clearly defined exterior boundary, but instead grows increasingly tenuous with altitude. The term can also be applied to a stellar atmosphere, referring to the visible outer layers of a star.
Also obliquity.
The angle between an object's rotational axis and its orbital axis, or, equivalently, the angle between its equatorial plane and orbital
plane. Axial tilt usually does not change considerably during a single orbital period; Earth's axial tilt is the cause of the seasons.
Axial tilt is distinct from orbital inclination.
The imaginary central line around which a compact body such as a star or planet undergoes circular rotation. On Earth, the points where this axis intersect the surface define the geographical poles. A rotational axis can "wobble" due to precession and nutation.
The branch of astronomy that studies the motions of all types of astronomical objects, including stars, planets, and natural and artificial satellites, among others.
A relatively small, icy body that displays extended features when it approaches the Sun. The energy from the Sun vaporizes volatiles on a comet's surface, producing a visible coma around the cometary body. Sometimes a comet can produce a long tail radiating away from the Sun.
An aura of plasma that surrounds cooler stars such as the Sun. It can be observed during a solar eclipse as a bright glow surrounding the lunar disk. The temperature of the corona is much higher than that of the stellar surface, and the mechanism that creates this heat remains subject to debate among astronomers.
An arch-like structure in the Sun's atmosphere made up of relatively dense plasma confined and isolated from the surrounding medium by magnetic flux tubes.
A significant release of plasma and the accompanying magnetic field from the Sun's corona, often following a solar flare or during a solar prominence eruption.
A type of radiation consisting of high-energy protons and atomic nuclei which move through space at nearly the speed of light, and which may originate from the Sun or from outside the Solar System. Collisions of cosmic rays with the Earth's atmosphere can produce dramatic effects both in the air and on the surface.
In the equatorial coordinate system, the celestial equivalent of terrestrial latitude. Coordinates north of the celestial equator are measured in positive degrees from 0° to 90°, while coordinates to the south are measured in negative degrees. See also right ascension.
Also distant detached object and extended scattered disc object.
A dynamical class of minor planet in the outer reaches of the Solar System whose point of closest approach to the Sun is so distant
that the object is only moderately or weakly affected by the gravitational influence of Neptune and the other known planets, such
that it appears to be "detached" from the rest of the Solar System. Detached objects are thus distinct from other populations of
trans-Neptunian objects, such as cubewanos and scattered disc objects.
small planetary-mass object that is in direct orbit around the Sun, massive enough to be gravitationally rounded, but insufficient to achieve orbital dominance like the eight classical planets of the Solar System. The prototypical dwarf planet is Pluto, which for decades was regarded as a planet before the "dwarf" concept was adopted in 2006.
See orbital eccentricity.
Also ecliptic plane or plane of the ecliptic.
The plane defined by the Earth's orbit around the Sun. Hence, the position of the Sun as viewed from the Earth defines the
intersection of this plane with the celestial sphere. The ecliptic is widely used as a reference plane for describing the
position of other Solar System bodies within various celestial coordinate systems. It differs from the celestial equator
because of the axial tilt of the Earth.
An astronomical coordinate system commonly used to specify the apparent positions, orbits, and axial orientations of objects within the Solar System, with an origin at the geometric center of either the Sun or the Earth, a fundamental plane defined by the plane of Earth's orbit around the Sun (i.e. the plane of the ecliptic), a primary direction towards the vernal equinox, and a right-handed convention. This system is convenient because most of the planets and many small Solar System bodies orbit the Sun with only slight inclinations to the ecliptic. It may be implemented in either spherical or rectangular coordinates.
Also elliptic orbit.
A type of Kepler orbit with an orbital eccentricity of less than 1 (often inclusive of circular orbits, which have
eccentricity equal to 0), or one with negative energy. Elliptical orbits take the shape of an ellipse, and are very
common in two-body astronomical systems.
The angular separation between the Sun and an orbiting body, such as a planet, as it appears from Earth.
The imaginary line on a gravitationally rounded spheroid such as a planet that represents the intersection of the spheroid's surface with a plane perpendicular to its axis of rotation and equidistant from its geographical poles. The plane of the Earth's terrestrial equator is the basis for the celestial equator. equatorial coordinate system An astronomical coordinate system defined by an origin at the geometric center of the Earth, a fundamental plane created by projecting the Earth's terrestrial equator onto the celestial sphere (forming the celestial equator), a primary direction towards the vernal equinox, and a right-handed convention. This system is widely used to specify the positions of celestial objects as viewed from Earth. It may be implemented in either spherical or rectangular coordinates.
Either of the two precise times of year when the imaginary plane of the Earth's equator, extended indefinitely in all directions, passes through the center of the Sun (i.e. the two points at which this plane intersects the plane of the ecliptic); or, equivalently, when the Sun's apparent geocentric longitude is either 0 degrees or 180 degrees. The two equinoxes, known as the vernal equinox and the autumnal equinox, occur on or near March 20 and September 22 each year. On the day of an equinox, the center of the visible Sun appears to be directly above the equator, and the durations of day and night are approximately equal all over the planet. Compare solstice.
The minimum speed that must be achieved for a free, non-propelled object to escape from the gravitational influence of a massive body, i.e. to achieve an infinite distance from it; more generally, escape velocity is the speed at which the sum of an object's kinetic energy and gravitational potential energy is equal to zero. It is a function of the mass of the body and of the distance between the object and the body's center of mass. An object which has achieved escape velocity is neither on the surface nor in a closed orbit of any radius.
A giant planet composed mainly of hydrogen and helium gases rather than heavier elements, e.g. Jupiter and Saturn in the Solar System.
With reference to, or pertaining to, the geometric center of the Earth; centered upon the Earth, e.g. a geocentric orbit.
Also known as a magnetic storm, is a temporary disturbance of the Earth's magnetosphere that is driven by interactions between the magnetosphere and large-scale transient plasma and magnetic field structures that originate on or near the Sun.
Also geosynchronous equatorial orbit (GEO).
A circular geosynchronous orbit, which maintains a constant altitude of 35,786 kilometres (22,236 mi) directly above Earth's
equator in the same direction as Earth's rotation such that, to an observer on Earth's surface, the orbiting object appears
motionless, in a fixed position in the sky. Artificial satellites are often placed in geostationary orbit so that antennas on
Earth do not have to rotate to track them.
geosynchronous orbit (GSO)
A synchronous orbit about the Earth, i.e. with an orbital period equal to Earth's rotational period, such that the orbiting
object appears to return to exactly the same position in the sky after a period of one sidereal day. All geosynchronous orbits
have a semi-major axis equal to 35,786 kilometres (22,236 mi); geostationary orbits are a special case of geosynchronous orbits.
The vast, bubble-like cavity in the interstellar medium which surrounds and is created by the plasma emanating from the Earth's Sun. The heliosphere encompasses the entirety of the Solar System and a vast region of space beyond it. Its outer limit is often considered the boundary between matter originating from the Sun and matter originating from the rest of the galaxy.
A natural satellite following a distant, inclined, and often eccentric and retrograde orbit about its primary. Irregular moons are thought to be captured from other orbits, as opposed to regular moons, which are thought to form in situ.
Also Edgeworth–Kuiper belt.
A circumstellar disc of small Solar System bodies such as asteroids, trojans, and centaurs in the outer Solar System, extending
between 30 and 50 AU from the Sun. It is similar to the asteroid belt but far larger, and is home to several dwarf planets,
including Pluto.
Also Lagrange point, libration point, or L-point.
Any of a set of points near two large bodies in orbit at which a smaller object will maintain a constant position relative to the
larger bodies. At other locations, a small object would eventually be pulled into its own orbit around one of the large bodies,
but at the Lagrangian points the gravitational forces of the large bodies, the centripetal force of orbital motion, and (in
certain scenarios) the Coriolis acceleration all align in a way that causes the small object to become "locked" in a stable or
nearly stable relative position. For each combination of two orbital bodies, there are five such Lagrangian points, typically
identified with the labels L1 to L5. The phenomenon is the basis for the stable orbits of trojan satellites and is commonly
exploited by man-made satellites.
A slight oscillating motion of the Moon as seen from the Earth, a result of the Moon's elliptical orbit. It can allow normally hidden parts of the Moon's far side to be visible along the limbs of the lunar disk.
Of or relating to the Earth's Moon.
Also Moon phase.
The shape of the portion of the Moon that is illuminated by direct sunlight as viewed from Earth. This shape is referred to
as a phase because it gradually changes in a regular cycle over the course of a synodic month: as the orbital positions of
the Moon around Earth and Earth around the Sun change, the visibility of the side of the Moon that constantly faces Earth
alternates between completely illuminated (known as a full moon) and completely darkened by the Moon's own shadow (known as
a new moon). There are also intermediate phases, during which the visible side may be only partially sunlit, e.g. when the
Moon appears as a crescent. During the part of the lunar cycle in which the illuminated portion is growing larger, the Moon
is said to be waxing; when the illuminated portion is becoming smaller, it is said to be waning. The phase of the Moon at any
particular time appears the same from every point on Earth.
A mostly convex region formed when a plasma, such as the solar wind, interacts with the magnetic field of a body, such as a planet or star.
Also shooting star or falling star.
The visible passage of a glowing meteoroid, micrometeoroid, comet, or asteroid through the Earth's atmosphere, usually as a
long streak of light produced when such an object is heated to incandescence by collisions with air molecules in the upper
atmosphere, leaving an ionization trail as a result of its rapid motion and sometimes also shedding material in its wake.
A solid piece of debris from a meteor that originated in outer space and survived its passage through the atmosphere to reach the surface of a planet or moon.
A small rock or boulder that has entered a planetary atmosphere. If it survives to reach the surface, it is then termed a meteorite.
A series of meteors that seemingly radiate from a single area in the night sky. These are produced by debris left over from a larger body, such as a comet, and hence they follow roughly the same orbit. This makes many meteor showers predictable events, as they recur every year.
A measure of the abundance of elements other than hydrogen and helium within an astronomical object. Note that this definition includes elements that are not traditionally considered metallic by chemical convention.
A very small meteorite that has survived its passage through the atmosphere to reach the surface of a planet or moon, usually ranging in size from 50 μm to 2 mm. Micrometeorites are a major component of cosmic dust.
A very small meteoroid, usually weighing less than one gram. If it survives to reach a planetary surface, it is then termed a micrometeorite.
An object in direct orbit around the Sun that is neither a dominant planet nor originally classified as a comet. A moon is not a minor planet because it orbits another body instead of the Sun.
A natural satellite that orbits a minor planet. See also moonlet and subsatellite.
Also minor moon or minor natural satellite.
An especially small natural satellite orbiting a planet, dwarf planet, or other minor planet. See also minor-planet moon and
subsatellite.
Also moon.
Any astronomical body that orbits a planet, minor planet, or sometimes another small Solar System body.
Any small Solar System body, such as an asteroid or comet, whose orbit brings it into proximity with Earth, generally by being less than 1.3 AU from the Sun at its closest approach.
A celestial event that occurs when a distant astronomical body or object is hidden by another, nearer body or object that passes between it and the observer, thereby blocking the first object from view. Solar and lunar eclipses are specific types of occultations.
Also the Öpik–Oort cloud.
A vast theoretical cloud of predominantly icy planetesimals hypothesized to surround the Sun at distances ranging from 2,000 to
200,000 AU. It is thought to be divided into two regions: a disc-shaped inner Oort cloud and a spherical outer Oort cloud. The
outer limit of the Oort cloud is often considered the cosmographical boundary of the Solar System.
The gravitationally curved trajectory of an object, such as the trajectory of a planet moving around a star or a natural satellite around a planet. Though the smaller body is often said to orbit the larger body itself, both bodies actually follow approximately elliptical orbits around a common center of mass positioned at a focal point of each ellipse. The word "orbit" can variously refer to the elliptical trajectory itself or the act of following this trajectory, and can refer to a stable, regularly repeating trajectory as well as a non-repeating trajectory.
A parameter that determines how much an orbit deviates from a perfect circle. For an elliptical orbit, the eccentricity ranges from greater than zero to less than one.
The tilt of an object's orbit around an astronomical body, expressed as the angle between the orbital plane or axis of direction of the orbiting object and a specified plane of reference.
Orbital mechanics or astrodynamics is the application of ballistics and celestial mechanics to rockets, satellites, and other spacecraft. The motion of these objects is usually calculated from laws of motion and of universal gravitation derived by Isaac Newton. Astrodynamics is a core discipline within space-mission design and control.
Also revolution period.
The time a given astronomical object takes to complete one orbit around another object. For objects in the Solar System, the
orbital period is often referred to as the sidereal period.
orbital plane
The imaginary geometric plane defined by the orbit of an astronomical body around its primary. The Earth's orbital plane, which
defines the ecliptic, is commonly used as a plane of reference for the orbits of other objects in the Solar System.
The speed at which an astronomical body or object orbits around a barycenter, or its speed relative to the center of mass of the most massive body in the system. The term may be used to refer to either the mean orbital speed, i.e. the average speed over the entire orbital period, or the instantaneous speed at a particular point in the orbit. Maximum instantaneous orbital speed typically occurs at periapsis.
Also pericenter.
The point at which an orbiting body is closest to its primary. Contrast apoapsis.
The point at which a body orbiting the Earth (such as the Moon or an artificial satellite) is closest to the Earth. Contrast apogee.
The point at which a body orbiting the Earth's Sun is closest to the Sun. Contrast aphelion.
A type of astronomical body orbiting the Sun, which is massive enough to be rounded by its own gravity (but not massive enough to achieve thermonuclear fusion) and has cleared its neighbouring region of all planetesimals. The term exoplanet is used in reference to a planet-like object that is not orbiting the Sun.
Also planetary object.
Any secondary body that is geologically differentiated or in hydrostatic equilibrium and therefore has a planet-like geology,
such as a planet, dwarf planet, or other planetary-mass object, but excluding smaller objects such as planetesimals.
Any solid object (generally larger than 1 kilometre (0.62 mi) in diameter) that arises during the formation of a planet whose internal strength is dominated by self-gravity and whose orbital dynamics are not significantly affected by gas drag. The term is most commonly applied to small bodies thought to exist in protoplanetary disks and debris disks during the process of planet formation, but is also sometimes used to refer to various types of small Solar System bodies which are left over from the formation process. There is no precise distinction between a planetesimal and a protoplanet.
Another name for a minor planet or dwarf planet.
An orbit in which the orbiting object passes directly over or nearly over both poles of the body being orbited during each revolution. It therefore has an inclination equal or nearly equal to 90 degrees to the body's equator.
A natural satellite following a relatively close and prograde orbit with little or no orbital inclination or orbital eccentricity. Regular moons are thought to form in situ about their primary, as opposed to irregular moons, which are thought to be captured.
A disk- or ring-shaped accumulation of various solid material such as dust and moonlets that orbits an astronomical body such as a planet. Ring systems are common components of satellite systems around giant planets, as with the Rings of Saturn. See also circumplanetary disk.
The time that an object takes to complete a single revolution about its own axis of rotation relative to the background stars. It is not necessarily the same as the object's synodic day or sidereal day.
An object in the Solar System that is neither a planet, a dwarf planet, nor a natural satellite. The SSSBs are: the comets; the classical asteroids, with the exception of the dwarf planet Ceres; the trojans; and the centaurs and trans-Neptunian objects, with the exception of the dwarf planets.
An intense, localized emission of electromagnetic radiation in the Sun's atmosphere.
A transient, collimated flow of plasma in the Sun's atmosphere.
A large, bright, transient feature, often in the shape of a loop, consisting of plasma extending outward from the Sun's photosphere into the corona. Prominences may be hundreds of thousands of kilometers long.
See geomagnetic storm.
A stream of charged particles, primarily protons, electrons, and alpha particles, released from the Sun's corona and flowing outwards at up to 900 kilometres per second (2,000,000 mph) into interplanetary space. Phenomena influenced by the solar wind include aurora, geomagnetic storms, and the plasma tails of comets.
The rate of travel of electromagnetic radiation through a medium. The speed of light in a vacuum is a universal physical constant, denoted by c. Massless particles and gravitational waves also travel at the speed of light. Light speed forms an upper limit for how fast information and matter can travel through space, while the large-scale expansion of space itself is not restricted.
Also standard acceleration due to gravity.
The nominal gravitational acceleration of an object in a vacuum near the surface of the Earth, as a result of Earth's gravity
and, less importantly, the centrifugal force generated by its rotation. It is by definition equal to 9.80665 m/s2
(approximately 32.17405 ft/s2).
Any natural or artificial satellite that orbits another natural satellite, i.e. "a moon of a moon".
The gravitational acceleration experienced at the equatorial surface of an astronomical body or other object, including that produced by the effects of rotation. It is typically expressed in units of acceleration such as meters per second squared (m/s2) or as a multiple of the Earth's standard gravity, which is equal to 9.80665 m/s2.
Any orbit in which an object orbits its primary with an orbital period equal to the average rotational period of the primary and in the same direction as the primary's rotation.
The boundary within the heliosphere, approximately 75 to 90 AU from the Sun, beyond which the solar wind slows to subsonic speeds (relative to the Sun) as a result of interactions with the local interstellar medium.
The line that divides the illuminated side of a moon or planet from its dark side. The line moves as the object rotates with respect to its parent star.
Any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune.
