Astronomical Terms

This is a list of common astronomical terms and their definitions. Many you hear often, but may not know the exact definition of. We hope this listing will be of help to you. If there is a term that you find you still don't know the meaning of, please use the form at the bottom of the page to send it to us. We will answer your question promptly.

A

accretion

Accumulation of dust and gas into larger bodies such as stars, planets and moons.

Adams, John Couch 1819-1892

English astronomer and mathematician who, at the age of 24, was the first person to predict the position of a planetary mass beyond Uranus. But, unfortunately, Adams did not publish his prediction. Galle confirmed the existence of Neptune based on independent calculations done by Le Verrier.

albedo

the ratio of the amount of light reflected by an object and the amount of incident light; a measure of the reflectivity or intrinsic brightness of an object (a white, perfectly reflecting surface would have an albedo of 1.0; a black perfectly absorbing surface would have an albedo of 0.0).

albedo feature

A dark or light marking on the surface of an object that may not be a geological or topographical feature.

antipodal point

the point that is directly on the opposite side of the planet

aphelion

the point in its orbit where a planet is farthest from the Sun; when refering to objects orbiting the Earth the term apogee is used; the term apoapsis is used for orbits around other bodies. (opposite of perihelion)

arcuate

having the form of a bow; curved; arc-shaped

Arago, Dominique François Jean 1786 - 1853

French astronomer and physicist and Director of the Paris Observatory, who discovered the phenomenon of the production of magnetism by rotation

d'Arrest, Heinrich Louis

Danish astronomer who assisted Galle with the first observations of Neptune. After receiving its predicted position from Le Verrier, Galle and d'Arrest began searching. With Galle at the eyepiece and d'Arrest reading the chart, they scanned the sky and checked that each star seen was actually on the chart. Just a few minutes after their search began, d'Arrest cried out, "That star is not on the map!" and earned his place in the history books.

asteroid

(also "planetoid") a medium-sized rocky object orbiting the Sun; smaller than a planet, larger than a meteoroid

asteroid number

asteroids are assigned a serial number when they are discovered. It has no particular meaning except that asteroid N+1 was discovered after asteroid N.

astronomical unit (AU)

= 149,597,870.691 km; the average distance from the Earth to the Sun. 1 AU is a long way -- at 100 miles per hour (160 kph) it would take over 100 years to go 1 AU.

atmosphere

= 1.013 bars = 1.03 kg/cm^2 = 14.7 pounds per square inch, standard atmospheric pressure at sea level on Earth.

aurora

a glow in a planet's ionosphere caused by the interaction between the planet's magnetic field and charged particles from the Sun

aurora borealis

the "Northern Lights"; caused by the interaction between the solar wind, the Earth's magnetic field and the upper atmosphere. A similar effect happens in the southern hemisphere where it is known as the aurora australis.

B

bar

= 0.987 atmosphere = 1.02 kg/cm^2 = 100 kilopascal = 14.5 lbs/square inch.

Barnard, Edward Emerson 1857-1923

American astronomer; discovered Jupiter's satellite Amalthea and Barnard's star, the second-nearest star system to the Sun.

Barsoom

The local name for Mars in Edgar Rice Burroughs' SF books.

billion

I use the American version of "billion" which means 1,000,000,000 (1e9); not the British version which means 1e12.

Bode, Johann Elert 1747-1826

German astronomer, known for the bogus "Bode's Law" which attempts to explain the sizes of the planetary orbits.

bolide

a fireball that produces a sonic boom

Bond, William Cranch 1789-1859

One of the earliest American astronomers of note; rose from poverty and overcame a lack of formal education to become the first director of the Harvard College Observatory where he studied Saturn and (with Lassell) discovered its moon Hyperion.

Brahe, Tycho 1546-1601

(a.k.a Tyge Ottesen) Danish astronomer whose accurate astronomical observations formed the basis for Johannes Kepler's laws of planetary motion.

brown dwarf

An object between 0.013 and 0.080 solar masses (13 to 80 Jupiter-masses): too small for normal nuclear fusion but big enough to fuse deuterium. Brown dwarfs are larger than planets but smaller than stars.

C

caldera

crater formed by an explosion or collapse of a volcanic vent.

carbonate

a compound containing carbon and oxygen (i.e. calcium carbonate a.k.a. limestone).

Cassini, Giovanni Domenico 1625-1712

(a.k.a. Jean Dominique) Italian-born French astronomer and first director of the Royal Observatory in Paris; discoverer of four of Saturn's moons (Tethys, Dione, Rhea and Iapetus) and the major division in its rings.

catena

chain of craters.

cavus

Hollow, irregular depression.

chaos

distinctive area of broken terrain.

chasma

canyon.

Christy, James W.

discoverer of Pluto's moon Charon 

chromosphere

the lower level of the solar atmosphere between the photosphere and the corona

colles

small hills or knobs.

coma

the dust and gas surrounding an active comet's nucleus

comet

a medium-sized icy object orbiting the Sun; smaller than a planet

conjunction

An inferior planet is said to be "in inferior conjunction" when it is directly between the Earth and the Sun. It is "in superior conjunction" when it is on the opposite side of the Sun from the Earth. A superior planet is "in conjunction" when it is on the opposite side of the Sun from the Earth. A superior planet obviously cannot have an inferior conjunction. When the Earth is at inferior conjunction with respect to an observer on a superior planet we say that planet is "in opposition" from Earth's perspective.

Congress

the legislative branch of the US Government; has proven to be a much more hostile environment for scientific spacecraft than the vastness of space.

convection

fluid circulation driven by temperature gradients in the presence of gravity; the transfer of heat by this mechanism.

Copernicus, Nicolaus 1473-1543

Polish/German astronomer who advanced the heliocentric theory that the Earth and other planets revolve around the Sun. This was highly controversial at the time as the Ptolemaic view of the universe, which was the prevailing theory for over 1000 years, was deeply ingrained in the prevailing philosophy and religion. (It should be noted, however, that the heliocentric idea was first put forth by Aristarcus of Samos in the 3rd century BC, a fact known to Copernicus but long ignored.)

corona

ovoid-shaped feature.

corona

the uppermost level of the solar atmosphere, characterized by low densities and high temperatures (> 1.0E+06° K).

coronagraph

a special telescope which blocks light from the disk of the Sun in order to study the faint solar atmosphere.

cosmic ray

an extremely energetic (relativistic) charged particle.

crater

bowl-shaped depression formed by the impact of a meteoroid; depression around the orifice of a volcano.

D

density

measured in grams per cubic centimeter (or kilograms per liter); the density of water is 1.0; iron is 7.9; lead is 11.3.

disaster

literally "bad stars"; particularly apt in reference to a major asteroid impact.

disk

the visible surface of the Sun (or any heavenly body) projected against the sky.

doppler effect

the apparent change in wavelength of sound or light caused by the motion of the source, observer or both.

dinosaurs

large reptiles that lived in the Mesozoic Era from 230 to 65 million years ago; most probably wiped out by the impact of a large asteroid or comet.

direct

rotation or orbital motion in a counterclockwise direction when viewed looking down from above the north pole of the primary (i.e. in the same sense to most satellites); the opposite of retrograde. The north pole is the one on the same side of the ecliptic as the Earth's north pole. (The word "prograde" is sometimes used to mean "direct" in this sense.)

dorsum

ridge.

E

eccentricity

the eccentricity of an ellipse (planetary orbit) is the ratio of the distance between the foci and the major axis. Equivalently the eccentricity is (ra-rp)/(ra+rp) where ra is the apoapsis distance and rp is the periapsis distance.

effusive eruption

a relative quiet volcanic eruption which puts out basaltic lava that moves at about the speed one walks; the lava is fluid in nature; the eruptions at the Kilauea volcano on the island of Hawaii are effusive

Einstein, Albert 1879-1955

German-American physicist; developed the Special and General Theories of Relativity which along with Quantum Mechanics is the foundation of modern physics. (See fusion, speed of light)

ellipse

oval. That the orbits of the planets are ellipses, not circles, was first discovered by Johannes Kepler based on the careful observations by Tycho Brahe.

erg/sec

= 1e-10 kilowatts.

explosive eruption

a dramatic volcanic eruption which throws debris high into the air for hundreds of miles; lava is low in silicate; can be very dangerous for people near by; an example is Mount St. Helens in 1980

exponential notation

"1.23e4" means "1.23 times 10 to the fourth power" or 12,300; "5.67e-8" means "5.67 divided by 10 to the eighth power" or 0.0000000567.

F

facula

bright spot.

farrum

pancake-like structure

filament

a strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the Sun; a filament on the limb of the Sun seen in emission against the dark sky is called a prominence.

fireball

a meteor brighter than magnitude -3

fissure

a narrow opening or crack of considerable length and depth.

flare

a sudden eruption of energy on the solar disk lasting minutes to hours, from which radiation and particles are emitted.

flexus

cuspate (pointed) linear feature.

fluctus

flow terrain.

fossa

long, narrow, shallow depression.

Franklin, Benjamin 1706-1790

American public official, writer, and scientist. Played a major part in the American Revolution and helped draft the Constitution. His numerous scientific and practical innovations include the lightning rod, bifocal spectacles, and a stove.

G

Gaia Hypothesis

named for the Greek Earth goddess Gaea, holds that the Earth as a whole should be regarded as a living organism and that biological processes stabilize the environment. First advanced by British biologist James Lovelock in 1969.

Galle, Johann Gottfried 1812-1910

German astronomer who, with Heinrich Louis d'Arrest, made the first observation of Neptune based on calculations by Le Verrier. Though Galle was the first to observe Neptune, its discovery is usually credited to Adams (who made an earlier calculation) and Le Verrier.

Galilean Moons

Jupiter's four largest moons: Io, Europa, Ganymede and Callisto; discovered independently by Galileo and Marius. (Galileo proposed that they be named the Medicean stars, in honor of his patron Cosimo II de Medici; the present names are due to Marius)

Galileo Galilei 1564-1642

Italian astronomer and physicist. The first to use a telescope to study the stars. Discoverer of the first moons of an extraterrestrial body (see above). Galileo was an outspoken supporter of Copernicus's heliocentric theory. In reaction to Galileo, the Church declared it heresy to teach that the Earth moved and imprisoned him. The Church clung to this position for 350 years; Galileo was formally exonerated in 1992.

gegenschein

a round or elongated spot of light in the sky at a point 180 degrees from the Sun. Also called counterglow.

George III 1738-1820

King of Great Britain and Ireland (1760-1820). His government's policies fed American colonial discontent, leading to revolution in 1776.

geosynchronous orbit

a direct, circular, low inclination orbit in which the satellite's orbital velocity is matched to the rotational velocity of the planet; a spacecraft appears to hang motionless above one position of the planet's surface.

granulation

a pattern of small cells seen on the surface of the Sun caused by the convective motions of the hot solar gas.

greenhouse effect

increase in temperature caused when incoming solar radiation is passed but outgoing thermal radiation is blocked by the atmosphere (carbon dioxide and water vapor are the major factors). Very important on Venus and Earth but very weak on Mars.

H

Hale, George Ellery 1868-1938

American astronomer who founded the Yerkes, Mt. Wilson and Palomar observatories.

Hall, Asaph 1829-1907

American astronomer who discovered the two moons of Mars, Deimos and Phobos.

Halley, Edmond 1656-1742

English astronomer who applied Newton's laws of motion to historical comet data and predicted correctly the reappearance of the comet which now bears his name.

heliocentric

Sun-centered; see Copernicus, Kepler, Galileo.

heliopause

the point at which the solar wind meets the interstellar medium or solar wind from other stars.

heliosphere

the space within the boundary of the heliopause containing the Sun and solar system.

Herschel, Sir William 1738-1822

British astronomer who discovered Uranus and cataloged more than 800 double stars and 2,500 nebulae.

Hubble, Edwin Powell 1889-1953

American astronomer whose observations proved that galaxies are "island universes", not nebulae inside our own galaxy. His greatest discovery was the linear relationship between a galaxy's distance and the speed with which it is moving. The Hubble Space Telescope is named in his honor.

Huygens, Christiaan 1629-1695

Dutch physicist and astronomer who first described the nature of Saturn's rings (1655) and discovered its moon Titan; also pioneered the use of the pendulum in clocks.

I

ice

used by planetary scientists to refer to water, methane, and ammonia which usually occur as solids in the outer solar system.

inclination

the inclination of a planet's orbit is the angle between the plane of its orbit and the ecliptic; the inclination of a moon's orbit is the angle between the plane of its orbit and the plane of its primary's equator.

Inquisition, The

A Renaissance Catholic court instituted to seek out and prosecute heretics.

inferior planets

the planets Mercury and Venus are called inferior planets because their orbits are closer to the Sun than is Earth's orbit. (The other planets are called "interplanetary magnetic field (IMF)

the magnetic field carried with the solar wind.

ionosphere

a region of charged particles in a planet's upper atmosphere; the part of the Earth's atmosphere beginning at an altitude of about 25 miles and extending outward 250 miles or more.

J

K

Keeler, James E. 1857-1900

American astrophysicist, probable discoverer of the dark narrow gap in the outer part of the A ring of Saturn, and the second director of Lick Observatory. Keeler was (probably accidentally) cheated of his rightful fame when the A ring gap became known as "Encke's Division". Encke had earlier seen a broad, poor contrast feature in the A-ring which is quite different from the sharp, distinct gap that Keeler recorded on the very first night of observing with the Lick 36-inch refractor. On the other hand, the gap may have been seen even earlier by Francesco De Vico, William Lassell and the Rev. William R. Dawes. The history of visual observations of gaps in Saturn's rings is very confusing.

Kelvin (K)

0 Kelvin is absolute zero; water melts at 273 K (= 0° C = 32° F); water boils at 373 K (= 100° C = 212° F). (developed by William Thomson).

Kepler, Johannes 1571-1630

German astronomer and mathematician. Considered a founder of modern astronomy. Using the positional data carefully amassed by Tycho Brahe, Kepler formulated the famous three laws of planetary motion. They comprise a quantitative formulation of Copernicus's theory that the planets revolve around the Sun. 

kilogram (kg)

= 1000 grams = 2.2 pounds, the mass of a liter of water.

kilometer (km)

= 1000 meters = 0.62 miles.

Kowal, Charles T. 1940-

American astronomer; discovered Leda and the comet-like object 2060 Chiron (aka 95 P/Chiron).

Kuiper, Gerard 1905-1973

Dutch-born American astronomer best known for his study of the surface of the Moon; discovered Miranda and Nereid, found an atmosphere on Titan. (Dr.Kuiper was solidly Americanized; his name is pronounced to rhyme with "viper.")

L

labes

landslide.

labyrinthus

intersecting valley complex.

lacus

lake.

Lagrange, Joseph Louis 1736-1813

French (originally Italian, Giuseppe Luigi Lagrangia; born in Turin, moved to Paris and became a French citizen) mathematician and astronomer; made a number of contributions to the study of celestial mechanics.

Lagrange points

Lagrange showed that three bodies can lie at the apexes of an equilateral triangle which rotates in its plane. If one of the bodies is sufficiently massive compared with the other two, then the triangular configuration is apparently stable. Bodies at such points are sometimes referred to as Trojans. The leading apex of the triangle is known as the leading Lagrange point or L4; the trailing apex is the trailing Lagrange point or L5. Collinear with the two large bodies are the L1, L2 and L3 unstable equilibrium points which can sometimes be useful places for spacecraft, eg SOHO.

Lassell, William 1799-1880

British astronomer, discovered Neptune's largest satellite, Triton and (with Bond) discovered Saturn's moon Hyperion. A successful brewer before turning to astronomy.

Le Verrier, Urbain Jean Joseph 1811-1877

French mathematician whose prediction of the position of an undiscovered planet (Neptune) that caused perturbations in the orbit of Uranus was the first to be confirmed (by Galle) though Adams had made a similar but unpublished prediction some months earlier.

lidar

an instrument similar to radar that operates at visible wavelengths.

limb

the outer edge of the apparent disk of a celestial body

light-year

= 9.46053e12 km (= 5,880,000,000,000 miles = 63,239 AU); the distance traveled by light in a year.

linea

elongate marking.

liter

= 1000 cm3 = 1.06 US quarts

Lowell, Percival 1855-1916.

American astronomer. He founded the Lowell Observatory in Arizona (1894), where his studies of Mars led him to believe that the linear markings (first noted by Schiaparelli) on the surface were "canals" and therefore that the planet was inhabited by intelligent beings. His successors later discovered Pluto.

lunar month

The average time between successive new or full moons, equal to 29 days 12 hours 44 minutes. Also called synodic month.

M

macula

dark spot.

magnetosphere

the region of space in which a planet's magnetic field dominates that of the solar wind.

magnetotail

the portion of a planetary magnetosphere which is pushed away from the Sun by the solar wind.

magnitude

The degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude -1.4 and the faintest visible star has magnitude 6, with the scale rule such that a decrease of one unit represents an increase in apparent brightness by a factor of 2.512. Also called apparent magnitude.

mare

literally "sea" (a very bad misnomer, still in use for historical reasons); really a large circular plain

Marius, Simon 1573-1624

(a.k.a. Mayr) German astronomer who gave Jupiter's "Galilean" moons their names. He and Galileo both claimed to have discovered them in 1610 and likely did so independently. Marius was also the first to observe the Andromeda Nebula with a telescope and one of the first to observe sunspots

mensa

mesa, flat-topped elevation.

metal

used by astrophysicists to refer to all elements except hydrogen and helium, as in: "the universe is composed of hydrogen, helium and traces of metals". (Note: this is quite different from the usual chemistry definition.)

meteor

(also "shooting star" or "falling star") a bright streak of light in the sky caused by the entry into Earth's atmosphere of a meteoroid or a small icy particle. Very large, bright ones are called fireballs and bolides

meteorite

a rock of extra-terrestrial origin found on Earth

meteoroid

a small rocky object orbiting the Sun; smaller than an asteroid

millibar

1/1000 of a bar. Standard sea-level pressure is about 1013 millibars.

minor planets

the official term used for asteroids.

mons

mountain (plural: montes)

N

Neujmin, Grigoriy N.

Ukrainian astronomer; discovered the asteroid 951 Gaspra.

neutrino

a fundamental particle supposedly produced in massive numbers by the nuclear reactions in stars. They are very hard to detect since the vast majority of them pass completely through the Earth without interacting.

Newton, Isaac 1642-1727

English cleric and scientist; discovered the classical laws of motion and gravity; the bit with the apple is probably apocryphal.

Nicholson, Seth Barnes 1891-1963

American astronomer; discovered Lysithea, Ananke, Carme and Sinope; also did important work on sunspots.

nuclear fusion

a nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the small ones. The difference in mass is converted to energy by Einstein's famous equivalence E=mc2. This is the source of the Sun's energy therefore ultimately of (almost) all energy on Earth.

O

oceanus

literally "ocean"; really a large circular plain

old

a planetary surface that has been modified little since its formation typically featuring large numbers of impact craters (compare young).

Oort, Jan Hendrik 1900-1992

Dutch astronomer made major contributions to knowledge of the structure and rotation of our galaxy. More or less as a sideline, Oort studied comets as well. The result of this work was a theory, now widely accepted, that the Sun is surrounded by a distant cloud of comet-stuff, now called the Oort cloud, bits of which are occasionally hurled into the solar system as comets.

opposition

A superior planet is said to be "in opposition" when it is directly on the opposite side of the Earth from the Sun. This is generally the closest it comes to the Earth and the time at which it is most easily visible.

ovoid

shaped like an egg

P

palus

literally "swamp"; really a small plain

parsec

= 206265 AU = 3.26 light year

patera

shallow crater; scalloped, complex edge.

penumbra

literally, "dim light"; the outer filamentary region of a sunspot.

perihelion

the point in its orbit where a planet is closest to the Sun. when referring to objects orbiting the Earth the term perigee is used; the term periapsis is used for orbits around other bodies. (opposite of aphelion)

Perrine, Charles Dillon 1867-1951

Argentine-American astronomer who discovered Himalia and Elara.

perturb

to cause a planet or satellite to deviate from a theoretically regular orbital motion .

photosphere

the visible surface of the Sun; sunspots and faculae are observed in the photosphere.

plage

bright regions seen in the solar chromosphere.

Pickering, William Henry 1858-1938

American astronomer. His photographs of Mars, among the earliest obtained, provided a basis for his opposition to Lowell's observations of supposed canals on Mars. Discovered Phoebe.

planet

An object orbiting a star that is not a brown dwarf but bigger than an asteroid. This is somewhat ambiguous. In our solar system it is really an historical definition which boils down to "a planet is a member of the set {Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto}". All the known planets orbiting other stars are large gas planets so no ambiguity yet arises there. But when we begin to discover terrestrial planets and then smaller objects it will become necessary to make some further arbitrary definition.

planitia

low plain.

planum

plateau or high plain.

Pope, Alexander 1688-1744

English writer best remembered for his satirical mock-epic poems The Rape of the Lock and The Dunciad.

prominence

a strand of relatively cool gas in the solar corona which appears bright when seen at the edge of the Sun against the blackness of space.

promontorium

cape; headland

Ptolemy 87-150

(aka Claudius Ptolemaeus) Alexandrian astronomer, mathematician, and geographer who based his astronomy on the belief that all heavenly bodies revolve around the Earth.

Q

R

red giant

a star that has low surface temperature and a diameter that is large relative to the Sun.

regio

region.

Relativity, Theory of

more accurately describes the motions of bodies in strong gravitational fields or near the speed of light than newtonian mechanics. All experiments done to date agree with relativity's predictions to a high degree of accuracy. (Curiously, Einstein received the Nobel prize in 1921 not specifically for Relativity but rather for his 1905 work on the photoelectric effect and "services to Theoretical Physics".)

resolution

the amount of small detail visible in an image; low resolution shows only large features, high resolution shows many small details

resonance

A state in which one orbiting object is subject to periodic gravitational perturbations by another.

reticulum

reticular (net-like) pattern

retrograde

rotation or orbital motion in a clockwise direction when viewed from above the north pole of the primary (i.e. in the opposite sense to most satellites); the opposite of direct. The north pole is the one on the same side of the ecliptic as the Earth's north pole.

rift valley

an elongated valley formed by the depression of a block of the planet's crust between two faults or groups of faults of approximately parallel strike.

rima

fissure.

Roche limit

the closest a fluid body can orbit to its primary without being pulled apart by tidal forces. A solid body may survive within the Roche limit if the tidal forces do not exceed its structural strength. The Roche limit is calculated with the equation

RL = 2.456*R*(p'/p)^(1/3)

where p' is the density of the planet, p is the density of the moon, and R is the radius of the planet.

rupes

scarp.

S

scarp

line of cliffs produced by faulting or erosion.

Schiaparelli, Giovanni Virginio 1835-1910

Italian astronomer who in 1877 first observed the "canals" on Mars. He believed that the features he observed included straight lines that joined in a complicated pattern. He called these lines 'canali', which means 'channels'. However, the Italian word was mistranslated into the English word 'canals'. That, combined with the suspicious straightness of the lines, bespoke of artificial structures, and this created a furor. Speculations concerning the possibility of intelligent life on Mars sprang up in the popular press. Even astronomers felt the pull of that dramatic possibility. Foremost among these was Percival Lowell, who carried matters far beyond Schiaparelli.

scopulus

lobate or irregular scarp.

semimajor axis

the semimajor axis of an ellipse (e.g. a planetary orbit) is 1/2 the length of the major axis which is a segment of a line passing thru the foci of the ellipse with endpoints on the ellipse itself. The semimajor axis of a planetary orbit is also the average distance from the planet to its primary. The periapsis and apoapsis distances can be calculated from the semimajor axis and the eccentricity by rp = a(1-e) and ra = a(1+e).

Shakespeare, William 1564-1616

English playwright and poet; wrote some good skits.

shepherd satellite

(or 'shepherd moon') a satellite which constrains the extent of a planetary ring through gravitational forces. 

sidereal

of, relating to, or concerned with the stars. Sidereal rotation is that measured with respect to the stars rather than with respect to the Sun or the primary of a satellite.

sidereal month

The average period of revolution of the moon around the earth in reference to a fixed star, equal to 27 days, 7 hours, 43 minutes in units of mean solar time.

silicate

a compound containing silicon and oxygen (e.g. olivine)

sinus

literally "bay"; really a small plain

solar cycle

the approximately 11-year quasi-periodic variation in frequency or number of solar active events.

solar nebula

the cloud of gas and dust that began to collapse about 5 billion years ago to form the solar system.

solar wind

a tenuous flow of gas and energetic charged particles, mostly protons and electrons -- plasma -- which stream from the Sun; typical solar wind velocities are near 350 kilometers per second.

speed of light

= 299,792,458 meters/second (186,000 miles/second). Einstein's Theory of Relativity implies that nothing can go faster than the speed of light; Scotty and Geordi know better.

spicules

grass-like patterns of gas seen in the solar atmosphere.

stellar classification

Stars given a designation consisting of a letter and a number according to the nature of their spectral lines which corresponds roughly to surface temperature. The classes are: O, B, A, F, G, K, and M; O stars are the hottest; M the coolest. The numbers are simply subdivisions of the major classes. The classes are oddly sequenced because they were assigned long ago before we understood their relationship to temperature. O and B stars are rare but very bright; M stars are numerous but dim. The Sun is designated G2.

sublime (or sublimate)

to change directly from a solid to a gas without becoming liquid

sulcus

subparallel furrows and ridges.

sunspot

an area seen as a dark spot on the photosphere of the Sun; sunspots are concentrations of magnetic flux, typically occurring in bipolar clusters or groups; they appear dark because they are cooler than the surrounding photosphere.

superior planets

the planets Mars, Jupiter, Saturn, Uranus, Neptune and Pluto are called superior planets because their orbits are farther from the Sun than Earth's orbit. (Mercury and Venus are called "inferior" planets.)

synchronous orbit radius

the orbital radius at which the satellite's orbital period is equal to the rotational period of the planet. A synchronous satellite with an orbital inclination of zero (same plane as the planet's equator) stays fixed in the sky from the perspective of an observer on the planet's surface (such orbits are commonly used for communications satellites).

synchronous rotation

said of a satellite if the period of its rotation about its axis is the same as the period of its orbit around its primary. This implies that the satellite always keeps the same hemisphere facing its primary (e.g. the Moon). It also implies that one hemisphere (the leading hemisphere) always faces in the direction of the satellite's motion while the other (trailing) one always faces backward. Most of the satellites in the solar system rotate synchronously.

T

tectonic

deformation forces acting on a planet's crust.

terminator

the dividing line between the illuminated and the unilluminated part of the moon's or a planet's disk.

terra

extensive land mass.

tessera

tile; terrain formed of polygonal pattern

tholus

small domical mountain or hill.

Thomson, William 1824-1907

aka Lord Kelvin, British physicist who developed the Kelvin scale of temperature. Also supervised the laying of a trans-Atlantic cable.

tidal heating

frictional heating of a satellite's interior due to flexure caused by the gravitational pull of its parent planet and possibly neighboring satellites.

Tombaugh, Clyde 1906-1997

American astronomer; discovered Pluto. 

Trekkie

(also "Trekker") a devotee of the science fiction program Star Trek.

Trojan

an object orbiting in the Lagrange points of another (larger) object. This name derives from a generalization of the names of some of the largest asteroids in Jupiter's Lagrange points: 588 Achilles, 624 Hektor, and 911 Agamemnon. Saturn's satellites Helene, Calypso and Telesto are also sometimes called Trojans.

U

umbra

the dark central region of a sunspot.

undae

dunes (literally 'waves').

V

vallis

sinuous valley (plural: valles)

Van Allen, James A.

American physicist who discovered the Earth's radiation belts (that now bear his name) with an instrument aboard the first successful American satellite, Explorer 1.

vastitas

widespread lowlands.

Verne, Jules 1828-1905

French writer who is considered the founder of modern science fiction. His novels include "Journey to the Center of the Earth" and "From the Earth to the Moon".

volatile

As a noun, this refers to substances that are gases at ordinary temperatures. In astronomy it includes hydrogen, helium, water, ammonia, carbon dioxide and methane.

W

white dwarf

a whitish star of high surface temperature and low intrinsic brightness with a mass approximately equal to that of a Sun but with a density many times larger.

X

Y

young

When used to describe a planetary surface "young" means that the visible features are of relatively recent origin, i.e. that older features have been destroyed (e.g. by erosion or lava flows). Young surfaces exhibit few impact craters and are typically varied and complex. In contrast an "old" surface is one that has changed relatively little over geologic time. The surfaces of Earth and Io are young; the surfaces of Mercury and Callisto are old.

Z

zodiacal light

a faint glow from light scattered off of interplanetary dust along the plane of the ecliptic.

Form

Please send suggestions or comments pertaining to this site to
The Astronomy Sight Staff.