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Astronomy glossary

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A

Albedo
The measure of reflectivity of a non-luminous body. A perfectly reflecting surface has an albedo of 1.0; a totally black, non-reflecting surface has an albedo of 0.0.
Angle of incidence
The angle at which a projectile (or light) strikes a surface as measured from a line perpendicular to the point of impact (incidence). An angle of incidence of 0° means the object's trajectory is perpendicular to the ground.
Antipode
A point on a planet that is diametrically opposite another point. A line that connects antipodes passes through the centre of the planet.
Apogee
A point in the path of a body (satellite or moon) orbiting around a planet that is farthest from the planet.
Apollo
American space program (Apollo 7 to 17). Between 1969 and 1972, 12 astronauts on six missions landed on the Moon and explored its surface.
Aristotle
Greek philosopher (384-322 B.C.) and author of many treatises on logic, politics, biology and physics. His work played a decisive role in the beginnings of science.
Asteroid
A small irregular rocky object that orbits the Sun; also called a minor planet. Ceres, the largest asteroid with a diameter of around 1,000 km, is also known as a dwarf planet because it is spherical. 
Asteroid belt
A ring of asteroids orbiting the Sun between the orbits of Mars and Jupiter; where most asteroids are found.
Astronaut
In Canada and the United States, a space pilot or passenger. See also cosmonaut.
Astronautics
The science and technology of spacefight and space navigation.
Astronomy
The study of celestial objects, the intervening regions of space and the universe as a whole.
Astronomical unit
Basic unit of length used to measure distances in the solar system. Equal to the average distance between the Sun and Earth. 1 AU = 149,597,870 kilometres.
Atmosphere
A gaseous envelope that surrounds a star, planet or moon.

B

Basalt
A dark coloured, finely granulated, igneous rock of volcanic origin, produced by the cooling and solidifying of lava flows.
Billion
One thousand million (1,000,000,000 or 109).
Binoculars
Optical instrument made of two lenses joined side by side, enabling simultaneous viewing with both eyes.
Bolide
An exceptionally bright meteor that is sometimes accompanied by an intense supersonic bang and a trail of smoke; also known as a fireball.

C

Cartography
The various operations (observations, development and design) needed as a whole to produce a geographical map.
Celsius
Centesimal temperature scale based on the freezing (0 degrees) and boiling (100 degrees) points of water.
Centre of mass
The point of a body which acts as if the entire mass were concentrated within it and all external forces were acting on it.
Circumference
The perimeter of a circle, the length all the way around it. The length of a large circle that runs all the way around a sphere.
Circular
That which has the form of a circle.
Comet
A small body of rock and ice that orbits around the Sun. When a comet approaches the Sun, the material it is composed of vaporizes, forming a long tail.
Core
The central region of a planet or moon, often composed of heavier, denser material than the crust or mantle. In the case of the Earth and Moon, it is thought that the core is composed of iron and nickel.
Cosmonaut
In Russia, a spaceship pilot or passenger.
Crater
A circular depression in the soil of a planet or satellite, generally caused by a meteorite crash.
Culmination
When a star reaches the highest point over the horizon of its diurnal or nocturnal trajectory.

D

Day
Period of the Earth’s rotation around itself.
Deceleration
Reduction in speed of a body in movement.
Density
Relationship between the mass of a body and its volume.
Diameter
A straight line through the center of a circle or sphere linking two diametrically opposed points on its surface or circumference.
Diffraction
The deviation of a ray of light when it encounters an obstacle or an opening of the same approximate size as its wavelength.
Diurnal (movement)
Apparent movement of the sky’s rotation, caused by the real movement of the rotation of the Earth around the axis of its poles.

E

Earthquake or Moonquake
Sudden movement of the terrestrial or lunar crust caused by a release of accumulated tension in the rocks beneath the ground.
Exit velocity
Minimum speed that an object (rocket, probe) must reach to escape the gravitational attraction of a planet or satellite.
Eclipse
Temporary full or partial eclipse of a celestial body caused by its passage through the umbra or penumbra of another celestial body.
Ecliptic
Course of the orbit of the Earth around the Sun.
Ejecta
Material ejected when a crater is formed that falls back to the surface around it.
Elliptical
That which is in the shape of an ellipsis, an oval shape.
Equator
Large circle around the terrestrial or lunar sphere whose course is perpendicular to the axis of the poles.
Erosion
Degradation of higher ground; transportation and accumulation of materials through the action of water, wind, ice, etc. On the Moon, the only form of erosion is caused by the solar wind and micrometeorites.
Eccentricity
The measurement of the “flattening” of one celestial body’s elliptical orbit around another body.

F

Foucault pendulum
Pendulum whose plane of oscillation changes direction as the Earth spins beneath it. Used in 1851 by French physicist Jean Léon Foucault to demonstrate the rotation of the Earth.

G

Galilei (Galileo)
Italian physicist and astronomer (1564-1642 A.D.), reported the first astronomical observations using a telescope he made himself, studied the laws of motion, and was a fervent defender of Copernicus’ heliocentric system.
Gaseous planet
Also known as Jovian planets. Giant planets composed almost exclusively of hydrogen and helium in liquid and gas form, of which Jupiter is the prototype. The gaseous planets in the solar system are Jupiter, Saturn, Uranus and Neptune.
Geology
A science that aims to describe the materials that make up the terrestrial world and study the past and present transformations that the Earth has undergone. See planetology.
Geometric centre
A point located at equal distance from all points of a perfect sphere of the same average radius as a celestial body.
Gibbous
Shaped like a hump. A celestial body is called gibbous when the visible illuminated surface is more than half of the disk.
Great Red Spot
A gigantic hurricane that has been raging in Jupiter’s atmosphere for the past 400 years.
Gravitation
The force of attraction that one celestial body holds over another. A phenomenon whereby bodies exert a mutual attraction proportional to their masses and inversely proportional to the square of the distance between them.
Gravitational field
The space around a celestial body which exercises its gravitational influence on other material bodies.
Gravitational resonance
Increase or decrease of the amplitude of oscillation (rotation, revolution, etc.) under the influence of a gravitational force, until a situation of balance is established.

H

Highlands
The oldest exposed region’s of the Moon’s surface, marked by numerous craters and with a chemical composition that is different from the lunar seas.
Hemisphere
Each of the halves of a terrestrial, lunar, or other globe.
Hour
A unit of time composed of 3,600 seconds (60 minutes) that occurs 24 times per day.
Hypothesis
A provisional proposal that aims to explain an observation, and which is then submitted to the control of an experiment or verified by deduction.

I

Igneous rock
Rocks formed by the cooling and solidification of volcanic magma.
Impact
Made by a body (such as a meteorite) hitting another (the surface of the Moon). Shock.
Impact basin
A large depression in a planet or satellite (generally with a diameter greater than 200 km) surrounded by mountains that was created by an explosive collision with an asteroid or comet. Very large crater.

J

Jovian planets
Also known as gaseous planets. Giant planets composed almost exclusively of hydrogen and helium in liquid and gas form, of which Jupiter is the prototype. The Jovian planets in the solar system are Jupiter, Saturn, Uranus and Neptune.

L

Latitude
North-south coordinate on the surface of the Earth. Distance in degrees measured from the equator towards the north or south along a meridian.
Lava
Liquid magma that flows to the surface of a planet or moon from a volcano or crack in the crust.
Libration
Oscillation of a celestial body around an average position, apparent swinging around its axis.
Longitude
East-west coordinate on the surface of the earth. Distance in degrees measured from the Greenwich mean (in England) to the east or west along the equator.
Lunar crust
Outer layer of the Moon’s surface, whose thickness varies from 60 km on the visible side to more than 100 km on the dark side.
Lunar eclipse
When the Moon passes into the shadow of the Earth.
Lunar sea
Vast reach of the surface of the moon, dark and somewhat sloping. Lunar seas are composed of cooled and hardened lava.

M

Magma
Molten rock formed by high temperatures inside a planet or moon. When it reaches the surface, magma becomes lava.
Mantle
Region inside a planet or moon located between the crust and the core, where rock reaches high temperatures. In extreme cases, rock may melt and form magma. See lunar crust.
Model
Three-dimensional representation of an object or system on a reduced scale but otherwise faithful to proportions and aspects.
Mass
Measurement of the quantity of material that a body contains. Mass is defined by the inertia of the body (its resistance to change) or the gravitational influence it exerts on its environment.
Meridian
Large circle running north to south around the Earth’s surface.
Meteor
Any light phenomenon in the Earth’s atmosphere (rainbow, lightning, shooting star, etc.)
Meteorite
A meteoroid that reaches the surface of a celestial body without having completely disintegrated.
Meteoroid
A small, solid object moving in space between planets.
Meteor shower
Time of year when an unusually large number of meteors seem to come from the same place in the sky. Meteor showers occur when swarms of meteors travelling together through space reach Earth.
Million
One thousand thousand (1,000,000 or 106).
Minute
Unit of time made up of 60 seconds that occurs 60 times per hour.

N

Nebula
Interstellar cloud made of gas and dust.
Node
Each of two points of intersection of a celestial body’s orbit with a reference plane (for example, the ecliptic). See ecliptic.
Nomenclature
All terms used to describe objects or places, presented according to specific and methodical rules.

O

Occultation
Momentary disappearance of a celestial body behind a celestial body with a greater apparent diameter.
Orbit
The closed trajectory of a body that revolves around another body. The curve made by a planet around the sun or by a satellite around its planet.

P

Penumbra
Portion of a shadow where only one part of a light source is hidden by an opaque body. See shadow cone.
Perigee
The point in the orbit of a body gravitating around the Earth (satellite, Moon) at which it is closest to the Earth.
Phase
Each of the different aspects presented by the Moon and some planets according to their position in relationship to the Earth and Sun.
Planet
Celestial body in orbit around a star that is sufficiently large and massive for its own weight to make it round, and which is by far the largest object in its orbit. Planets do not produce their own light, but reflect that of their star. There are officially eight planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
Planetology
Science that focuses on comparative study of the planets and their satellites as well as their geological processes. Planetology is an extension of geology in other planets of the solar system.
Polar axis
The rotation axis of a celestial body that passes through its centre and its North and South poles.
Precession
Very slow conical movement of an axis around an average position.
Pressure suit
Hermetically sealed protective suit that reproduces the temperature and pressure conditions of Earth around the body of an astronaut, protecting the astronaut from the vacuum and coldness of space.
Prism
A solid, triangular object made of transparent material that deviates and decomposes rays of light.
Probe
An uninhabited aircraft launched outside the Earth’s atmosphere to study a body in the solar system or interplanetary space.

R

Radius
In geography, a straight segment with one extremity that is the centre of a circle or sphere and the other extremity that is a point of the circumference of the circle or the surface of the sphere.
Ray
In geology, material ejected during a meteorite impact that falls radially around a crater. See crater.
Refraction
A change of direction of a light ray passing from one medium to another of a different density (for example, from the vacuum of space to the Earth’s atmosphere).
Regolith
A fairly loose deposit of debris of all sizes resulting from the fragmentation of underlying rocks. On the Moon, regolith was produced by a large number of meteoritic impacts and covers the entire surface.
Revolution
Periodic movement of a celestial body around another with a greater mass.
Rim
The raised edge that surrounds a crater caused by the impact of a meteor. See crater.
Ring
A circular area where a multitude of small fragments of rock and ice orbit around a planet.
Ring eclipse
Eclipse of the Sun that takes place when the apparent diameter of the Moon is slightly smaller than the apparent diameter of the Sun.
Rotation
Periodic movement of a celestial body that spins around an axis that runs through its centre (polar axis).

S

Satellite
A body in orbit around a planet (moon) or another celestial body with a greater mass. Aircraft launched by a rocket or spaceship in orbit around the Earth or another body (artificial satellite).
Scale factor
A factor of proportionality that, when multiplied by the dimensions of a scale model, gives the real dimensions of the object.
Scale model
See model.
Science
A consistent body of knowledge that aims to explain the order of nature using laws proven by the scientific method.
Scientific method
Procedure that consists of (1) observing natural phenomena or the results of experiments, (2) formulating hypotheses to describe the phenomena or results, (3) verifying the hypotheses and noting whether they correctly predict new phenomena or the results of new experiments.
Seismometer
A device used to record the time, duration and intensity of earthquakes or moonquakes.
Shadow cone
A cone-shaped shadow created by a spherical object (Earth, the Moon, etc.) on which sunlight or another broad source of light is shining.
Shooting star
A light phenomena caused by a solid object from space (meteoroid) passing through the Earth’s atmosphere.
Sidereal month
Interval of time between two successive passages of the moon using a reference star.
Sidereal rotation
Rotation measured in relation to faraway, stationary stars.
Sidereal revolution
Revolution measured in relation to faraway, stationary stars.
Simulation
Method of measurement and study that consists of replacing a phenomenon or system to be studied by a model that is simpler but behaves in a similar way.
Solar corona
External region of the atmosphere of the sun, very hot and with little density.
Solar eclipse
When the Earth passes into the shadow of the moon.
Solar system
The Sun and all objects in orbit around it: planets, satellites, rings, comets, asteroids, meteoroids, dust, solar wind.
Solar wind
Continual flow of charged particles emitted by the Sun.
Sphere
Closed surface on which all points are equidistant (radius) to an internal point called the centre.
Star
A sphere of gas that shines by producing its own light.
Stylus
Inclined shaft whose shadow marks the time on a sundial. The stylus is generally parallel to the Earth’s rotation axis (pole axis).
Sun
The star around which the Earth and other planets of the solar system revolve.
Sundial
An instrument for measuring time whose surface is divided according to the hours of the day and which indicates the time according to the projection of the shadow of a pointer (stylus) cast by the Sun.
Sunspot
Region of the surface of the Sun that is temporarily cooler than its immediate surroundings (4000 degrees instead of 5500). The spots look darker in contrast with the hotter (and therefore brighter) surrounding areas.
Surface gravity
The gravitational force that a planet holds over objects on its surface. The weight of a unit of mass measured on a planet’s surface.
Synodical month
Interval of time between two successive new moons.
Synchronous
Describes movements that happen at the same time.
System of reference
A group of bodies considered to be stationary by which the movement of other bodies is defined.

T

Telescope
An astronomical observation instrument which uses a convex lens (refractor telescope) or a concave mirror (reflector telescope).
Telluric planet
Also known as a terrestrial planet. Medium-sized planet with a solid surface and hot core, of which Earth is the prototype. In the solar system, the Telluric planets are Mercury, Venus, Earth and Mars.
Terminator
Line that separates the bright and dark parts of the disk of the Moon or another planet.
Terrestrial planet
Also known as a Telluric planet. Medium-sized planet with a solid surface and hot core, of which Earth is the prototype. In the solar system, the terrestrial planets are Mercury, Venus, Earth and Mars.
Thales of Miletus
Greek philosopher and mathematician (c. 625-c. 547 B.C.) who brought elements of geometry and algebra back from Egypt and Babylon, and measured time correctly for the first time using a gnomon.
Theory
A group of hypotheses and laws whose veracity has been demonstrated through the scientific method and which aim to establish the truth of a scientific system.
Time
Measurable duration that makes it possible to identify events in their succession.
Time zone
Each of the imaginary divisions of the surface of the Earth in which all points share a standard time of day.

U

Umbra
Dark area due to the interception of light by an opaque body. See cone shadow.

V

Vaporize
To transform into vapor, to project as fine droplets.
Volatile
Describes chemical elements or molecules (such as water) that vaporize at relatively low temperatures.
Volcano
A mountain with fairly rugged terrain resulting from the infiltration of molten (lava) seeping through cracks in a planet’s crust, which then escapes through an opening whose shape is usually conical (crater).
Volume
Three-dimensional space occupied by a body or limited by surfaces.

W

Weight
A force resulting from the gravitational action of a celestial body (planet or satellite) on a mass on its surface.

Y

Year
A period during which the Earth makes a complete revolution around the sun. A year has 365.26 days.

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