General Glossary A - macombso.org

1 General Glossary Page 1 of 5 General Glossary A aphelion - The point in its orbit where a planet is farthest from the Sun; when referring to objects orbiting the Earth the term apogee is used. (opposite of perihelion) astronomical unit (AU) = 149,597, ; the average distance from the Earth to the Sun. One AU is a long way -- at 100 miles per hour (160 kph) it would take over 100 years to go one AU. atmosphere the gaseous area surrounding a planet or other body. aurora - (Latin for "dawn") 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.

2 Axis An imaginary straight line on which an object rotates. C caldera - Crater formed by the explosion or collapse of a volcanic vent. celestial meridian - The imaginary circle that passes through the north and south points on our horizon and through the zenith. celestial poles Two imaginary points in the sky directly above the Earth s north and south poles, that intersects the imaginary sphere of stars called the celestial sphere. Polaris (the North Star) is about 7 degree from the north celestial pole. celestial equator An imaginary circle on the imaginary celestial sphere, in the same plane as the Earth s equator. celestial sphere An imaginary sphere surrounding Earth. It is used to help astronomers descibe where objects are found in the sky. chasma - canyon chromosphere - The lower level of the solar atmosphere between the photosphere and the corona.

3 Circumpolar star A star which never sets, but can be viewed year round. 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. constellation a grouping of stars which have been given names by ancient astronomers because of the patterns they form. General Glossary Page 2 of 5 convection Fluid circulation driven by temperature gradients in the presence of gravity. corona The crown , or uppermost layer of the Sun's atmosphere that lies above the chromosphere. It is visible during a total solar eclipse as a white crown with plasma streaming outward, forming the points of the crown.

4 Characterized by low densities and high temperatures (over a million degrees K). cosmic rays - Extremely energetic charged particles (protons or atomic nuclei) that reach the Earth from outer space. crater - Bowl-shaped depression formed by the impact of a meteorite; or the depression around the mouth of a volcano. D day Amount of time it takes a planet to spin once on its axis. differential rotation Seen when different parts of a rotating body move at different speeds. For example, the Sun and Jupiter rotate faster at the equator than at their poles. direct rotation or orbit - Motion in a counterclockwise direction when viewed looking down from above the north pole of the primary; also called prograde motion; the opposite of retrograde. disaster - literally "bad stars"; particularly apt in reference to a major asteroid impact.

5 Disk - when talking about a sun or planet: The visible surface of the Sun (or any heavenly body) projected against the sky. E eccentricity - The degree to which an orbit is elliptical rather than circular. eclipse When our view of one object in the sky is blocked or partially blocked by either another object or the Earth s shadow. ecliptic - The apparent path of the Sun on the celestial sphere; or the plane of the Earth s orbit around the Sun. ellipse - Oval. Johannes Kepler first discovered that the orbits of the planets are ellipses, not circles. equinox literally, equal night ; the dates when day and night are the same length. Occurs once in the spring and once in the fall. F filament - A strand of cool gas suspended over the Sun s photosphere by magnetic fields, which appears dark as seen against the solar disk.

6 A filament above the limb of the Sun seen against the dark sky is called a prominence. fissure - A narrow opening or crack of considerable length and depth. General Glossary Page 3 of 5 flare - A sudden eruption of energy on the solar disk lasting minutes to hours, from which radiation and particles are emitted. G gibbous when the moon is more than half full, but less than completely full. 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. H heliocentric - the astronomical model in which the Earth and planets revolve around the Sun at the center of the solar System. First proposed by Aristarchus (310 BC - 230 BC), later made popular by Copernicus in the 16th century.

7 Horizon - The great circle midway between zenith and nadir. 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. 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 "superior" planets.) 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. L labyrinthus An intersecting valley complex. light-year - The distance traveled by light in a year (63,241 AU), or about trillion trillion miles).

8 Limb - The outer edge of the visible disk of a celestial body. lunar month - The average time between successive new or full moons, equal to 29 days 12 hours 44 minutes. Also called synodic month. M magnitude - The degree of brightness of a celestial body designated on a numerical scale, on which the brightest star has magnitude and the faintest stars visible to the naked eye have magnitude 6. Each magnitude is about brighter than the next lower magnitude. The Sun is magnitude -25 and the Moon is magnitude mare - Literally "sea" (a very bad misnomer, still in use for historical reasons); really the large darker gray circular plains seen on the moon. (plural: "maria"). General Glossary Page 4 of 5 mons - Mountain (plural: montes) O old surface A planetary surface that has been modified little since its formation typically featuring large numbers of impact craters.

9 Mercury, Callisto (compare young ). 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 appears largest in a telescope. orbit The path one object takes around another. P penumbra - Literally, "dim light." The lighter part of a shadow found around a shadow s edge. 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. (opposite of aphelion) perturb - To cause a planet or satellite to deviate from a theoretically regular orbital motion. phases The apparent change in the shape of the Moon, Mercury, and Venus due to how much of the sunlit side is visible from the Earth.

10 Photosphere - The visible surface of the Sun the part we see ; sunspots are observed in the photosphere. prominence - A strand of relatively cool gas that protrudes outward from the limb of the Sun, and appears bright against the blackness of space. When seen within the solar disk, it appears as a dark line and is called a filament. R retrograde - Rotation or orbital motion in a clockwise direction when viewed from above the north pole of the primary object ( in the opposite sense to most satellites); the opposite of direct. revolution to go around something When a planet or moon travels around an object. It takes Earth 365 days to make one complete revolution around the sun. rotation to spin When a planet or moon spins on its axis. Earth completes one rotation every 24 hours. S solar wind A stream of high-energy, charged particles (plasma), primarily made of electrons and protons, flowing outward from the Sun at high speed (250-750 km/sec).