Astronomy Key Terms

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apparent solar time

Back 1

Time kept according to the actual position of the Sun in the sky. Apparent solar noon occurs when the Sun crosses an observer’s meridian

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autumnal equinox

Back 2

The point in the sky where the Sun appears to cross the celestial equator moving from north to south. This happens on approximately September 22

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autumnal equinox

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autmn equinox

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celestial equator

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The circle where the Earth’s equator, if extended outward into space, would intersect the celestial sphere.

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conjunction

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The appearance of two celestial bodies, often a planet and the Sun, in approximately the same direction

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crescent phase

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The phase of the Moon at which only a small, crescentshaped portion of the near side of the Moon is illuminated by sunlight. Crescent phase occurs just before and after new Moon.

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declination

Back 7

The angular distance of a celestial body north or south of the celestial equator. Declination is analogous to latitude in the terrestrial coordinate system

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direct motion

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The eastward apparent motion of a solar system body with respect to the stars. Direct motion is interrupted by regular episodes of retrograde (westward) motion

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diurnal

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daily

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diurnal circle

Back 10

The circular path that a celestial body traces out as it appears to move across the sky during an entire day. Diurnal circles are centered on the north and south celestial poles.

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ecliptic

Back 11

The plane of the Earth’s orbit about the Sun. As a result of the Earth’s motion, the Sun appears to move among the stars, following a path that is also called the ecliptic

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equatorial system

Back 12

A coordinate system, using right ascension and declination as coordinates, used to describe the angular location of bodies in the sky.

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full phase

Back 13

The phase of the Moon at which the bright side of the Moon is the face turned toward the Earth.

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gibbous phase

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The phase of the Moon at which the near side of the Moon is more than half illuminated by sunlight. Gibbous phase occurs just before and after full Moon

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leap years

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ue

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local years

Back 16

asa

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local hour angle

Back 17

The angle, measured westward around the celestial equator, between the meridian and the point on the equator nearest a particular celestial object

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mean solar time

Back 18

Time kept according to the average length of the solar day

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meridian

Back 19

The great circle passing through an observer’s zenith and the north and south celestial poles

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new phase

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The phase of the Moon in which none or almost none of the near side of the Moon is illuminated by sunlight, so the near side appears dark.

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nodes

Back 21

The points in the orbit of the Moon where the Moon crosses the ecliptic plane

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north celestial pole

Back 22

The point above the Earth’s North Pole where the Earth’s polar axis, if extended outward into space, would intersect the celestial sphere. The diurnal circles of stars in the northern hemisphere are centered on the north celestial pole

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north circumpolar region

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The region of the northern sky within which the diurnal circles of stars do not dip below the horizon. The size of the north circumpolar region varies with the latitude of the observer.

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opposition

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The configuration of a planet or other body when it appears opposite the Sun in the sky

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orbit

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The elliptical or circular path followed by a body that is bound to another body by their mutual gravitational attraction. (p. 31)

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prograde motion

Back 26

The eastward (normal) revolution of a solar system body.

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quarter phase

Back 27

The phase of the Moon in which half of the near side of the Moon is illuminated by the Sun

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retrograde motion

Back 28

The westward revolution of a solar system body around the Sun.

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right ascension

Back 29

the greater the richness. (p. 602) right ascension Angular distance of a body along the celestial equator from the vernal equinox eastward to the point on the equator nearest the body. Right ascension is analogous to longitude in the terrestrial coordinate system.

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sidereal clocks

Back 30

A clock that marks the local hour angle of the vernal equinox.

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sidereal day

Back 31

The length of time (23 hours, 56 minutes, 4.091 seconds) between successive appearances of a star on the meridian

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sidereal month

Back 32

The length of time required for the Moon to return to the same apparent position among the stars.

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solar day

Back 33

The amount of time that passes between successive appearances of the Sun on the meridian. The solar day varies in length throughout the year

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south celestial pole

Back 34

The point above the Earth’s South Pole where the Earth’s polar axis, if extended outward into space, would intersect the celestial sphere. The diurnal circles of stars in the southern hemisphere are centered on the south celestial pole.

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standard time

Back 35

The time kept throughout one of Earth’s approximately 15-degree-wide time zones.

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summer solstice

Back 36

The point on the ecliptic where the Sun’s declination is most northerly. The time when the Sun is at the summer solstice, around June 21, marks the beginning of summer.

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synodic month

Back 37

The length of time (29.53 days) between successive occurrences of the same phase of the Moon

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synodic period

Back 38

The length of time it takes a solar system body to return to the same configuration (opposition to opposition, for example) with respect to the Earth and the Sun. (p. 32)

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tropical year

Back 39

The interval of time, equal to 365.242 solar days, between successive appearances of the Sun at the vernal equinox.

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vernal equinox

Back 40

The point in the sky where the Sun appears to cross the celestial equator moving from south to north. This happens approximately on March 21.

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waning crescent

Back 41

The Moon’s crescent phase that occurs just before new Moon.

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waxing crescent

Back 42

The Moon’s crescent phase that occurs just after new Moon

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winter solstice

Back 43

The point on the ecliptic where the Sun has the most southerly declination. The time when the Sun is at the winter solstice, around December 22, marks the beginning of winter.

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year

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year

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zodiacal constellaltions

Back 45

The band of constellations along the ecliptic. The Sun appears to move through the 12 zodiacal constellations during a year.

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Astrology

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A pseudoscience that holds that people and events are in fluenced by the configurations of the Sun, Moon, and planets with respect to each other and the stars.

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deferent

Back 47

One of the circles on which a planet moved according to the Ptolemaic model of the solar system

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epicycle

Back 48

One of the circles upon which a planet moved according to the Ptolemaic (geocentric) model of the solar system. The center of the epicycle moved on a larger circle, called the deferent.

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equant

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In the Ptolemaic system, the point from which the motion of the epicycle around the deferent is uniform.

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geocentric

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Centered on the Earth. In a geocentric model of the solar system, the planets moved about the Earth

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precession

Back 51

The slow, periodic conical motion of the rotation axis of the Earth or another rotating body

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aphelion

Back 52

The point in the orbit of a solar system body where it is farthest from the sun

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astronomical unit

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The average distance between earth and the sun

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eccentricity

Back 54

A measure of the extent to which an orbit departs from circularity. Eccentricity ranges from 0.0 for a circle to 1.0 for a parabola.

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ellipse

Back 55

A closed, elongated curve describing the shape of the orbit that one body follows about another.

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foci

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One of two points from which an ellipse is generated. For all points on the ellipse, the sum of the distances to the two foci is the same

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greatest elongation

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The position of Mercury or Venus when it has the greatest angular distance from the Sun.

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heliocentric

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Centered on the Sun. In the heliocentric model of the solar system, the planets move about the Sun

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impetus

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A theory of motion, developed in the fourteenth and fifteenth centuries, that motion could continue only so long as a force was at work.

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inferior planet

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A planet whose orbit lies inside the Earth’s orbit.

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kepler's laws of planetary motion

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Three laws, discovered by Kepler, that describe the motions of the planets around the Sun

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major axis

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The axis of an ellipse that passes through both foci. The major axis is the longest straight line that can be drawn inside an ellipse.

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perihelion

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The point in the orbit of a body when it is closest to the Sun.

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semimajor axis

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Half of the major axis of an ellipse. Also equal to the average distance from the focus of a body moving on an elliptical orbit

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sidereal period

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The time it takes for a planet or satellite to complete one full orbit about the Sun or its parent planet

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stellar parallax

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The shift in the direction of a star caused by the change in the position of the Earth as it moves about the Sun.

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superior planet

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A planet whose orbit lies outside the Earth’s orbit

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transverse velocity

Back 68

The part of the orbital speed of a body perpendicular to the Sun between the body and the Sun

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universe

Back 69

All the matter and space there is

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Acceleration

Back 70

The rate of change of velocity. An acceleration may involve a change of speed, direction of motion, or both

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acceleration of gravity

Back 71

The acceleration of a body, equal to 9.8 meters per second per second, caused by the force of gravity near the surface of the Earth.

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angular momentum

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The momentum of a body associated with its rotation or revolution. For a body in a circular orbit, angular mo mentum is the product of orbital distance, orbital speed, and mass. When two bodies collide or interact, angular momentum is conserved.

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central force

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A force directed at the center of motion of a body. Gravity is the central force that accounts for the orbital motion of solar system bodies.

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centripetal acceleration

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The acceleration toward the center of motion, that causes the path of an orbiting body to continually bend away from a straight-line path.

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centripetal force

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The central force that produces centripetal acceleration.

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circle

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A curve on which all points are equidistant from the center

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circular speed

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The speed that causes an orbiting body to have a circular orbit rather than an elliptic one.

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conic section

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One of four kinds of curves (circle, ellipse, hyperbola, and parabola) that can be formed by slicing a right circular cone with a plane.

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ellipse

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A closed, elongated curve describing the shape of the orbit that one body follows about another.

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escape velocity

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The speed that an object must have to achieve a parabolic trajectory and escape from its parent body

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force

Back 81

A push or a pull

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gravitational potential energy

Back 82

The energy stored in a body subject to the gravitational attraction of another body. As the body falls, its gravitational potential energy decreases and is converted into kinetic energy

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gravity

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The force of attraction between two bodies generated by their masses.

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hyperbola

Back 84

A curved path that does not close on itself. A body moving with a speed greater than escape velocity follows a hyperbola

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inertia

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The tendency of a body at rest to remain at rest and a body in motion to remain in motion at a constant speed and in constant direction

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inertial motion

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Motion in a straight line at constant speed followed by a body when there are no unbalanced forces acting on it

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kinetic energy

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Energy of motion. Kinetic energy is given by one-half the product of a body’s mass and the square of its speed.

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mass

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A measure of the amount of matter a body contains. Mass is also a measure of the inertia of a body

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momentum

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A quantity, equal to the product of a body’s mass and velocity, used to describe the motion of the body. When two bodies collide or otherwise interact, the sum of their momenta is conserved.

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neap tide

Back 90

An unusually low high tide and unusually high low tide that occur when the tidal forces of the Sun and Moon act at right angles to one another

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parabola

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A geometric curve followed by a body that moves with a speed exactly equal to escape velocity

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spring tide

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Unusually high, high tide and unusually low, low tide that occur when the tidal forces of the Sun and Moon are aligned. This occurs at full Moon and new Moon.

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tidal forces

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The differences in gravity in a body being attracted by another body.

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tides

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Distortions in a body’s shape resulting from tidal forces.

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vector

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A quantity that has both direction and magnitude. Velocity is a vector, whereas speed is not.

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velocity

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A physical quantity that gives the speed of a body and the direction in which it is moving.

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weight

Back 97

The gravitational force exerted on a body by the Earth (or another astronomical object).

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adaptive optics

Back 98

A system for modifying the shape of the mirror of a telescope to compensate for atmospheric seeing and to produce sharp images.

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charge coupled device

Back 99

An array of photosensitive electronic elements that can be used to record an image falling on it

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detector

Back 100

A device used to measure light once it has been brought into focus by a telescope

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dispersion

Back 101

The separation of white light according to wavelength. Dispersion produces a rainbowlike spectrum.

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doppler effect

Back 102

The change in the frequency of a wave (such as electromagnetic radiation) caused by the motion of the source and observer toward or away from each other

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electromagnetic spectrum

Back 103

The range of frequency or wavelength of all possible electromagnetic radiation. The electromagnetic spectrum includes (in order of increasing wavelength) gamma ray, X ray, ultraviolet, visible, infrared, and radio.

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electromagnetic wave

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A periodic electrical and magnetic disturbance that propagates through space and transparent materials at the speed of light. Light is an example of an electromagnetic wave

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energy flux

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The rate at which a wave carries energy through a given area.

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focal length

Back 106

The distance between a mirror or lens and the point at which the lens or mirror brings light to a focus.

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focal plane

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The surface where the objective lens or mirror of a telescope forms the image of an extended object

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focal point

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The spot where parallel beams of light striking a lens or mirror are brought to a focus

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frequency

Back 109

The number of oscillations per second of a wave.

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gamma ray

Back 110

The part of the electromagnetic spectrum having the shortest wavelengths

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index of refraction

Back 111

The ratio of the speed of light in a vacuum to the speed of light in a particular substance. The index of refraction, which always has a value greater than 1.0, describes how much a beam of light is bent on entering or emerging from the substance.

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infared

Back 112

The part of the electromagnetic spectrum having wavelengths longer than visible light but shorter than radio waves.

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interferometry

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The use of two or more telescopes connected together to operate as a single instrument. Interferometers can achieve high angular resolution if the individual telescopes of which they are made are widely separated.

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light

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The visible form of electromagnetic radiation

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light-gatheriing power

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A number, proportional to the area of the principal lens or mirror of a telescope, that describes the amount of light that is collected and focused by the telescope

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objective

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The main lens or mirror of a telescope

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photometer

Back 117

An instrument used to measure the brightness of a source of electromagnetic radiation such as a planet, star, or galaxy.

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photon

Back 118

A massless particle of electromagnetic energy

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pixel

Back 119

A “picture element,” consisting of an individual detector in an array of detectors used to capture an image

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reflection

Back 120

The bouncing of a wave from a surface

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reflectivity

Back 121

The ability of a surface to reflect electromagnetic waves. The reflectivity of a surface ranges from 0% for a surface that reflects no light to 100% for a surface that reflects all the light falling on it.

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reflector

Back 122

A telescope in which the objective is a mirror

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refraction

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The bending of light when it passes from a material having one index of refraction to another material having a different index of refraction.

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refractor

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A telescope in which the objective is a lens

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resolution

Back 125

The ability of a telescope to distinguish fine details of an image.

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seeing

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A measure of the blurring of the image of an astronomical object caused by turbulence in the Earth’s atmosphere

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speckle interferometry

Back 127

A technique to overcome seeing by analyzing very brief images.

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spectograph

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A device used to produce and record a spectrum

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spectroscopy

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The recording and analysis of spectra

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ultraviolet

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The part of the electromagnetic spectrum with wavelengths longer than X rays, but shorter than visible light

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wave

Back 131

A regular series of disturbances that moves through a material medium or through empty space

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wavelength

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The distance between crests of a wave. For visible light, wavelength determines color

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X ray

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The part of the electromagnetic spectrum with wavelengths longer than gamma rays but shorter than ultraviolet

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absolute zero

Back 134

The lowest possible temperature. At absolute zero, atoms and other particles stop moving

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accretion

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The growth in the mass of a body by the infall of matter gravitationally attracted to the body

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accretional heating

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The heating of a body by the impacts that occur as it grows by adding infalling material.

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albedo

Back 137

The ratio of the light reflected in all directions by a surface to the light incident on it. A perfectly reflecting surface has an albedo of 1, a perfectly absorbing surface has an albedo of 0

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atom

Back 138

A particle consisting of a nucleus and one or more surrounding electrons.

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atomic numer

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The number of protons in the nucleus of an atom. Unless the atom is ionized, the atomic number is also the number of electrons orbiting the nucleus of the atom.

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blackbody

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An object that is a perfect absorber of radiation falling on it.

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blackbody radiation

Back 141

The electromagnetic radiation emitted by a blackbody. The spectrum and intensity of blackbody radiation are controlled by the temperature of the blackbody. Many stars and other celestial bodies approximate blackbodies.

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conduction

Back 142

The transfer of heat by means of direct collisions between adjacent atoms, molecules, or ions

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convection

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The process of energy transport in which heat is carried by hot, rising and cool, falling currents or bubbles of liquid or gas.

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density

Back 144

The mass of a body divided by its volume.

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deuterium

Back 145

An isotope of hydrogen. The nucleus of a deuterium atom is a deuteron

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electron

Back 146

A low-mass, negatively charged particle that can either orbit a nucleus as part of an atom or exist independently as part of a plasma

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element

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A substance that cannot be broken down into a simpler chemical substance. Oxygen, nitrogen, and silicon are examples of the approximately 100 known elements.

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exosphere

Back 148

The outer part of the thermosphere. Atoms and ions can escape from the exosphere directly into space

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fission

Back 149

A nuclear reaction in which a nucleus splits to produce two less massive nuclei. (p. 143) (B) A possible explanation for the origin of a close binary pair of stars in which a star splits into two pieces, each of which becomes a star.

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fusion

Back 150

A nuclear reaction in which two nuclei merge to form a more massive nucleus.

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half-life

Back 151

The time required for half of the atoms of a radioactive substance to disintegrate

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hydrostatic equlinrium

Back 152

The balance between the inward directed gravitational force and the outward directed pressure force within a celestial body

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ion

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An atom from which one or more electrons has been removed.

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isotope

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Nuclei with the same number of protons but different numbers of neutrons

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kelvin temperature scale

Back 155

A temperature scale (like Fahrenheit and Celsius) in which 0 K is defined as absolute zero and 273.15 K is defined as the melting point of ice

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mass number

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A measure of the mass of a nucleus given by the total number of protons and neutrons in the nucleus.

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neutral gas

Back 157

A gas containing atoms and molecules but essentially no ions or free electrons.

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neutron

Back 158

A nuclear particle with no electric charge.

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nucleus

Back 159

The massive, positively charged core of an atom. The nucleus of an atom is surrounded by one or more electrons. A nucleus missing one or more accompanying electrons is called an ion. Nuclei consist of protons and electrons. (p. 142) (B) An irregularly shaped, loosely packed lump of dirty ice several kilometers across that is the permanent part of a comet.

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number density

Back 160

The number of particles in a given volume of space.

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planetology

Back 161

The comparative study of the properties of planets

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plasma

Back 162

A fully or partially ionized gas.

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pressure

Back 163

The force exerted per unit area

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proton

Back 164

A positively charged nuclear particle.

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radiative transfer

Back 165

The transport of energy by electromagnetic radiation.

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radioactive

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yea

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solar constant

Back 167

The solar energy received by a square meter of surface oriented at right angles to the direction to the Sun at the Earth’s average distance (1 AU) from the Sun. The value of the solar constant is 1372 watts per square meter.

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stefan-boltzmann law

Back 168

The relationship between the temperature of a blackbody and the rate at which it emits radiant energy.

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1

Back 169

1

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temperature

Back 170

A measure of the average energies of the particles in a system. For a gas, temperature is a measure of the motions of the particles. (

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thermal equilibrium

Back 171

The condition in which a body or a portion of a body gains energy (by generating it or absorbing it) at the same rate at which energy is transported away from it.

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wien's law

Back 172

The relationship between the temperature of a blackbody and the wavelength at which its emission is brightest.