| A |
|
| absolute magnitude |
the apparent brightness an object would have if it were 10
parsecs (32.6 light-years) from Earth |
| albedo |
the percentage of light that an object reflects |
| altitude |
1. the height above sea level
2. the angle between an object’s position on the celestial sphere and the horizon |
| angular size |
the apparent width of an object as seen by an observer,
usually expressed in degrees, arcminutes, or arcseconds |
| annular eclipse |
a solar eclipse in which the moon does not fully cover the
disk of the sun, leaving a thin ring of sunlight visible |
| aphelion |
the point farthest from the sun in an object’s orbit |
| apogee |
the point in a satellite’s orbit when it is farthest
from Earth |
| apparent magnitude |
the measure of the brightness of an object as seen from
Earth |
| arcminute |
a unit of angular size equal to 1/60 of a degree |
| arcsecond |
a unit of angular size equal to 1/3,600 of a degree (or 1/60
of an arcminute). |
| asterism |
a small grouping of stars |
| astronomical unit |
the average distance from Earth to the sun, equal to about
93,000,000 miles (150,000,000 km) |
| autumnal equinox |
the time of year around September 23 when the sun crosses
the celestial equator heading south. |
| averted vision |
a technique that uses the more light-sensitive rods in the
eye to better see a faint object by looking at it indirectly |
| azimuth |
the angle along the horizon measured eastward from due north
to the point on the horizon directly below an object |
| B |
|
| bolide |
a brilliant meteor or fireball that explodes in mid-air |
| C |
|
| celestial pole |
the imaginary projection of Earth’s rotational axis
onto the celestial sphere |
| celestial sphere |
the apparent sphere of the sky; an imaginary sphere of
immense radius centered on Earth often used to plot the coordinates of objects in the sky |
| circumpolar stars |
stars which don't drop below the horizon from a given
observing point on Earth. At Earth's Geographical North Pole (90° north latitude), all
stars in the sky are circumpolar. On Earth's equator, no stars are circumpolar. |
| conjunction |
a time when two or more bodies appear close together in the
sky |
| constellation |
one of the 88 patterns of stars in the sky, often named for
a mythological god, hero, or animal |
| convection |
the transfer of heat energy by moving currents of material |
| corona |
the outer atmosphere of the sun or a star |
| crescent |
the phase of a planet or moon during which less than half
the surface is illuminated |
| D |
|
| dark adaptation |
the process by which the human eye becomes well adjusted to
seeing dim objects in the dark. |
| declination |
the angular distance of a celestial object above or below
the celestial equator; the celestial sphere equivalent of latitude |
| deep sky objects |
|
| degree |
1. a unit of angular size equal to 1/360 the circumference
of the celestial sphere; the sun and full moon both appear about half a degree wide
2. a unit of measure for temperature along a graded scale |
| diffraction |
the spreading out of light as it passes the edge of an
obstacle |
| E |
|
| eclipse |
an event in which one body passes in front of another,
blocking it partially or completely from view, a specific type of occultation. |
| elongation |
the apparent angular separation of an object from the sun |
| ephemeris |
a table that gives the positions of astronomical objects at
certain intervals of time. |
| equinox |
the two times of year when the sun crosses the celestial
equator, giving day and night an equal 12-hour length everywhere on Earth. |
| evening star |
the planet Venus when it appears in the evening sky |
| extragalactic |
beyond the Milky Way Galaxy |
| F |
|
| field of view |
the area of sky visible in a telescope or binoculars |
| fireball |
an extremely bright meteor; generally brighter than
magnitude –4 |
| first quarter |
the phase of the moon a quarter of the way around its orbit
from new moon; the eastern half is illuminated. |
| full moon |
the phase of the moon when it is halfway around its orbit
from new moon and opposite the sun in the sky; the full disk is illuminated |
| G |
|
| galaxy |
an enormous gravitationally bound assemblage of millions or
billions of stars |
| gibbous |
the phase of the moon between first quarter and last
quarter, when the moon appears more than half illuminated |
| globular cluster |
a roughly spherical congregation of hundreds of thousands of
stars; most globular clusters consist of old stars and exist in a galaxy’s halo |
| H |
|
| helical rising |
the period of time when an object, such as a star, is
briefly seen in the eastern sky before dawn and is no longer hidden from the glare of the
sun |
| I |
|
| inclination |
the angle between a planet’s orbit and the ecliptic
plane; or the angle between a satellite’s orbit and its host planet’s rotational
plane |
| inferior conjunction |
the configuration of an inferior planet when it lies between
the sun and Earth |
| inferior planet |
a planet that orbits the sun inside of Earth’s orbit;
includes Mercury and Venus |
| International Space Station |
a global cooperative program between the United States,
Russia, Canada, Japan, and Europe, for the joint development, operation, and utilization
of a permanently habitated space station in low-Earth orbit |
| interplanetary |
the space between the planets |
| interstellar |
the space between the stars |
| J |
|
| Jovian planet |
a planet with characteristics similar to Jupiter |
| K |
|
| Kelvin |
a unit of temperature equal to one degree on the Celsius
scale and 1.8 degrees on the Fahrenheit scale; also the absolute temperature scale defined
so that 0 Kelvin is absolute zero |
| L |
|
| last quarter |
the phase of the moon three-quarters of the way around its
orbit from new moon; the western half is illuminated |
| latitude |
the angular distance north or south from the equator to a
point on Earth's surface, measured on the meridian of the point |
| libration |
the small oscillations in the moon’s motion that allow
Earth-based observers to see slightly more than half the moon’s surface |
| light pollution |
light, typically from artificial sources, that reaches the
night sky, obscuring the view of faint astronomical objects |
| light-year |
the distance light travels in one year, equivalent to
approximately 5.9 trillion miles (9.5 trillion km) |
| limb |
the apparent edge of a celestial object |
| limiting magnitude |
the apparent magnitude of the faintest objects that can be
seen given the local observing conditions and any telescope, film, or other detector you
may be using |
| longitude |
the angular distance of a particular place on Earth as
measured east or west from the prime meridian running through Greenwich, England |
| luminosity |
the total amount of light that an object radiates |
| lunar eclipse |
a phenomenon caused by the Earth passing between the sun and
moon |
| lunar month |
the period of one complete revolution of the moon around
Earth, 29.5 days |
| lunation |
the time between two successive new moons; approximately
29.5 days |
| M |
|
| magnetosphere |
the dynamic region around a planet where the magnetic field
traps and controls the movement of charged particles from the solar wind |
| magnitude |
the measurement of an object's brightness; the lower the
number, the brighter the object |
| main sequence |
the band of stars on a Hertzsprung-Russell diagram
stretching from the upper left to the lower right; stars spend most of their lives in the
main sequence phase, in which they are fusing hydrogen into helium in their cores |
| mare |
a dark and relatively smooth area on the surface of the moon
or a planet. |
| megaparsec |
one million parsecs; equal to 3.26 million light-years |
| meridian |
an imaginary circle on the celestial sphere that connects
the zenith to the north (or south) celestial |
| Messier objects |
the 107 bright deep-sky objects that belong to a catalog
compiled by French astronomer Charles Messier in the 1700s |
| meteor |
a flash of light that occurs when a meteoroid burns up in
Earth’s atmosphere; also popularly known as a shooting star |
| meteor shower |
a period of enhanced meteor activity that occurs when Earth
collides with a swarm of meteoroids; an individual shower happens at the same time each
year and has all its meteors appearing to radiate from a common point |
| meteorite |
a rock from space that survives passage through Earth’s
atmosphere and falls to the ground |
| meteoroid |
a small rock that orbits the sun |
| Milky Way |
the band of light that encircles the entire sky and results
from the combined light of billions of stars in our galaxy’s disk |
| Milky Way Galaxy |
the spiral galaxy to which Earth belongs |
| moon |
a smaller body orbiting a larger body; often refers to
Earth’s moon |
| morning star |
the planet Venus when it appears in the morning sky |
| N |
|
| nebula |
a cloud of interstellar gas and dust; some nebulae represent
stellar nurseries, others represent stellar graveyards |
| new moon |
the phase in which the moon is in the same direction as the
sun in Earth’s sky, so it is unilluminated and invisible |
| O |
|
| obliquity |
the angle between a planet’s equator and the plane of
its orbit |
| occultation |
the passage of one object in front of a smaller one,
temporarily obscuring all or part of the background object from view |
| open cluster |
a system containing a few dozen to a few thousand stars that
formed from the same stellar nursery. |
| opposition |
the moment when a planet farther from the sun than Earth
appears opposite the sun in the sky; it is the best time to observe a planet |
| orbit |
the path an object follows around a more massive object or
common center of mass; usually elliptical in shape |
| orbital period |
he length of time it takes one body to orbit another |
| P |
|
| parallax |
the apparent shift of a relatively nearby object against a
fixed background due to the motion of the observer; astronomers observe the parallax of
stars to measure their distances |
| parsec |
the apparent shift of a relatively nearby object against a
fixed background due to the motion of the observer; astronomers observe the parallax of
stars to measure their distances |
| penumbra |
the region of a shadow from which part of the light source
remains visible |
| perigee |
the point in a satellite’s orbit when it is closest to Earth |
| perihelion |
the point in an object’s orbit when it’s closest
to the sun |
| period |
the time interval for a regular event to take place |
| phase |
the regular cycle of changes in the appearance of a moon or
planet |
| planet |
a large rocky or gaseous body that orbits a star |
| planisphere |
a two-dimensional map of the sky with an adjustable overlay
to show the part of the sky visible at any time of the night or year |
| position angle |
the direction in the sky of one celestial object from
another, measured eastward from due north. |
| precession |
the slow, periodic change in the direction an object’s
rotational axis caused by the gravitational influence of another body |
| prime meridian |
the line of longitude that runs through Greenwich, England |
| proper motion |
the apparent yearly motion of a star across the sky |
| R |
|
| retrograde |
objects that move or appear to move in the opposite
direction of most solar system bodies; for example planets that appear to move
east-to-west in the sky or objects that revolve or rotate clockwise as seen from north of
the solar system |
| revolution |
the orbital motion of one body around another body or a
common center of mass |
| right ascension |
the angular distance of a celestial object east of the
vernal equinox; the celestial sphere equivalent of longitude |
| rotation |
the spin of a galaxy, star, planet, moon, or asteroid about
a central axis |
| rotation period |
the length of time it takes a body to complete one rotation |
| S |
|
| satellite |
a small body that orbits a planet or asteroid |
| seeing |
the quality of observing conditions induced by turbulence in
Earth’s atmosphere, which blurs the images of astronomical objects |
| sidereal |
relating to or measured with respect to the stars |
| sidereal year |
the amount of time it takes one body to revolve about
another with respect to the stars |
| solar eclipse |
an eclipse of the sun caused by the moon passing between
Earth and the sun |
| solar filter |
a filter used to block almost all of the sun’s light so
our star can be viewed safely and comfortably. |
| solar system |
the system containing the sun and all the smaller bodies in
orbit around it |
| solar wind |
the stream of charged subatomic particles emanating from the
sun |
| solstice |
either of the two points on the celestial sphere where the
sun is farthest north or south of the celestial equator; when the sun is at a solstice,
the amount of daylight hours is greatest for summer and least for winter |
| South Celestial Pole |
the point in the sky to which Earth's Geographical South
Pole points |
| spectral class |
the designation of a star based on its spectrum, which is
determined by its surface temperature |
| spectral type |
the designation of a star based on its spectrum, which is
determined by its surface temperature |
| spectroscopy |
the study of spectra from astronomical objects |
| spectrum |
1. the energy emitted by a radiant source 2. the entire
range of electromagnetic radiation (light) |
| speed of light |
the fastest possible speed in a vacuum, equivalent to
186,000 miles per second (300,000 km per second). |
| star |
a self-luminous sphere of hot gas held together by gravity;
ordinary stars generate energy by nuclear fusion in their cores |
| star atlas |
an collection of maps that marks the positions of stars,
nebulae, galaxies, and other astronomical objects on a coordinate system |
| star hopping |
the technique of using recognizable patterns of stars to
“hop” from one part of the sky to another; useful in observing both with the
naked eye and a telescope |
| star party |
a gathering of people to observe the night sky |
| sunspot |
a dark, temporary, relatively cool spot on the surface of
the sun |
| sunspot cycle |
a cycle averaging 11 years in which the number of sunspots
increases and decreases. |
| superior conjunction |
the configuration of an inferior planet when it lies on the
far side of the sun |
| superior planet |
a planet farther from the sun than Earth; includes Mars,
Jupiter, Saturn, Uranus, Neptune, and Pluto |
| T |
|
| telescope |
a tubed instrument used to brighten and magnify the view of
astronomical objects (telescopes gather more light than the eye) |
| terminator |
the boundary on a planet or moon separating the illuminated
side from the unilluminated. |
| terrestrial |
of or relating to Earth |
| terrestrial planet |
a small, rocky planet such as Mercury, Venus, Earth, and
Mars |
| transit |
the passage of a smaller body in front of a larger body;
also, the passage of a celestial body across an observer’s meridian. |
| transparency |
the clarity of the sky. |
| tropical year |
the time it takes Earth to revolve around the sun with
respect to the vernal equinox |
| U |
|
| umbra |
the dark, central region of a shadow from which none of the
light source can be seen |
| universal time |
the local time of day on a line of longitude centered on
Greenwich, England (also known as Greenwich Mean Time); it forms the basis for all civil
timekeeping |
| V |
|
| variable star |
a star that varies in luminosity |
| visible light |
the portion of the electromagnetic spectrum visible to the
human eye |
| W |
|
| waning |
the period between full moon and new moon |
| waxing |
the period between new moon and full moon |