Definitions for Astronomy
An Illustrated Dictionary
A technical glossary for the Solar System can also be found here.
[A] [B]
[C] [D] [E]
[F] [G] [H]
[I] [J] [K]
[L] [M] [N]
[O] [P] [Q]
[R] [S] [T]
[U] [V] [W]
[X] [Y] [Z]
A
- Analemma
- If you took a picture of the Sun at the same time each day, would it remain in the same position? The answer is no, and the shape traced out by the Sun over the course of a year is called an analemma. The Sun's apparent shift is caused by the Earth's motion around the Sun when combined with the tilt of the Earth's rotation axis. (From ASTRONOMY PICTURE OF THE DAY)
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- Asteroid
- Small moonlets that orbit the sun. They are made of rock and they shine by reflected sunlight. Many of them rotate and their irregular shapes cause variations of brightness in a day or so. Even the brightest asteroids are too faint to be seen with the naked eye.
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- The surface of an asteroid, Eros, is shown in this movie taken by a space craft as it landed on the asteroid. It shows rocks of all sizes on the surface.
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- Double asteroid --
Asteroids "Ida and Dactyl". In this remarkable system, a small asteroid (about a mile or so) is in orbit about a larger one.
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B
- Big Bang
- According to Einstein's General Theory of Relativity, the expanding universe originated from a singular point in space. This explosive event (about 13 billion years ago) was dubbed the Big Bang by the skeptical British Astrophysicist, Fred Hoyle, and the name stuck. Hoyle was an advocate of the steady state universe, but that theory has very few advocates today. Evidence for the Big Bang is overwhelming.
- Blue Moon
- For one thing, most blue moons aren't really blue! There are two types: those that are and those that are not. Originally, this expression meant something that would hardly ever happen, as in "We will have a blue moon, before such and so...." Then it became the name for the third full moon in a season (three months) of four full moons. Then about 50 years ago it became the name of the second full moon in a month, as Mar. 31, 1999. They are not very rare. They happen every two or three years. The other kind, when the moon really is blue, is a bit more rare. See Blue Sun for an explanatioin of the color.
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- Blue Sun
- The sun can look blue on rare occasions when the atmsophere is filled with peculiar dust from a dessert dust storm or oil droplets produced by a forest fire. Here is an actual picture of a blue sun in 2006, taken by Aymen Ibrahem of the Biblioteca Alexandrina in Egypt.
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- Bok Globules
- Named for the Dutch-American astronomer, Bart J. Bok, who first drew attention to them, these a small black clouds seen against star-filled regions of the Milky Way. They are dense clouds of molecules and dust, and are the sites of active star formation.
BHR 41
Photo Credit & Copyright: J. Alves (ESO), E. Tolstoy (Groningen), R. Fosbury (ST-ECF), & R. Hook (ST-ECF), VLT.
- Brown Dwarf
- An object smaller than a star and larger than a planet. They have so little mass (less than about 0.07 suns) that the nuclear burning of hydrogen did not ignite. Hence they shine by energy released by shrinking, and they will rapidly become cool. Many brown dwarfs have been identified by their low temperatures. (See Fig. below.) The dividing line btween brown dwarfs and planets is somewhat uncertain.
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C
- Celestial Equator
- The projection of the Earth's equator on the celestial sphere.
This circle divides the sky into its northern and southern portions.
- Celestial Pole
- The point where the Earth's axis of daily rotation strikes the celestial sphere. The sky appears to rotate about this point. Polaris, the North Star is close to the North Celestial Pole.
- photo by Gene Steeves
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- Celestial Sphere
- An imaginary sphere against which we see the stars and on
which we measure their positions.
- Cluster
- A group of stars held together by
gravity since they were formed together. There are two main types: Open star clusters and Globular star clusters. One is tight and round, the other is loose and irregular. (See Illustrations.)
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- Globular star Cluster Open star cluster
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- Comet Nucleus
- The long tail of a comet originates from ice and dust particles that are shed from the comet nucleus under the heating action of sunlight. This is a closeup of Comet Tempel 1 at the instant it was struck by a probe which dislodged a cloud of particles.
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- Conjunction
- Two objects are said to be in conjunction when they arrive
at the same longitude on the ecliptic. See Inferior Conjunction
and Superior Conjunction.
- Constellation
- A dot-to-dot picture in the sky. Most groups have no physical
significance.
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D
- Declination
- The celestial equivalent of latitude.
It is measured in degrees, and its symbol is the Greek letter,
delta.
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E
- Earth
- I have capitalized the name of our planet in this book because
I am dealing with astronomy, not soil.
- Eclipse
- One one body covers another or casts its shadow on it. During an eclipse of the sun (upper animation), the moon covers the sun.
- During an eclipse of the moon (below), the earth's shadow falls on the face of the moon. This animation shows the moon passing through the earths shadow.
- Animation copyright by Scot R. Anderson (Open Course)
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- Ecliptic
- The path of the sun as seen against the stars. The band of
sky in which eclipses can occur. It is also the path of the planets
and is tilted 23 degrees from the equator of the Earth.
- Elongation
- Angular distance from the sun. Mercury, for example, is visible
only within a week or two of the times of its greatest elongations.
- Equator
- The great circle halfway between the North and South Poles of the Earth.
- Equinox
- The instant when the sun crosses the celestial equator on
its path around the ecliptic. Fall equinox occurs on September
21 and spring equinox on March 21, although the times vary slightly
from year to year because our calendar has leap years.
- Evening Sky
- The half of the sky opposite the sun.
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F
- Focal Length
- The distance between a lens and the image it makes of a distant
object. You can determine the focal length of your camera lens
by taking it out and casting an image onto a piece of paper.
(One inch is 25.4 millimeters.)
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G
- Galaxy
- An "island universe," or an independent system
of stars. Our Milky Way is one. It contains hundreds of billions
of stars and unknown millions of planets
and "solar systems."
- The following diagram shows Hubble's scheme for classification of galaxies as seen on visible photographs, developed in the 1920s.
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- This scheme is shaped like a tuning fork, with the ellipticals (E) at the stem and the normal spirals (S) and barred spirals (SB) along the two arms. It was originally interpreted as a sequence of aging, but this is no longer the case. The actual evolution of shapes of galaxies is still poorly understood.
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- Gamma Ray
- A high-energy bundle of light; a photon produced by nuclear
reactions or gas at extremely high temperatures. A Gamma Ray
Burster is a mysterious cosmic explosion which is observed to
take place in galaxies at the outer reaches of the universe.
They are considered the greatest explosions since the Big
Bang, but their cause is still uncertain. See NASA Web
link for latest news.
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- Gravitational Lens
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Abell 2218: A Galaxy Cluster Lens
Picture Credit: NASA, HST, WFPC2, W. Couch (UNSW)
- The gravity of a mass, in this case a giant cluster at the center, can cause light to bend and refocus, in much the way a glass bottle-bottom can focus light passing through it. This amazing photo shows the refocused (and distorted) images of dozens of very distant galaxies that might otherwise be too faint for detection. Even the thinnest streaks shown here are images of galaxies. By detailed study it is often possible to identify their types and their distances (by measuring the red-shift of their spectrum).
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H
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I
- Inferior Conjunction
- The instant when Mercury or Venus comes between us and the
sun. More precisely, when one of them arrives at the sun's longitude and is on the near side of its
orbit.
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J
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K
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L
- Latitude
- A measure of distance from the equator, expressed as the
angle between the equator and a point
as seen from the center of the Earth. Simimlar to declination.
- Light Echo
- Expanding reflection of a pulse of light as it moves out through a nebula.
- In early 2002, a relatively undistinguished star named V838 Monocerotis flared twice in February and March before fading away. The series of five images above were captured by the Hubble Space Telescope. They show the outward journey of the light from the flares.
The casual viewer might think these frames show an expanding shell of glowing dust and gas moving away from V838 Mon. In truth, the flareups are merely illuminating progressively larger spheres of preexisting, stationary material in space. We are seeing a "light echo" bouncing off this material, the same way echoes of a sound can be heard bouncing off objects near the sound's source.
- Longitude
- A measure of distance from Greenwich, England, expressed
as an angle along the equator from the meridian of Greenwich
to the meridian of the point in question. Similar to Right
Ascension.
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M
- Magnification
- The ratio of angular size with and without an optical device.
With 10X magnification, an object appears ten times as broad.
Equivalently, it appears to be at one-tenth the distance.
- Magnitude
- A measure of the faintness of a star. Magnitude 6.0 is the
faintest we can ordinarily see, and the average of the twenty
brightest stars defines magnitude 1.0. Each magnitude step is
a factor of 2.5 in brightness.
- Meridian
- An observer's meridian passes through his zenith
at the celestial poles. It is his north-south line.
- Meteor
- A flash of light produced by a particle in the atmosphere
moving so rapidly that it becomes luminous.
- Meteorite
- A rock that once looked like a meteor.
- Morning Sky
- That half of the celestial sphere containing the sun.
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N
- Near Earth Asteroid
- As the name implies, this is an asteroid (rock) that passes near to the earth. The movie below shows an NEA that came within 35,000 miles of the earth, about 1/8 of the distance to the moon! These images were captured in less than a quarter of an hour.
- Nebula
- A cloud of gas or dust among the stars. A nebula may be luminous
(like a neon sign) if excited by a nearby star, or, if it is
dusty it may blot out the background of starlight. In the past,
this term was also used for galaxies because they appeared nebulous.
This is called the "Eagle Nebula"
- Noctilucent Cloud
- Seen in the twilight, a noctilucent cloud is produced by atmospheric particles at heights of 80 to 100 km above the earth. Their exact origin is not understood. In the picture below,
the cloud is the blue wisps seen above the normal sunset orange.
Photo Credit Pekka Parviainen
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O
- Opposition
- The moment when one of the outer planets is directly opposite
the sun. Because the orbits are fairly circular, this nearly
coincides with the time of closest approach to the Earth.
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P
- Planet
- (literally, a "wanderer"): A solid object in motion
about a star. If it is very small, it is called an asteroid
See also Brown Dwarf .
- Planetary Nebula
- As the name might imply, this is a nebula with a broad face, somewhat like a planet. They actually come in all sorts of remarkable shapes, usually roundish, spiralling, or bi-lobed like a butterfly, and nearly as colorful. They are produced in the death throes of an old star shedding its outer layers and revealing a hot interior. So they all have a blue star at the center. The planetary nebula in Lyra is the brightest and is a good challenge for small telescopes. (At right is NGC 6543.)
- Polar Cap
- Ice and frost covered region at the pole of a planet, esp. Mars. The polar caps are round and vary with the planetary season. The photo shows a Martial polar cap (at bottom of image) as seen from Earth in a modest telescope.
- Pole
- The intersection of the Earth's axis of daily rotation with the surface of the Earth or the celestial sphere.
- Precession of the Equinoxes
- The Earth's axis of daily rotation slowly swings about the
perpendicular to the sun's path. The motion is similar to that
of a spinning top and is caused by the moon's gravitational pull
on the Earth's equatorial bulge. The equinoxes slide around the
ecliptic in 26,000 years.
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- Protoplanetary disc
- The ring-filled dust disk around HD 141569A, in which planets may or may not be forming.
In this computer-enhance image, the shape has been stretched to circular and some artifacts
have been removed. The black areas are omitted because of poor data, and the arcs on the
upper image were added to highlight the rung structure. The part of the disk seen here
extends 400 astronomical units from the star. (For reference, the earth-sun distance
is 1 astronomical unit.) The Hubble Space Telescope's new Advanced Camera for
Surveys (ACS) has captured a detailed image of spiral rings in a distant protoplanetary
disk — the pancake-shaped cloud of gas and dust around a young star
in which planets are expected to condense. But contrary to earlier suggestions,
the intricate structure of this particular disk is probably caused by a
nearby companion star rather than by embedded planets starting to form.
Image Courtesy Mark Clampin. Text courtesy Grovert Schilling and Sky &
Telescope Magazine
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- Pulsar
- A rapidly pulsing radio signal produced by a rotating neutron star with a magnetic field. This is an artistic animation of what it is thought to look like. The magnetic field guides the emission of the radio signals which come out in two opposite narrow beams. Animation by Andy Anderson
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- Q
- Quasar
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- A Quasar Portrait Gallery
Credit J. Bahcall (IAS, Princeton), M. Disney (Univ. Wales), NASA
Quasars (QUASi-stellAR objects) lie near the edge of the observable Universe. Discovered in 1963, astronomers were astounded that such objects could be visible across billions of light-years, as this implies they must emit prodigious amounts of energy. Where does the energy come from? Many believe the quasar's central engine is a giant black hole fueled by tremendous amounts of infalling gas, dust, and stars. This gallery of quasar portraits from the Hubble Space Telescope offers a look at their local neighborhoods: the quasars themselves appear as the bright star-like objects with diffraction spikes. The images in the center and right hand columns reveal quasars associated with disrupted colliding and merging galaxies which should provide plenty of debris to feed a hungry black hole. From Astronomy Picture of the Day.
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R
- Retrograde
- Backward or reversed. Mar, Jupiter and Saturn show retrograde
motions among the stars.
- Right Ascension
- The equivalent of longitude on the
celestial sphere. It is measured in hours from the spring equinox,
where the ecliptic and equator intersect. The symbol for right
ascension is alpha, a. One hour of right ascension is 15°
of longitude.
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S
- Sky
- The visible portion of the celestial sphere. On a dark night,
the average person can see about 2000 stars.
- Solstice
- The moment when the sun farthest from the Earth's equator.
The summer solstice (in June) marks the official first day of
summer. The winter solstice (in December) marks the official
first day of winter. As soon as the solstice is passed, the sun
starts accelerating toward the equator.
- Spectrum
- The rainbow of colored light from an object. By a measurement
of the amount of light of different colors in the spectrum, astronomers
can determine the temperature and the chemical composition of
the outer layers of stars and planets. The moon and asteroids,
that shine by reflected light, show a spectrum very much like
that of the sun.
- Sprite
- Red sprites are recently discovered and still poorly understood optical flashes seen dancing at altitudes of 30 to 90 kilometers above thunderstorms. Cousins to lightning bolts, red sprites occur near the edge of the atmosphere and have been glimpsed by astronauts from orbit. What ever their cause, the red sprite flashes usually last only tenths to hundredths of a second and characteristically take on shapes which researchers describe as columns, fingers, trees, or carrots.This dramatic, garishly colored image was captured with a low-light level camera on 2001 June 7. It shows what appears to be a "burning tree" above the National Cheng Kung University campus in Tainan City, Taiwan ... but the burning tree is actually a fleeting red sprite 300 kilometers away
- Star Cluster
- A group of stars within a galaxy. Some clusters contain dozens
of stars; others contain millions.
- Sun Dogs
- Patches of light in the sky near the sun. They are produced
by crystals of snow and ice high in the atmosphere.
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- Sun Pillar
- Vertical column of light in the sky near the sun. It is produced by crystals of snow and ice falling through the atmosphere. (See photo)
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- Sunrise and Sunset
- The moment when the sun's upper edge touches the horizon.
In an almanac, the computed time corresponds to an imaginary
horizon 90 degrees from the overhead point known as the zenith.
- Superior Conjunction
- The instant when a planet on the far side of the sun has
the longitude of the sun. This is a time of invisibility, as
is inferior conjunction.
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T
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U
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V
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W
- Waning Crescent
- Decreasing crescent. This occurs between full moon and new
moon.
- Waxing Crescent
- Increasing crescent. This occurs when the moon is not waning, that is, from new moon to full moon, when the crescent is concave to the east (toward left in this picture).
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X
Y
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Z
- Zenith
- Straight up.
- Zodiac
- A band of 12 constellations along the ecliptic,
or sun's annual path around the celestial sphere. The moon and
planets remain in the zodiac.
- Zodiacal Light
- A wedge-shaped band of light seen (on the right) in the twilight, rising up from the sun, which is below the horizon. It is produced by dust in the solar system in the plane of the planets and the earth's orbit. It is often too faint to be seen. In this picture, a few Leonids can be seen. Note the Big Dipper standing vertically on the left.
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David Harvey Photo: 15 mm. fisheye mounted on telescope, 30 min exposure.
Copyright
© 1996, Charles A. Whitney. E-mail: charleswhitney@comcast.net
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