مسرد مصطلحات المذنبات والفلك
Glossary of (comet and) astronomical terms
Below are listed alphabetically some terms,
with explanations, that one might encounter frequently in reading
about topics on these ICQ/CBAT/MPC World Wide Web pages.
The angular distance from the
observer's horizon, usually taken to be that horizon that is
unobstructed by natural or artificial features (such as mountains or
buildings), measured directly up from the horizon toward the zenith;
positive numbers indicate values of altitude above the horizon, and
negative numbers indicate below the horizon --- with negative
numbers usually being used in terms of how far below the horizon the
sun is situated at a given time [for example, the boundary between
civil twilight and nautical twilight is when the sun is at altitude
The size of the primary
optical surface of an astronomical instrument (telescope), usually
given in inches, centimeters, or meters. In the case of a reflecting
telescope, the aperture usually refers to the size of the main
mirror; in the case of a refracting telescope (of which binoculars
are one example), the aperture refers to the size of the primary
lens (which in binoculars is usually given in millimeters).
For an object orbiting the
sun, the point (distance and time) where/when the object is furthest
from the sun in its elliptical orbit.
- Arc minutes.
There are 60 minutes (denoted
as 60') of arc in 1 degree. In the sky, with an unobstructed horizon
(as on the ocean), one can see about 180 degrees of sky at once, and
there are 90 degrees from the true horizon to the zenith. The full
moon is about 30' (30 arc minutes) across, or half a degree. There
are 60 seconds (denoted 60") of arc in one minute of arc.
The careful, precise
measurement of astronomical objects, usually made with respect to
standard catalogues of star positions. For comet orbit computations,
astrometry good to 1" or 2" (1 or 2 arc seconds), or better, is the
- Astronomical Unit (AU).
Approximately equal to the
mean earth-sun distance, which is about 150,000,000 km or 93,000,000
miles. Formally, the AU is actually slightly less than the earth's
mean distance from the sun (semi-major axis) because it is the
radius of a circular orbit of negligible mass (and unperturbed by
other planets) that revolves about the sun in a specific period of
see Astronomical Unit.
Angular distance measured
clockwise around the observer's horizon in units of degrees;
astronomers usually take north to be 0 degrees, east to be 90
degrees, south to be 180 degrees, and west to be 270 degrees.
The center of mass of a system
of bodies, such as the solar system. When a comet, for example, is
well outside the orbit of Neptune (the farthest major planet), it
sees the sun and major planets essentially as a single object of
summed mass, and the center of this mass (called the barycenter of
the solar system) is offset somewhat from the sun; "original" and
"future" orbits of long-period comets are computed for this
barycenter, while perturbed, osculating orbits of currently-observed
objects in the inner solar system are computed for heliocentric
- Barycentric Dynamical Time
Differing from TDT only via
periodic variations, TDB is used in ephemerides and equations of
motion that refer to the barycenter of the solar system.
- Besselian year.
A quantity introduced by F. W.
Bessel in the nineteenth century that has been used into the
twentieth century. Bessel introduced a system whereby it would be
convenient to identify any instant of time by giving the year and
the decimal fraction of the year to a few places, but the starting
time of the year was not convenient for dynamical studies that
utilize Julian dates (see definition for Julian date), differing by
0.5 day, and the Besselian year varies slowly. The recent change to
Julian year usage in dynamical astronomy (and the J2000.0 equinox)
took effect in solar-system ephemerides of the Minor Planet Center
and Central Bureau for Astronomical Telegrams on Jan. 1, 1992. (See
Charge-coupled device, a very
sensitive electronic device that is revolutionizing astronomy in the
1990s. CCD cameras are composed of silicon chips that are sensitive
to light, changing detected photons of light into electronic signals
that can then be used to make images of astronomical objects or to
analyze how much light is being received from such objects. CCDs
require computers for reduction of data, so the expense can be much
greater than for, say, photography --- but CCDs can detect much
fainter objects than can photographs. Unfiltered CCDs tend to be
more red-sensitive than the human eye.
- Celestial sphere.
An imaginary sphere of great
(or infinite) radius that is centered on the earth and is used for
practical purposes in astronomical observing. Since stars (other
than our own sun!) are very distant from us, they make up a
background that is essentially unchanging from year to year; of
course, over a period of years, the closer stars will move very
slightly and factors such as precession cause a change in the
appearance of the stars in our skies over many years. But we create
a map grid on the celestial sphere for identifying, referring to,
and locating objects in the sky; some of these map grids include
equatorial coordinates (right ascension and declination), ecliptic
coordinates (ecliptic longitude and latitude), and galactic
coordinates (galactic longitude and latitude) --- which refer to the
earth's rotation, the earth's revolution about the sun, and the
Milky Way galaxy's plane, respectively.
A comet's atmosphere (composed
of dust and/or various gases) surrounding its nucleus. The coma is
rather tenuous (except very close to the nucleus), and stars can be
occasionally easily seen through it, shining from behind. And yet,
the coma is usually thick enough that it masks our view of the true
nucleus of the comet, as seen from the earth. As a comet's nucleus
is usually quite small, it is not able to retain its coma for long
periods of time, and the coma material gradually drifts away into
space (helped out by the solar wind). Much coma material is thrown
back into what we see as the comet's tail. But all coma material
originates in the comet's nucleus, and solar sublimation due to
heating causes gases to move outward, often in jets, taking dust
material with them to form the coma and tail.
A celestial body orbiting the
sun (though some may be ejected from the solar system by planetary
perturbations) that displays (at least during a portion of its
orbit) some diffuseness and/or a "tail" of debris that points
generally in the anti-solar direction. Both the diffuseness
(generally called a coma) and the tail are composed of gas and/or
dust of various atomic or molecular compositions, as is ascertained
by spectroscopy. The coma and tail material come from a much smaller
nucleus that is usually invisible due to the bright surrounding coma
activity. Close-up pictures of a cometary nucleus did not occur
until spacecraft fly-bys of Halley's comet in 1986. A more detailed
explanation of what a comet is may be found in the
Press Information Sheet
on comet C/1995 O1 (Hale-Bopp).
One element of the
astronomical coordinate system on the sky that is used by
astronomers. Declination, which can be thought of as latitude on the
earth projected onto the sky, is usually denoted by the lower-case
Greek letter delta and is measured north (+) and south (-) of the
celestial equator in degrees, minutes, and seconds of arc. The
celestial equator is defined as being at declination zero (0)
degrees; the north and south celestial poles are defined as being at
+90 and -90 degrees, respectively. When specifying a comet's
location on the sky, one must state the right ascension and
declination (with equinox), along with date and time (since a comet
moves with respect to the background stars). For examples of how
right ascension (R.A.) and declination (Decl.) appear on a star
the Millennium Atlas.
A unit used in the measurement
of angles, heavily used particularly in astronomy. Due to ancient
Babylonian mathematics, we still divide a circle into 360 even units
of arc and call each of these units one degree. The entire sky,
therefore, spans 360 degrees. Up to about 180 degrees of sky is
visible from any given point on earth with an unobstructed horizon
(as measured from, say, east to west, or north to south). The degree
is used to make measurements of distance, or position (as with
declination) in astronomy. In turn, a degree is composed of 60
minutes of arc, and also of 360 seconds of arc.
The upper-case Greek letter
used to denote an object's geocentric distance in ephemeris tables;
see "ephemeris". (Note that lower-case delta is used to denote
The apparent path of the sun
against the sky background (celestial sphere); formally, the mean
plane of the earth's orbit about the sun.
Angular distance of a
celestial object from the sun in the sky. In standard ephemerides,
this is usually denoted by the Greek letter epsilon (or by the
abbreviation "Elong."). A celestial (usually solar-system) object's
"phase angle" is the elongation of the earth from the sun, as would
be seen by an observer on that third celestial object.
- Ephemeris (plural:
(ef-fi-MARE-uh-deez). A table listing specific data of a moving
object, as a function of time. Ephemerides usually contain right
ascension ("R.A." in these web pages) and declination ("Decl." in
these web pages), apparent angle of elongation ("Elong." in these
web pages) from the sun (in degrees), and magnitude (brightness) of
the object; other quantities frequently included in ephemerides
include the objects distances from the sun and earth (in AU),
usually given as Roman letter "r" and Greek letter "Delta",
respectively; phase angle; and moon phase.
- Ephemeris Time (ET).
Determined in principle from
the sun's apparent annual motion, ET is the numerical measure of
uniform time, which is the independent variable in the gravitational
theory of the earth's orbital motion, coming from Simon Newcomb's
Tables of the Sun. In practice, ET was obtained by comparing
observing positions of the Moon with gravitational ephemerides
calculated from theories. In 1992, standard (apparent geocentric)
ephemerides of comets and minor planets changed from using Ephemeris
Time to Terrestrial Dynamical Time (TDT, or TT).
Either of the two points
(vernal, autumnal) on the celestial sphere where the ecliptic (which
is the apparent path of the sun on the sky) intersects the celestial
equator. Due to precession, this point moves over time, so positions
of stars in catalogues and on atlases are usually referred to a
"mean equator and equinox" of a specified standard epoch. For the
purposes of the positions of objects dealt with in these
ICQ/CBAT/MPC Web pages, the positions are almost always given for
"equinox J2000.0", meaning that the reference system is that at the
beginning of the year 2000; prior to 1992, most astronomers were
using "equinox B1950.0". Many older star atlases and catalogues
still in use refer to equinox 1950.0, so observers must be careful
when plotting positions (and when reporting positions) to note the
proper equinox. (The "B" and "J" preceding the equinox years
indicate "Besselian" and "Julian", respectively. See separate
definitions for Besselian year and Julian year.) The differences in
an object's position when given in equinoxes 1950.0 and 2000.0
amounts to several arc minutes.
- Extinction, atmospheric.
The diminishing of light from
astronomical objects due to the earth's atmosphere, in which
molecules (air, dust, etc.) of the atmosphere absorb, reflect, and
refract light before it reaches the ground. Extinction becomes a
severe problem for astronomers when objects are viewed close to
(especially within 20 degrees of) the local horizon. There are
various methods that have been developed for astronomers to try and
compensate for this extinction, but it is always best to make
measurements of astronomical objects when they are as high in the
sky as possible (to minimize errors).
Referring to the sun. A
heliocentric orbit is one based on the sun as one of the two foci of
the (elliptical) orbit (or as the center of a circular orbit); a
heliocentric magnitude is the brightness of an object as would be
seen from a heliocentric distance of 1 AU (which means a distance of
1 AU from the sun).
- Julian date (JD).
The interval of time in days
(and fraction of a day) since Greenwich noon on Jan. 1, 4713 BC. The
JD is always half a day off from Universal Time, because the current
definition of JD was introduced when the astronomical day was
defined to start at noon (prior to 1925) instead of midnight. Thus,
1995 Oct. 10.0 UT = JD 2450000.5.
- Julian year.
- Exactly 365.25 days, in which a century
(100 years) is exactly 36525 days and in which 1900.0 corresponds
exactly to 1900 January 0.5 (from the Julian-date system, which is
half a day different from civil time or UT). The standard epoch
J2000.0, now used for new star-position catalogues and in
solar-system-orbital calculations, means 2000 Jan. 1.5 Barycentric
Dynamical Time (TDB) = Julian Date 2451545.0 TDB. When this
dynamical, artificial "Julian year" is employed, a letter "J"
prefixes the year.
kilometer = 0.6 mile.
- Light pollution.
The emission of stray light or
glare from lighting fixtures in manners that counter the purpose of
the light (which is to light what is below); also known as the waste
of money and energy in the form of electric light, usually meant in
the form of outdoor night lighting. Such light trespass causes
severe safety problems for motorists, pedestrians, and cyclists at
night from lighting that shines onto streets and highways and
sidewalks from poorly-designed or poorly-mounted lighting. Such
glare also imposes on privacy, by shining brightly into bedroom
windows at night and into backyards where adults and children are
trying to observe the night sky. While most people have accepted
such bad, glare lighting without question and assumed that nothing
could be done about it, dedicated groups of volunteers around the
world are now showing that effective laws and guidelines can be
instated at the local and regional levels of government (and in
planning and engineering offices), which mean that proper outdoor
night lighting can be a norm so that everybody benefits --- auto
drivers, sleeping residents, government budgets, and skygazers
alike. Laws mandating full-cutoff light fixtures are already in
place in states such as Maine and Connecticut and are pending
elsewhere. For more information on the Web, see URL
Total, integrated magnitude of
a comet's head (meaning coma + nuclear condensation). This can be
estimated visually, as the comet's "total visual magnitude". The
variable m1 is usually found in ephemerides predicting a
comet's future motion, position on the sky, and brightness. See also
definition for "Magnitude", below. [Note that m1
is also used by stellar spectrophotometrists to define a "metal
index" on the Stroemgren ubvy photometric system.]
The magnitude value measured
(or predicted) for a comet's nuclear condensation. Note that the
true comet nucleus is rarely, if ever, directly observed from the
earth because of the large amount of gas and dust that is
ever-present in the inner coma close to the nucleus, serving to hide
the true nucleus' surface. So-called "nuclear magnitudes" are
therefore fraught with problems as to true meaning, especially
because such nuclear magnitudes are extremely dependent upon
instrumentation (aperture, focal-ratio, magnification) and
wavelength. Nuclear magnitudes are chiefly used for astrometric
purposes, in which predictions are made for the brightness of the
comet's nuclear condensation so that astrometrists can gauge how
faint the condensation is likely to be and thus how long an exposure
is needed to get a good, measurable image. (Astrometrists are only
concerned about measuring the nuclear condensation, which is
considered to be the site of the main mass of any comet.) See also
definition for "Magnitude", below.
The units used to describe
brightness of astronomical objects. The smaller the numerical
brighter the object. The human eye can detect stars to 6th or 7th
magnitude on a dark, clear night far from city lights; in suburbs or
cities, stars may only be visible to mag 2 or 3 or 4, due to light
pollution. The brightest star, Sirius, shines at visual magnitude
-1.5. Jupiter can get about as bright as visual magnitude -3 and
Venus as bright as -4. The full moon is near magnitude -13, and the
sun near mag -26. Comet C/1996 B2 (Hyakutake) reached magnitude
about 0 in late March 1996. The magnitude scale is logarithmic, with
a difference of one magnitude corresponding to a change of about 2.5
times in brightness; a change of 5 magnitudes is defined as a change
of exactly 100 times in brightness. In the case of comets, we speak
of a magnitude that is "integrated" over an observed coma diameter
of several arc minutes; this is called the comet's "total (visual)
magnitude", and is usually denoted by the variable m1.
Thus, a 7th-magnitude comet is much harder to see than a
7th-magnitude star -- the latter having all its light in a pinpoint,
and the former having the same amount of light spread out over a
large area (imagine defocussing a 7th-magnitude star to the size of
a diffuse comet). Typically, however, when comets become very
bright, their apparent coma sizes shrink so that the majority of
visible light is in a small, intense core of the comet's head (and
the comet may appear starlike with a tail emanating from the comet's
publications, ephemerides for solar-system objects usually give
predicted/projected magnitudes of comets and minor planets in the
last column, denoted m1 and m2
for cometary "total" and "nuclear" magnitudes, or V for
minor-planet V-band ("visual") magnitudes.
Small rocky and/or icy
particles that are swept up by the earth in its orbit about the sun.
Also called "shooting stars", they travel across the sky in a very
short time, from less than a second to several seconds, and they do
so because they are only a matter of tens of miles above the surface
of the earth. Meteor showers are generally thought to be produced by
the debris left by comets as the latter orbit the sun. (Comets, on
the other hand, are not in our atmosphere but are much further away
than is our own Moon; therefore, comets do not "streak" across the
sky as do meteors -- a common misconception among the general
The path of one object about
another (used here for an object orbiting the sun).
- Orbital elements.
Parameters (numbers) that
determine an object's location and motion in its orbit about another
object. In the case of solar-system objects such as comets and
planets, one must ultimately account for perturbing gravitational
effects of numerous other planets in the solar system (not merely
the sun), and when such account is made, one has what are called
"osculating elements" (which are always changing with time and which
therefore must have a stated epoch of validity). Six elements are
usually used to determine uniquely the orbit of an object in orbit
about the sun, with a seventh element (the epoch, or time, for which
the elements are valid) added when planetary perturbations are
allowed for; initial ("preliminary") orbit determinations shortly
after the discovery of a new comet or minor planet (when very few
observations are available) are usually "two-body determinations",
meaning that only the object and the sun are taken into account ---
with, of course, the earth in terms of observing perspective) work
with only the following six orbital elements: time of perihelion
passage (T) [sometimes taken instead as an angular measure called
"mean anomaly", M]; perihelion distance (q), usually given in AU;
eccentricity (e) of the orbit; and three angles (for which the mean
equinox must be specified) --- the argument of perihelion
(lower-case Greek letter omega), the longitude of the ascending node
(upper-case Greek letter Omega), and the inclination (i) of the
orbit with respect to the ecliptic.
- Nuclear magnitude.
See definition for m2,
The apparent displacement or
the difference in apparent direction of an object as seen from two
different points not on a straight line with the object (as from two
different observing sites on earth).
The point where (and when) an
object's orbit about the earth in which it is closest to the
earth; only applicable to objects orbiting the earth (not to objects
orbiting the sun --- a common error).
The point where (and when) an
object orbiting the sun is closest to the sun.
- Gravitational influences ("tugging" and
"pulling") of one astronomical body on another. Comets are strongly
perturbed by the gravitational forces of the major planets,
particularly by the largest planet, Jupiter. These perturbations
must be allowed for in orbit computations, and they lead to what are
known as "osculating elements" (which means that the orbital element
numbers change from day to day and month to month due to continued
perturbations by the major planets, so that an epoch is necessarily
stated to denote the particular date that the elements are valid.
- Phase angle.
For a solar system object
besides the earth and sun, the angle between the earth and the sun
(or the earth's elongation from the sun) as seen from that third
object. The phase angle is given in ephemerides on IAU Circulars
and Minor Planet Circulars is denoted by either of the
lower-case Greek letters beta or phi.
In astronomy, the measurement
of the light emitting from astronomical objects, generally in the
visible or infrared bands, in which a specific or general wavelength
band is normally specified. An excellent reference on this topic is
Astronomical Photometry: A Guide, by C. Sterken and J.
Manfroid (1992, Dordrecht: Kluwer Academic Publishers).
A slow but relatively uniform
motion of the earth's rotational axis that causes changes in the
coordinate systems used for mapping the sky. The earth's axis of
rotation does not always point in the same direction, due to
gravitational tugs by the sun and moon (known as lunisolar
precession) and by the major planets (known as planetary
The alphabetic letter
("variable") used to denote the distance between the sun and the
object being discussed, also called the object's heliocentric
distance; in most ephemerides of objects such as comets and minor
planets, r is given in AU. Similarly, the upper-case Greek
letter Delta gives the distance between the object and the earth
(its geocentric distance).
A telescope that uses as its
primary optical element a mirror. Nearly all large telescopes in use
today by amateur and professional astronomers are reflecting
A telescope that uses as its
primary optical element a lens. Binoculars are a type of refractor.
In general, refractors are much more expensive to build and buy than
- Right ascension.
One element of the
astronomical coordinate system on the sky, which can be though of as
longitude on the earth projected onto the sky. Right ascension is
usually denoted by the lower-case Greek letter alpha and is measured
eastward in hours, minutes, and seconds of time from the vernal
equinox. There are 24 hours of right ascension, though the 24-hour
line is always taken as 0 hours. More rarely, one sometimes sees
right ascension in degrees, in which case there are 360 degrees of
right ascension to make a complete circuit of the sky. When
specifying a comet's location on the sky, one must state the right
ascension and declination (with equinox), along with date and time
(since a comet moves with respect to the background stars). For
examples of how right ascension (R.A.) and declination (Decl.)
appear on a star atlas, see
the Millennium Atlas
that, at the celestial equator, there are 15 arc seconds in one
second of R.A. (often stated as "one second of time"); as one moves
away from the celestial equator, one must multiply this factor of 15
by an additional factor (cosine of the declination), because the
lines of right ascension get closer and closer as one nears the
celestial poles, to get straight-line distances between two
celestial objects that are close to each other (for long distances
across the celestial sphere, a more complex formula is used). Thus,
when the R.A. is given in h, m, and s, it is usually given in
seconds of time to one more significant digit than is the Decl. in
arcsec (i.e., if R.A. is given to 0s.01, the Decl. should be only
given to 0".1, though this significant-figures requirement
disappears as one approaches the celestial poles).
- Secular motion.
Secular variations in the
motions of the planets are those that have very slow changes that
proceed through ages (secula) in a way such that they are
nearly proportional with time for a relatively large number of
years. Precession is considered a secular variation, arising from
the motions of the mean equator and the mean ecliptic. Compare this
with periodic variations, which are rather rapid changes; nutation
is a periodic variation.
The change of a solid (such as
ice) directly into a gaseous state (bypassing the liquid state).
This happens in the vacuum of space with comets, as the heating
effects of solar radiation cause ices in comets to "steam off" as
gasses into space. The ice molecules present in the nucleus actually
break up (or dissociate) into smaller atoms and molecules after
leaving the nucleus in gas form.
- Terrestrial Dynamical Time
(TDT or TT).
Time scale used in orbital
computations; TDT is tied to atomic clocks (International Atomic
Time, TAI), whereas Universal Time is tied to observations. Prior to
1992, Ephemeris Time (ET) was used in publications of the
ICQ/CBAT/MPC; since then, TT has been used. The difference between
TDT and UTC in 1994 was 60 seconds (i.e., UT + 60 seconds = TDT).
- Total (visual) magnitude.
Total, integrated magnitude of
a comet's head (meaning coma + nuclear condensation). This can be
estimated visually, as the comet's "total visual magnitude". The
variable m1, usually found in comet ephemerides, is used
to denote the total (often predicted) magnitude. See also definition
for "Magnitude", above.
- Universal Time (UT, or UTC).
A measure of time used by
astronomers; UT conforms (within a close approximation) to the mean
daily (apparent) motion of the sun. UT is determined from
observations of the diurnal (daily) motions of the stars for an
observer on the earth. UT is usually used for astronomical
observations, while Terrestrial Dynamical Time (TDT, or simply TT)
is used in orbital and ephemeris computations that involve
geocentric computations. Coordinated Universal Time (UTC) is that
used for broadcast time signals (available via shortwave radio, for
example), and it is within a second of UT.
- Vernal equinox.
The point on the celestial
sphere where the sun crosses the celestial equator moving northward,
which corresponds to the beginning of spring in the northern
hemisphere and the beginning of autumn in the southern hemisphere
(in the third week of March). This point corresponds to zero (0)
hours of right ascension.
The point directly overhead in
Good references for some of the
above definitions include the annual Astronomical Almanac
(Washington: U.S.G.P.O.); the Explanatory Supplement to the
Astronomical Almanac, ed. by P. K. Seidelmann (1992, Mill Valley,
CA: University Science Books); A Manual of Spherical and Practical
Astronomy, by W. Chauvenet (1960, New York: Dover), 1, 603;
and Spherical Astronomy by E. W. Woolard and G. M. Clemence
(1966, New York: Academic Press).