Glossary of (comet and) astronomical terms.
Below are listed alphabetically some terms, with explanations, that
one might encounter frequently in reading my Web page, or handling
the program GPS.EXE. You might want to print them.
- 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 -6 degrees].
- 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
- 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 standard nowadays.
- 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 time.
- 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 orbits.
- Barycentric Dynamical Time (TDB).
- 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 times 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 Julian year.)
- 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
- A comet's atmosphere 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.
- 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. A more
detailed explanation is available 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).
- 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 apparent path of the sun against the sky background
(celestial sphere); formally, the mean plane of the earth's orbit about
- 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
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: ephemerides).
- Pronounced ee-FEM-er-is,
ef-fi-MARE-uh-deez. A table listing specific
data of a moving object, as a function of time. Ephemerides usually contain
right ascension and declination, apparent angle of elongation 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),
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
- 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,
- 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 m(sub)1 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
m(sub)1 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
value, the 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 m(sub)1. 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 head).
In ICQ/CBAT/MPC publications, ephemerides for solar-system
objects usually give predicted/projected magnitudes of comets and
minor planets in the last column, denoted m(sub)1 and m(sub)2
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
- 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 m(sub)2, above.
- 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
- 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
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 precession).
- The alphabetic letter ("variable") used to denote the
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 telescopes.
- 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 are reflectors.
- 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
- 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
- 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
m(sub)1, 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 the sky.
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); and Spherical Astronomy
by E. W. Woolard and G. M. Clemence (1966, New York: Academic Press).
Author: V. Okan
Last Updated: June, the 1st of 1998