International Comet Quarterly

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.

Altitude.

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].

Aperture.

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).

Aphelion.

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.

Astrometry.

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.

AU.

see Astronomical Unit.

Azimuth.

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.

Barycenter.

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 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 Julian year.)

CCD.

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.

Coma.

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.

Comet.

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).

Declination.

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 atlas, see the Millennium Atlas.

Degree.

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 3600 seconds of arc.

Delta.

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 declination.)

Ecliptic.

The apparent path of the sun against the sky background (celestial sphere); formally, the mean plane of the earth's orbit about the sun.

Elongation.

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: 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 ("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).

Equinox.

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).

Heliocentric.

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.

km.

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 http://www.cfa.harvard.edu/iau/nelpag.html .

m₁.

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.]

m₂.

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.

Magnitude.

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.

It is customary in astronomy to precede a magnitude value with a left bracket ([) and no space to indicate a limiting magnitude, when an object was not seen. Note that those careless people (notably dealing with variable stars) who use a less-than sign (<) instead of a left bracket to indicate limiting magnitude are, in fact, erroneously indicating that the object is brighter than the specified magnitude, because a smaller number in astronomical magnitudes means brighter than; if they wish to use a mathematical symbol to denote limiting magnitude, the only proper option is to use a greater-than sign, >, but this alone does not tell the reader whether the object was visible (and somewhat fainter than the specified value) or invisible.

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 head).

"Traditionally", in ICQ/CBAT/MPC publications, ephemerides for solar-system objects have usually given 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. However, in 2003, a subcommittee of IAU Commission 20 charged with assessing cometary magnitudes for ephemerides decided that the concept of "nuclear" magnitudes should be done away with because few astrometrists ever observe a true nuclear magnitude (and in fact generally observe something between a true "total" and a true "nuclear" magnitude); CBAT publications have since then used the heading "Mag." alone instead of the old m₁ and m₂ headings to more appropriately refer to the predicted brightness of comets.

Meteors.

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 public.)

Orbit.

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. The six orbital elements used for comets are usually the following: 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, above.

Parallax.

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).

Perigee.

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).

Perihelion.

The point where (and when) an object orbiting the sun is closest to the sun.

Perturbations.

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.

Photometry.

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).

Precession.

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).

r.

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).

Reflector.

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.

Refractor.

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 stars). For examples of how right ascension (R.A.) and declination (Decl.) appear on a star atlas, see the Millennium Atlas (also here and here). Note 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.

Sublimation.

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.

Zenith.

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); 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).

Index to the CBAT/MPC/ICQ pages.