Comet Definitions

The following are generally accepted definitions for terms related to cometary astronomy.

absolute magnitude (Ho)

The brightness of a comet when it is at 1 AU from both the Earth and Sun. As this virtually never happens, this quantity is calculated from the comet's light curve. Unfortunately, this quantity is far from absolute. It can be different pre- and post-perihelion. It can also change from apparition to apparition (for periodic comets).

anti-tail or anomalous tail

When a comet's tail appears to be pointing toward the Sun, this is called an anti-tail or anomalous tail. In reality, the tail only appears to be pointing toward the Sun. To get an anti-tail, the comet must produce large ("heavy") dust particles. If this happens, these particles are left along the comet's orbit instead of being pushed away from the Sun and the comet's orbit by light pressure. Often dusty comets will produce particules of different sizes creating a fan-shaped appearance. The smallest dust will be pushed directly away from the Sun (like the gas tail) and the largest will be left in the comet's orbit. When a comet is close to the Sun, the angle of this fan can be 90 degrees or larger. If the Earth-comet-Sun geometry is correct, the dust in the comet's orbit will appear to point toward the Sun. [Try this...make a right (90 degree) angle with your thumb and index finger. Your index finger is the main tail and your thumb is the dust left in the comet's orbit. Point your finger and thumb directly away from you (keeping the angle 90 degrees). Your finger seems to be going in exactly the opposite direct from the thumb. This is what causes an anti-tail.]

astronomical unit (AU)

Standard unit for measuring distance within the solar system. One AU is equal to the average distance between the Sun and Earth or about 93 million miles.

coma or the comet's head

The comet's coma or head is the fuzzy haze that surrounds the comet's true nucleus. The coma (and tail) are really all that we see from Earth.

The shape of the coma can vary from comet to comet and for the same comet during its apparition. The shape depends on the comet's distance from the Sun and the relative amount of dust and gas production. For faint comets or bright comets producing little dust, the coma is usually round. Comets, which are producing significant quantities of dust, have a fan-shaped or parabolic comae. This is because different size dust is being released. The larger dust gets left along the comet's orbital path while smaller dust gets pushed away from the Sun by light pressure. The smaller the dust, the more directly away from the Sun the dust is pushed. With a distribution of both large and small dust sizes, a fan is created. For comets within 1 AU, the coma of a dusty comet often becomes parabolic in shape. Clearly, for comets with fan-shaped or parabolic comae, there is no obvious boundary between the coma and tail.

coma diameter

The diameter of the coma is usually given in minutes of arc ('). If the coma is round, this is a straightforward definition. If the coma is elongated or has a tail, the measurement represents the smallest dimension of the coma (usually at a right angle to the tail) and transecting the brightest part of the coma.

degree of condensation (DC)

DC is an indicator of how much the surface brightness of the coma increases toward the center of the coma. In general, DC=0 indicates totally diffuse and DC=9 means "stellar." As the DC increases, the coma size usually decreses and becomes more sharply defined. A totally diffuse comet, with no brightening toward the center, is rated DC=0. With DC=3-5, there is a distinct brightening. By DC=7 you have a steep overall gradient and by DC=8 the coma is very small, dense,and intense with fairly well defined boundaries. With DC=9 the comet looks like a soft star or a planet in bad seeing.

geocentric distance (delta)

The comet's distance from the Earth in astronomical units.

heliocentric distance (r)

The comet's distance from the Sun in astronomical units.

long-period comets

Comets with orbital periods greater than 200 years.


The photometric parameter n in the power-law formula for comet brightness, m1 = Ho + 5 log (delta) + 2.5n log (r), indicates how fast the comet's brightness is changing with heliocentric distance, r. Specifically, n is the power in the power-law formula. That is, the comet's brightness varies as r to the -n power. When the comet's heliocentric brightness, m1 - 5 log (delta), is plotted against log (r), the slope of the straight line (assuming it is a straight line) is 2.5n.


The true nucleus of a comet has only been seen once (P/Halley by spacecraft). From the ground, the star-like nucleus always includes a cloud of dust and gas around the true nucleus. Hence, terms such as stellar condensation and nuclear condensation are often used when a star-like object is seen in the comet's coma. The magnitude of the "nucleus" is denoted m2 and usually isn't of much use because one is really not such what m2 represents. In general, the value of m2 will get fainter as more magnification is applied.

observed magnitude (m1)

The observed magnitude of the comet represents the integrated brightness of the comet's coma or head as seen from Earth. This is normally obtained by comparing the comet's average surface brightness with that of defocused stars (matching the comet's size) of known brightness. Because comets have size (in contrast to stars which are pinpoints of light), a comet of a given brightness will appear less obvious than a star of the same brightness.

periodic or short-period comets

Any comet with an orbital period of less than 200 years. These comets are indicated by a "P/" before the names. For example, P/Halley is Halley's comet or more properly known as periodic Comet Halley. Recently, the International Astronomical Union has started numbering periodic comets that have been seen at more than one apparition. Thus, Halley's Comet is 1P/Halley and P/de Vico is now known as 122P/de Vico.

position angle (PA)

The PA of a tail or other cometary feature represents the direction on the sky (in degrees from north) toward which it is pointing. Thus, a comet in the morning sky (in the east) that has a tail pointing due west will have a PA of 270 degrees. A comet in with a tail poining toward the south-east will have a PA of 135 degrees. It must be stressed that the determination of the PA of a tail (or other feature) requires plotting it on an atlas and measuring the angle with a protractor. PAs should be measured to at least five degree resolution. It is not possible to look in an eyepiece and accurately estimate the PA of a tail. Also, the determination of PA in the polar regions of the sky is very tricky and may not be intuitive.


The comet's tail is its most distinctive feature. Generally pointing away from the Sun, these appendages come in a variety of shapes and lengths. The lengths can vary from a small fraction of a degree (tails are always measured as the angular length either in degrees or minutes of arc [', 60' = one degree]) to the rare few that cover a significant fraction of the sky.

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Charles S. Morris /