Glossary

The brightness a comet would have if it were placed one astronomical unit from both Earth and the Sun. Converting observed brightness estimates to absolute magnitudes helps astronomers compare a comet’s activity at different distances.

Comet brightness models use a photometric parameter, n, to describe how brightness changes with distance from the Sun. H10 uses a conventional value of n = 4, while H0 is quoted with the adopted value of n.

A comet tail that appears to point toward the Sun because of viewing geometry. This effect can occur when Earth passes through the plane of a comet’s orbit and its dust is viewed nearly edge-on.

The point in the sky from which meteors belonging to a shower appear to originate, as seen by an observer.

Units of angular measurement used to describe apparent sizes and separations in the sky. One arc minute is 1/60 of a degree. The apparent size of a comet’s coma or tail is often expressed in arc minutes.

Units of angular measurement equal to 1/60 of an arc minute, or 1/3,600 of a degree. They are used to describe small apparent sizes, such as a comet’s central condensation.

A standard unit of distance approximately equal to the average distance between Earth and the Sun. One AU is approximately 149.6 million kilometers (93 million miles).

A term commonly used for a very bright meteor that produces a conspicuous flare or explosion, often with fragmentation. Some events also produce sonic booms audible from the ground.

The diffuse cloud of gas and dust surrounding an active comet’s nucleus. It develops as solar heating causes the comet to release gas and dust.

The apparent angular width of a comet’s coma. It is usually recorded in arc minutes, although small, distant comas may be measured in arc seconds and very large, nearby comas in degrees.

A meteoroid stream whose shower radiant lies close to the Sun in the sky, making its activity primarily observable during daylight. Such showers are studied mainly using radar and radio techniques; some also produce meteors visible during twilight.

The celestial equivalent of latitude. Declination measures angular distance north or south of the celestial equator, which is at 0°. The north celestial pole is at +90° and the south celestial pole is at −90°.

A visual estimate of how strongly a comet’s brightness is concentrated toward the center of its coma. The scale runs from 0, indicating a diffuse coma without a central concentration, to 9, indicating a starlike appearance.

An exceptionally bright meteor, commonly defined as one brighter than magnitude −4, approximately the brightness of Venus.

The distance of an object from Earth’s center, usually expressed in astronomical units when describing comets and other solar system objects.

The distance of an object from the Sun’s center, usually expressed in astronomical units.

Comets with orbital periods longer than 200 years.

A measure of an object’s brightness. Smaller or more negative magnitudes indicate brighter objects. Under dark skies, the faintest stars visible to the unaided eye are typically around magnitude +6.

For comets, m1 describes the total or integrated brightness of the coma, while m2 describes the brightness of the central or nuclear condensation. This central condensation may include surrounding material rather than only the solid nucleus.

Meteor magnitudes are estimated by comparing their brightness with stars or planets of known magnitude.

The light phenomenon produced when a solid particle from space enters Earth’s atmosphere at high speed. A meteor is commonly called a shooting star.

A solid piece of material from space that survives its passage through the atmosphere and reaches the ground.

A small natural solid object traveling through space. When it enters Earth’s atmosphere, it may produce a meteor; any surviving material that reaches the ground is called a meteorite.

Meteor activity produced when Earth passes through a stream of meteoroids. The meteors belonging to a shower appear to radiate from a common region of the sky called the radiant.

An exceptionally intense meteor shower with rates reaching at least 1,000 meteors per hour. Storms can occur when Earth passes through a particularly dense concentration of debris within a meteoroid stream.

A collection of meteoroids traveling along similar orbits around the Sun, usually released by a comet and, in some cases, an asteroid. Also called a meteoroid stream, it can produce a meteor shower when Earth passes through it.

A luminous trail that remains along a meteor’s path after the meteor itself has passed. It can fade quickly or remain visible for seconds or minutes.

Showers with relatively low activity, typically producing fewer than 10 shower meteors per hour at maximum under favorable conditions. Their activity can be difficult to distinguish from the sporadic meteor background.

The comet or asteroid that supplied the debris forming a meteoroid stream. When Earth passes through this stream, the particles entering our atmosphere produce a meteor shower. Some showers have an uncertain or unidentified parent object.

A parameter used in comet brightness models to describe how a comet’s brightness changes with its distance from the Sun. It helps characterize brightening during approach and fading during recession.

The solid body of a comet, composed of a mixture of ices, dust, and rocky material. Many comet nuclei are only a few kilometers across, although their sizes vary considerably.

When a comet is active, its nucleus is usually hidden within the much larger, brighter coma.

The time an object takes to complete one orbit. For comets orbiting the Sun, this is usually expressed in years.

The date and time when an object reaches its closest point to the Sun in its orbit.

The distance between an object and the Sun at the closest point in its orbit, usually expressed in astronomical units.

A meteor train that remains luminous for more than a second after the meteor disappears. Some persist for several minutes and change shape as upper-atmosphere winds distort them.

The direction of one feature relative to another on the sky, measured from celestial north through east. North corresponds to 0°, east to 90°, south to 180°, and west to 270°.

In comet observations, position angle commonly describes the direction of a tail or the location of a fragment relative to the main nucleus.

The point in the sky from which the paths of meteors belonging to a shower appear to diverge. This is a perspective effect: the particles follow nearly parallel paths through space.

The gradual movement of a meteor shower’s radiant against the background stars during its activity period. It results from changes in encounter geometry as Earth travels through the meteoroid stream.

The celestial equivalent of longitude, measured eastward along the celestial equator from the March equinox point. Together with declination, it identifies a position in the sky.

Right ascension is commonly expressed in hours, minutes, and seconds, with 24 hours covering a full circle. Meteor radiant coordinates are also frequently given in degrees: one hour equals 15°.

Comets with orbital periods shorter than 200 years.

The Sun’s angular position along the ecliptic as seen from Earth, measured from the March equinox point. It provides a way to describe Earth’s position in its annual orbit.

Meteor astronomers use solar longitude to compare shower activity between years more precisely than calendar dates alone allow.

An extended feature of gas or dust released by a comet. Comet tails generally extend away from the Sun rather than simply trailing behind the comet along its orbit.

Dust tails are often curved or broad and may appear yellowish. Ion tails, formed from electrically charged gas, are typically straighter and can appear blue.

A sudden flare in brightness near the end of a meteor’s visible path, often associated with fragmentation. Also called a terminal flash.

The speed at which a shower’s meteoroids encounter Earth, usually expressed in kilometers per second (km/s). Meteor speeds are measured relative to Earth, rather than relative to the Sun.

Faster meteors generally streak across the sky more quickly, although their apparent motion also depends on their distance and the direction from which they are viewed.

A standardized estimate of the number of shower meteors an observer would see in one hour under a clear, dark sky, with an unobstructed view and the radiant directly overhead at the zenith.

ZHR assumes a stellar limiting magnitude of +6.5. Actual observed rates are usually lower because of moonlight, light pollution, obstructions, or a radiant below the zenith.