Glossary

Absolute magnitude (H10 and H0)

This is the brightness a comet would exhibit if placed 1 AU from both the Earth and sun. Using a special formula, the brightness estimates made for comets can be converted to absolute magnitudes which aids in the study of how a comet reacts as it approaches or recedes from the sun. For H0, an accompanying value designated “n” determines how fast a comet brightens or fades when approaching or receding from the sun.

Anti-tail

The name given to a tail when it points towards the sun. This is a rare event and typically occurs when the Earth crosses the plane of the comet’s orbit when that comet is relatively close to the sun and exhibiting notable tail activity.

Apparent Radiant

The point from which meteors appear to originate in the sky.

Arc Minutes

This is an angular measure frequently used to denote the size of a comet’s coma and tail. One arc minute is 1/60th of a degree.

Arc Seconds

This is an angular measure frequently used to denote the size of a comet’s nuclear condensation. One arc second is 1/60th of an arc minute.

Astronomical unit (AU)

This is a standard unit of measure to represent the distances of objects within our solar system from the sun. One AU is 149,597,870 kilometers.

Bolide

A very bright meteor which fragments or explodes. Sounds of the explosion can be heard if the observe is close enough.

Coma

A diffuse, luminous cloud of dust and gas that develops around a comet’s nucleus as it nears the sun.

Coma Diameter

One of the important pieces of information an observer can determine when viewing comets. The size is usually given in arc minutes, although distant comets can display a coma that is measured in arc seconds and near-Earth comets can display a coma that is measured in degrees.

Daylight Stream

A meteor stream that is active and above the horizon at the same time as the sun. They can only be observed by radar and radio-echo techniques.

Declination (DECL)

The celestial equivalent of latitude. The celestial equator is zero degrees, while the celestial north and south poles are 90 and -90 degrees, respectively.

Degree of Condensation (DC)

A term used to denote how the comet’s surface brightness is concentrated within the coma. A DC of 0 indicates the comet surface brightness is evenly distributed with no apparent concentrated. A DC of 9 indicates the comet essentially looks like a planet.

Fireball

A meteor which is brighter than any planet or star, i.e. brighter than magnitude -4.

Geocentric distance (delta)

The distance of an object from Earth, usually given in astronomical units.

Heliocentric distance (r)

The distance of an object from the sun, usually given in astronomical units.

Long-period comets

Comets moving in orbits having periods greater than 200 years.

Magnitude (m1 and m2 for comets)

One of the most important pieces of information gathered by amateur astronomers. This represents the estimated brightness of the comet when compared to stars around it. The term “m1” is used to represent the total or integrated brightness of the comet’s coma, while “m2” represents the brightness of the nucleus. Observers estimate “m1” by usually memorizing the comet’s appearance and then defocusing the surrounding stars to a size equivalent to the comet’s coma diameter. The memorized comet’s appearance is then compared to the defocused stars to determine the comet’s brightness. In meteor astronomy the magnitude of a meteor is compared to that of other stars in the sky. The magnitude scale is set up so that the brighter magnitudes are actually represented by smaller numbers. The sun is about magnitude -26, the moon -12, Venus is -4, and the faintest naked-eye star is generally about +6.

Meteor

In particular, the light phenomenon which results from the entry into the Earth’s atmosphere of a solid particle from space.

Meteorite

A meteor that is large enough to survive its passage through the atmosphere and hit the ground.

Meteor Shower

A shower of meteors occurs when Earth’s orbit intersects the orbit of a meteor stream.

Meteor Storm

This is a rare event that occurs when Earth encounters closely grouped meteors within a meteor stream. Such events can see meteor rates exceeding 1000 per minute.

Meteor Stream

This represents the orbit of meteoroids as they travel around the sun. Meteors are the by-product of comets, so it is possible for the parent comet to be traveling in the same orbit–if it still exists.

Meteor Train

A trail of ionized dust and gas that remains along the path of a meteor.

Minor Meteor Showers

Meteor showers that produce less than 10 meteors per hour at the time of maximum activity.

“n”

This is the photometric parameter generated when calculating a comet’s absolute magnitude. It indicates the comet’s rate of brightening and fading as it approaches or recedes from the sun, respectively.

Nucleus

The actual solid body of a comet. The nucleus is rarely visible when the comet is in the inner solar system because of the coma. It has been referred to as a “dirty snowball” because it is believed to be composed of about 75 percent of various ices and about 25 percent of various dusts. A photo of the nucleus of Halley’s comet by the Giotto probe revealed the nucleus as an asteroidal-looking body. Most comet’s have a nucleus that measures only a few kilometers across.

Period

The amount of time, usually given in years, that it takes an object to orbit the sun.

Perihelion Date (T)

The date an object reaches its closest distance from the sun.

Perihelion Distance (q)

The point in an orbit when an object is closest to the sun. The value is usually given in astronomical units.

Persistent Train

Train luminosity that lasts more than a second.

Position Angle (PA)

An angular measurement indicating which side of the nucleus something is located. A “PA” of 0 degrees indicates an object is located north of the nucleus, while 90 degrees indicates east, 180 degrees is south, and 270 degrees is west. It is most commonly used to indicate the direction the tail is pointing. On occasion, when a comet’s nucleus has broke up, the position angle is used to indicate which direction the nuclear fragments are located from the primary nucleus.

Radiant

The point from which a meteor appears to emanate.

Radiant Drift

The movement of a meteor shower’s radiant against the star background. This characteristic is common to all meteor showers and is caused by Earth’s passage through a meteor stream.

Right Ascension

The celestial equivalent of Earth’s longitude, beginning at a line running pole to pole and cutting through eastern Pegasus. In meteor astronomy the right ascension is handled in degrees, starting at zero degrees and advancing eastward around the sky for a full 360 degrees ending in eastern Pegasus. For the rest of astronomy, the right ascension is handled as a time measurement. Since it takes 24 hours for the Earth to rotate, the sky is divided into 24 one hour wide bands. Each hour of right ascension equals 15 degrees.

Short-period comets

Comets moving in orbits having periods less than 200 years.

Solar Longitude

This is an angular measurement that specifies the location of Earth in its orbit around the sun. More precisely, it is the longitude of the sun as given in geocentric coordinates. The evaluation of meteor data strongly relies on this figure rather than a conventional date.

Tail

The most distinctive feature of comets, especially great ones. It is typically directed away from the sun. Ancient and medieval observers frequently described a comet as a broom or sword, depending on the look of the tail. Typically, telescopic comets will exhibit either no tail or one extending a few arc minutes. Naked-eye comets can show a tail extending up to several tens of degrees. Comets can display two basic types of tails: one gaseous and the other largely composed of dust. The dust tail can be curved, spread out, and yellowish in appearance, while the gas tail is usually very straight and bluish.

Terminal Burst

The flare at the end of a meteor’s path.

Zenithal Hourly Rate (ZHR)

This is the rate a meteor shower would produce if seen by an observer with a clear, dark sky, and with the radiant at the zenith.