Ice and Stone 2020 Companion Glossary

Comet Hale-Bopp captured on April 1, 1997, from the deck of my former residence in Cloudcroft, New Mexico.

We hope you are enjoying reading the weekly Ice and Stone 2020 content from astronomer Alan Hale. For those who might not be as versed in astronomy terms, he’s put together this glossary of technical terms, many of which are regularly used within the weekly educational content.


Absolute magnitude: for a comet or asteroid, the apparent magnitude it would have if it were located 1 AU from both the earth and the sun (and, for an asteroid, at a phase angle of zero degrees).

Albedo: the measure of the “reflectivity” of an object, usually expressed as a decimal fraction (the amount of light reflected divided by the amount of light received). Objects with a low albedo are “dark,” and those with a high albedo are “bright.”

“Annual” comet: a periodic comet which, because of its small orbit and/or modern observational techniques, is observable throughout its entire orbit.

Anti-tail: an apparent sunward-pointing tail of a comet, in actuality a geometrical effect caused by the scattering of sunlight by dust grains within the plane of the comet’s orbit.

Aphelion: that point in an object’s orbit which is farthest from the sun.

Apollo-type asteroid: an asteroid whose orbit brings it to within the earth’s orbit (i.e., its perihelion distance is less than 1 AU).

Apparent magnitude: the magnitude of an object that is measured from the earth (or wherever the observer is located).

Apparition: the passage of a comet through the inner solar system, specifically the period of time around its perihelion passage. For a periodic comet, “apparition” is synonymous with “return.”

Arcminute: 1/60 of a degree.

Arcsecond: 1/60 of an arcminute.

Asteroid belt: the region of the solar system, between the orbits of Mars and Jupiter, wherein the majority of the asteroids orbit the sun. This is sometimes referred to as the “main belt.”

Astrometry: the practice of measuring precise and accurate coordinates of objects (usually comets and asteroids) on the celestial sphere, primarily for usage in calculation of the orbital elements.

Astronomical Unit (AU): a unit of distance equivalent to the average distance between the earth and the sun (93 million miles, or 149 million km). The distances of objects within the solar system are usually expressed in AU.


Brownlee particles: very tiny, porous dust grains that have been collected in the earth’s upper atmosphere, and believed to be the same type of dust grains that make up cometary comae and dust tails.


Celestial sphere: the sky as seen from the earth’s surface, imagined as being projected upon the inside surface of a sphere that encircles the earth. Positions upon the celestial sphere can be described via the coordinates of right ascension and declination, which are analogous to the coordinates of longitude and latitude used on the earth’s surface.

Centaurs: objects, which may be asteroids and/or comets, which travel in orbits that remain in the general vicinity of the outer planets (i.e., Jupiter to Neptune). 

Central condensation: a dense cloud of material surrounding a comet’s nucleus, and which usually appears as a bright spot within the coma.

Charge Coupled Devices (CCDs): electronic imaging systems which utilize a computer chip to convert received light into electrical charges, and then store this information as a computer-readable image.

Coma: the “fuzzy” head of a comet, composed of gas and dust which has been ejected off the nucleus as it approaches the sun.

Conjunction: literally, a meeting or close gathering of two or more objects. When applied to astronomical objects, the term usually means that the two objects are located along the same line of sight as seen from the earth. Unless specifically indicated otherwise, an object which is stated as being “in conjunction” is usually understood as being in conjunction with the sun.

Corona: the faint, tenuous (but hot) outer atmosphere of the sun, usually visible only during a total solar eclipse or via the use of a coronagraph.

Coronagraph: a device (either on the ground or aboard a spacecraft) which utilizes a central opaque “occulting disk” to block out the light of the sun and create an artificial solar eclipse, in turn allowing the sun’s corona and objects within the sun’s immediate vicinity to be detected.


Declination: the coordinate used on the celestial sphere which is analogous to the terrestrial coordinate of latitude. 

Degree: a unit of angular measure, equal to 1/360 of a full circle. Sizes of objects in the sky, and distances between them, are usually expressed in degrees.

“Dirty snowball” (more formally, the “icy conglomerate”): the theory originally put forth by Fred Whipple in 1950 concerning the structure of a comet’s nucleus, which was verified when the Giotto spacecraft flew by the nucleus of Comet P/Halley in 1986.

Disconnection event: a “break” or disconnection in a comet’s ion tail, generally believed to be caused by changes in the solar wind and/or variations in the interaction between the solar wind and the comet’s magnetic environment.

Dust tail: the tail of a comet that is composed primarily of dust grains that have been ejected off the nucleus.


Eccentricity (e): a measure of how elongated an orbit is. Orbits with an e of 0 are circles; those with e between 0 and 1 are ellipses; those with an e of 1 are parabolas, and those with e greater than 1 are hyperbolas.

Ecliptic: the path across the constellations upon which the sun travels.

Electromagnetic spectrum: a term that encompasses all the various forms of light which can be given off by means of physical processes and which can (in theory, at least) be detected with scientific instruments. On the high-energy, high-frequency, short-wavelength end of the electromagnetic spectrum are gamma rays; proceeding “downward” to lower energies, lower frequencies, and longer wavelengths are x-rays, ultraviolet, “visible” light, infrared, microwaves, and radio waves.

Elements: the mathematical quantities which define the parameters of an object’s orbit around another object, for example, a comet’s orbit around the sun. These elements include the date and location of perihelion, the inclination of its orbit with respect to Earth’s orbit, etc.

Ellipse: an oval-shaped closed curve. The orbits of all planets, asteroids, and (most) comets around the sun are ellipses. The ellipses within which the planets and most of the asteroids travel are nearly circular, whereas most comets travel in orbits which are far more “elliptical.”

Elongation: the angular separation (in degrees) between an object on the celestial sphere and the sun, as seen from the earth.

Ephemeris (plural ephemerides): a table listing an object’s position in the sky (usually its right ascension and declination) on different dates. The ephemeris for an object like a comet is computed from its orbital elements.


“Flying sandbank”: a now-discredited theory of a cometary nucleus first proposed during the 19th Century wherein the nucleus was postulated as being composed of a large number of loosely-bound dust particles of sizes up to perhaps a few meters in diameter.


Gas tail: see “ion tail” below.


Halley-type comet: a comet normally with an orbital period between 20 and 200 years. Most Halley-type comets have orbital inclinations and eccentricities that are significantly larger than those of Jupiter-family comets.

Hyperbola: a large open-ended curve; an object traveling in a hyperbolic orbit will never return to the object it is orbiting. 


Icy conglomerate”: see “dirty snowball” above.

Inclination: a measure of how much an orbit is tilted with respect to the earth’s orbit, usually expressed in degrees. An orbit with an inclination less than 90 degrees is said to be “prograde,” or “direct,” whereas an orbit with an inclination greater than 90 degrees is said to be “retrograde.”

Intrinsic brightness: when referring to a comet, the brightness that that comet exhibits as a result of its own size and activity, without regard for any external factors, such as its distance from the sun or the earth. This is often quantified through the use of a comet’s “absolute magnitude.”

Ionized: electrically charged. Specifically, an atom that has been ionized either has had negatively-charged electrons stripped from it, resulting in a positive ion, or has extra electrons added to it, resulting in a negative ion.

Ion tail: the tail of a comet that is composed of gas molecules that have been ejected off the nucleus and then ionized by the solar wind. Also called the “gas tail” or the “plasma tail.”


Jet: an emission of material off a comet’s nucleus, similar to the eruption of a geyser.

Jupiter-family comet: a comet that has been “captured” by Jupiter’s gravity into a very small short-period orbit. Most Jupiter-family comets have orbital periods of six to eight years and relatively small orbital inclinations.


Kreutz sungrazer: one of a “family” of comets first extensively studied by Heinrich Kreutz during the late 19th Century which are characterized by very similar orbits and extremely small perihelion distances, usually less than 0.01 AU.

K-T event: the supposed impact of a comet or asteroid onto the earth at the end of the Cretaceous Period 65 million years ago, and believed to be responsible for the extinction of the dinosaurs and many other species living at that time.

Kuiper Belt: a broad band of objects (comets and/or asteroids) orbiting the sun at distances between 30 AU and a few hundred AU. 


Lagrangian point: one of five points in a two-body gravitational system where the gravitational attractions from the two bodies are equal. In the absence of any perturbing bodies these points can be considered as being very stable.

Long-period comet: by definition, a comet with an orbital period of greater than 200 years.


Magnitude: the scheme currently used to describe the brightness of an astronomical object. The brighter an object, the smaller is its magnitude number, and the fainter an object, the larger is its magnitude.

Meteor shower: those occasions when the earth intersects a stream of interplanetary dust particles, creating the effect that numerous meteors appear to emanate from the same location in the sky.


Near-Earth asteroid: an asteroid that can come relatively close to the earth; primarily used to indicate those with perihelion distances less than approximately 1.2 AU.

Node: the point in an object’s orbit where it crosses the plane of the earth’s orbit. The “ascending node” is where the object crosses the plane from south to north, and the “descending node” is where it crosses from north to south.

Non-gravitational forces: small changes in a comet’s orbital motion, caused by the eruptions of material off the nucleus. These eruptions act as small rocket engines, and can cause a comet to deviate slightly from its predicted course.

Nucleus: the “solid” object in the center of a comet. A comet’s nucleus is composed primarily of dust mixed in with various ices, and when it approaches the sun, the sun’s heat causes the ices to sublimate, creating the coma and the tail.


Occultation: an event wherein one solar system object (e.g., the moon or a planet) passes in front of a more distant object, as seen from our vantage point on the earth.

Oort Cloud: a large, spherical cloud of comets believed to enshroud the solar system at distances of 1000 to 10,000 AU.

Opposition: located in a position in the sky opposite to that of the sun. A solar system object at opposition will usually rise around sunset, be at its highest point above the horizon around midnight, and will set around sunrise.

Organic molecules: complex molecules composed of chains of carbon atoms, upon which are attached atoms of various other elements. Organic molecules form the basis for all known life forms, although the presence of such molecules in an environment does not necessarily imply the presence of life.

Outburst: a sudden, dramatic (and usually short-lived) increase in a comet’s brightness.

Outgassing: when applied to a comet, the release of gas and dust from the nucleus through sublimation.


Parabola: an open-ended curve; an object traveling in a parabolic orbit will never return to the object it is orbiting. Most long-period comets have orbits which are close to being parabolas.

Perihelion: that point in an object’s orbit that is closest to the sun.

Periodic comet: see “short-period comet” below.

Perturbations: disturbances of an object’s orbit as a result of gravitational forces from other objects.

PHA: “Potentially Hazardous Asteroid.” By definition, an asteroid larger than 100 to 200 meters in diameter which can approach to within 0.05 AU of the earth’s orbit.

Phase: the angular separation between the earth and sun, as viewed from the object in question. For a solid object like an asteroid, there is an inverse correlation between its phase angle and the fraction of its sunlit surface that is visible from Earth.

Photometry: the practice of measuring the brightness, or magnitude, of an astronomical object.

Planetesimal: one of the “building blocks” that formed the planets during the early stages of the solar system. Leftover planetesimals became the objects we know today as comets and asteroids.

Plasma tail: see “ion tail” above.

Polymer: A relatively large, complex molecule, formed by linking chains of smaller, simpler molecules.

Precession: the “wobbling” motion of a rotating object caused by an external gravitational field. The earth’s precessional cycle takes approximately 25,800 years to complete one “wobble.”

Pre-discovery (photograph): a photograph of a comet (or other object) taken before that object’s discovery, but not noticed until after the discovery has been reported. Often these images are found as a result of a deliberate search after an object’s orbit has been computed.


Radiant: the point in the sky from which the meteors in a meteor shower appear to radiate.

“Resisting medium”: a now-discredited idea proposed in the 19th Century to explain what is now called non-gravitational forces, wherein interplanetary “material” that a comet encountered would act to slow it down into a smaller orbit.

Resonance: a condition wherein the ratio of the orbital periods of two objects is a simple fraction, e.g., 1:2, 2:3, etc.

Retrograde: moving in a direction opposite that of the earth (see “inclination” above).

Right ascension (RA): the coordinate used on the celestial sphere which is analogous to the terrestrial coordinate of longitude.

Roche radius: the distance from an object (such as a planet) within which the tidal forces acting upon a smaller object (such as a comet) are stronger than the internal material strength of that smaller object, and thus it is ripped apart.


Semi-major axis: half of the distance between an object’s perihelion and aphelion points. This is also equivalent to an object’s average distance from the sun.

Solar wind: a stream of highly energetic charged particles (such as protons, electrons, and ions) constantly “blowing” off the sun’s “surface.”

Short-period comet: by definition, a comet with an orbital period of 200 years or less.

Spectrophotometry: the practice of measuring an object’s brightness at different wavelengths in the electromagnetic spectrum, and combining these to produce its spectrum.

Spectroscopy: the practice of observing and recording an object’s spectrum.

Sublimate: to change directly from a solid to a gas. Substances such as water and carbon monoxide ice do this in space when they are heated, since the vacuum around them does not “force” them into a liquid state.

Sungrazer: see “Kreutz sungrazer” above.

Synchronic bands: parallel ray-like structures in a comet’s dust tail, believed to be caused by the disruption of large dust grains.


Trans-Neptunian objects (TNOs): an alternative name given for objects which inhabit the Kuiper Belt.


Volatile: a substance that is highly reactive and/or undergoes reactions or other changes as a result of only mild stimuli (e.g., temperatures, etc.)


Zodiac: the group of constellations within which the sun, moon, and planets travel.

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