The Great Christ Comet

by The Great Christ Comet- Revealing the True Star of Bethlehem (retail) (epub)

  Coma. The comet’s head, consisting of a gas and dust cloud enveloping a nucleus.

  Comet. A celestial body consisting of dust and ice that, when it is close to the Sun, produces gas and dust, which form a head (coma) and tail(s) that point away from the Sun.

  Cometary. Of or relating to comets.

  Conjunction. The coming together of two celestial objects in the sky so that they seem to observers to be in the same location or very close to one another; or, the occasion when the celestial longitude of two astronomical bodies is the same; or, the occasion when a celestial body is too close to the Sun to be visible.

  Constellation. A grouping of stars that seem to observers to form a pattern.

  Culmination. The arrival of a celestial body at the meridian and therefore its highest daily altitude (relative to the horizon).

  Daytime comet. A comet so bright that it is visible, for a while, during the daytime.

  Degree. A unit for measuring angles. 180 degrees of the sky are visible above an unobstructed horizon. From the zenith to the unobstructed horizon there are 90 degrees; 1 degree is equal to 60 arcminutes or 3,600 arcseconds.

  Delta Effect. The theory that, as a comet makes a close approach to Earth (within about 0.4 AU), the outer edges of the growing coma may go undetected by the human eye.

  Descending node. The point at which an orbit crosses the plane of the ecliptic, as the comet moves from the north of it to the south of it.

  Dust. A variety of, among other things, magnesium-rich silicates, sulfides, and carbon expelled from a comet.

  Dust tail. The tail of a comet, consisting of dust particles lagging behind the comet’s head (or coma). The dust is illuminated by the Sun and may become bright enough to be visible to Earth-dwellers.

  Earth-Sun line. The imaginary straight line connecting Earth and the Sun.

  Eccentricity (e). The extent to which a celestial body’s orbit deviates from perfect cir­cu­larity—the eccentricity of a circle is 0; the more stretched the oval (ellipsis) is, the higher the eccentricity is, up to 1; an eccentricity of greater than 1 means that the celestial body is incapable of completing a orbital revolution.

  Ecliptic. The plane on which Earth orbits the Sun.

  Elliptical. Oval.

  Encke-type comets. Short-period comets that, like Comet Encke, have orbits so small that the comets do not have close encounters with Jupiter.

  Fireball. A meteor at least as bright as Jupiter or Venus.

  First Point of Aries. The location of the vernal equinox, a base line for astronomical measurements.

  Forward-scattering. As an active comet moves closer to the imaginary line between Earth and the Sun, the coma and the dust tail are subject to an increasingly large spike in brightness, because the Sun’s light hits the small dust particles and is scattered forward.

  Full Moon. The lunar phase during which the whole of the Moon is illuminated, which occurs when it is in opposition to the Sun.

  Galaxy. A gravitationally bound system of stars and associated matter.

  Gas tail. The usually bluish, straight tail formed when electrically charged gas particles that exploded from the comet nucleus are pushed by the solar wind straight back behind the nucleus to point away from the Sun.

  Gravitational effect. The effect of the gravitational pull of planets (such as Jupiter and Saturn) on a solar system object’s orbit.

  Great comet. A comet judged to be exceptional by virtue of its brightness and/or size.

  Halley-type comet. A comet that, like Halley’s Comet, has an orbital period of between 20 and 200 years.

  Heliacal rising. The first visible rising of a celestial body over the (usually eastern) horizon after being invisible because of proximity to the Sun.

  Heliacal setting. The final visible setting of a celestial body in the run-up to becoming invisible because of proximity to the Sun.

  Hui-hsing. A Chinese expression for a broom-star comet, that is, a comet with a tail.

  Hyperbolic orbit. A solar system object’s orbit when it has greater than escape velocity (that is, an eccentricity value of greater than 1.0).

  Inclination (i). The angle between the plane of a solar system object’s orbit and the ecliptic (in degrees).

  Inferior planets. Mercury and Venus.

  Inner planets. Planets inside the asteroid belt, namely Mercury, Venus, Earth, and Mars.

  Inner solar system. The region of the solar system up to and including the asteroid belt, incorporating the Sun, Mercury, Venus, Earth, Mars, the asteroid belt, and Ceres.

  Intercalary month. A leap lunar month inserted into a lunar calendar to calibrate it with the solar calendar.

  Julian calendar. A calendar that assumes that a full year consists of precisely 365.25 days and that, unlike the Gregorian calendar, does not allow for the fact that a full year is actually 11 minutes less than 365.25 days. From 45/44 BC to 9/8 BC a leap day was added every three years instead of four, with the result that three leap days too many had been intercalated. The Emperor Augustus remedied this by suspending leap years until AD 4 or 8.

  Jupiter-family comets. Comets that complete one revolution around the Sun in less than 20 years.

  Kracht Group. A group of comets that come as close as 0.047 AU to the Sun and have a relatively low inclination (roughly 13.4 degrees).

  Kreutz Family. A large group of bright sungrazing comets, including the great comets of 1843 and 1882 and Ikeya-Seki in 1965, which have high-inclination and 600- to 1,100-year orbits.

  Longitude of the ascending node (Ω). The angle from the First Point of Aries (the vernal equinox) to a celestial object’s ascending node.

  Longitude of perihelion. The angle between the First Point of Aries (the vernal equinox) and the point of perihelion.

  Long-period comet. A comet that takes more than 200 years to complete one revolution around the Sun.

  Lunar eclipse. When the Moon, or a part of it, moves into Earth’s shadow, directly behind Earth.

  Magnitude. A measurement of the relative brightness of celestial bodies. The brighter the object, the lower the magnitude value. A star of magnitude +1 is 2.5 times brighter than a star of magnitude +2. A star five magnitudes brighter than another star is 100 times brighter.

  Major axis. The longest diameter of an ellipse (oval), measuring from the widest points through the foci and the center.

  Massing. A grouping of three or more planets in the same area of sky.

  Meridian. The great imaginary circle that passes through the observer’s zenith and nadir, the celestial north and south poles, and the observer’s due north and due south.

  Meteor. The streak of light, or “shooting star,” that appears when a body of matter from outer space (a meteoroid) is heated until incandescent due to friction as it encounters resistance from Earth’s atmosphere.

  Meteor shower. When a number of meteors seem to radiate from one particular point in the sky at one particular time of the year.

  Meteor storm. When more than 1,000 meteors per hour radiate out of one point in the sky (or, more technically, when more than 1,000 meteors would occur per hour if the radiant were at the zenith).

  Meteorite. A piece of space debris that survives its encounter with Earth’s atmosphere and surface.

  Meteoroid. A piece of rocky or metallic debris in space, commonly from comets or asteroids.

  Meteoroid stream. A stream of particles ejected from a comet or asteroid.

  Minor axis. The shortest diameter of an ellipse (oval), measuring from the narrowest points through the center.

  Minor planet. A solar system object that is not classified as a comet or a planet. Included in the minor planets are asteroids, centaurs, and trans-Neptunian objects.

  Morning star. A bright celestial entity, particularly Venus, that is present in the eastern sky in the predawn period.

  Nebulous star. A cluster of hazy stars, or a star in a haze.

  New Moon. The initial phase of the Moon’s cycle, when the Moon firs
t becomes visible after being in conjunction with the Sun.

  Nongravitational effects. The acceleration or deceleration of a comet due to the recoil effect of its degassing.

  Nova. A cataclysmic nuclear explosion on the surface of a white dwarf that causes a sudden brightening that lasts for weeks or months.

  Nucleus. The icy and dusty core of a comet, which becomes active when near the Sun.

  Occultation. When one celestial body is hidden by another body that moves between it and the observer.

  Orbital elements. The six pieces of data that fully describe a solar system body’s orbit.

  Orbital period (P). The time a solar system body takes to complete one revolution around the Sun.

  Orbital plane. The plane on which a body in the solar system orbits the Sun.

  Outburst. The sudden, unexpected explosive release of dust by a comet, causing significant brightening.

  Outer planets. The planets in the solar system that are beyond the asteroid belt, consisting of Jupiter, Saturn, Uranus, and Neptune.

  Outer solar system. The region of the solar system beyond the asteroid belt, as far as Neptune.

  Outgassing. The release of jets of gas from the comet nucleus when it is close to the Sun.

  Parabolic orbit. A solar system object’s orbit when it has an eccentricity value of 1.0; the boundary between a capture (elliptical) orbit and an escape (hyperbolic) orbit.

  Partial lunar eclipse. When a portion of the Moon passes behind Earth through Earth’s shadow.

  Perigee. The point in a celestial body’s orbit when it is closest to Earth.

  Perihelion. The point in a celestial body’s orbit when it is closest to the Sun.

  Perihelion distance (q). The distance from a solar system body to the Sun when it is closest to it.

  Perihelion time (T). The time in a celestial body’s orbit when it is closest to the Sun.

  Periodic comet. A comet that completes a single revolution around the Sun in less than 200 years. (Also called a “short-period comet.”)

  Phase angle. The angle formed where a line extending to a solar system body from the Sun and one extending to the same body from Earth converge.

  Planetarium software. Software that simulates the sky at different points in history.

  Planetary perturbations. A solar system body’s deviations from its calculated orbit due to the gravitational forces exerted by other solar system bodies, especially planets such as Jupiter and Saturn.

  Precession of the equinoxes. The slow westward shift of the equinoxes along the ecliptic, caused by the spinning-top-like wobble of Earth’s axis. One cycle lasts about 26,000 years.

  Progenitor comet. A parent or ancestor comet.

  Prograde. Orbital movement that is in the same direction as most solar system bodies. When viewed from the vantage point of Earth’s North Pole, it is counterclockwise motion.

  Pseudonucleus. A cometary coma’s star-like zone of concentrated brightness in the vicinity of the nucleus.

  Radiant. The point from which the meteors in a given meteor shower seem to emanate.

  Retrograde. Orbital movement in a direction opposite to that of most solar system bodies. When viewed from the vantage point of Earth’s North Pole, it is clockwise motion.

  Return. A comet’s reappearance in connection with a particular perihelion pass.

  Semi-major axis. Half of the longest diameter of an ellipse, measuring from the widest points through the foci and the center.

  Short-period comet. A comet that completes one revolution around the Sun in less than 200 years. (Also called a “periodic comet.”)

  Solar wind. The constant stream of charged particles emanating from the Sun.

  Spin axis. The imaginary line around which an object rotates.

  Sporadic meteors. Meteors that have not been associated with a particular meteor shower.

  Sungrazer. Any of the group of comets that make very close passes by the Sun, coming within 0.01 AU (or 0.05 AU) of it.

  Sunskirter. Any of the group of comets that have a perihelion distance that is greater than 0.01 AU (or 0.05 AU) but less than 0.1 AU.

  Superbolide. A bolide (meteor) that attains to an apparent magnitude of -17 or brighter.

  Superior planets. Mars, Jupiter, Saturn (and, in recent times, Uranus and Neptune).

  Supernova. The enormous, catastrophic nuclear explosion of a star that greatly intensifies its brightness for a period up to about three years. A supernova may be due to the cataclysmic ignition of nuclear fusion in a white dwarf’s core, or due to a massive star’s running out of fuel and then catastrophic collapse under its own gravity.

  Surface brightness. The brightness of a celestial body’s brightness per unit area, expressed in magnitudes per square arcsecond/arcminute.

  Tail. Streams of dust and gas expelled from and pushed behind a coma that may become bright enough to be seen on Earth.

  Trans-Neptunian object. Any solar system object with an orbit farther away from the Sun than Neptune (for example, Pluto).

  Vernal equinox. The moment, in the Spring, when the Sun crosses the plane of Earth’s equator, with the result that night and day are of equal length. Also, the celestial location of the Sun at the moment of the vernal equinox—known as the First Point of Aries.

  Waning gibbous Moon. The phase of the lunar cycle after a full Moon when the Moon is less than fully, but more than half, illuminated.

  Waxing gibbous Moon. The phase of the lunar cycle before a full Moon when the Moon is more than half, but less than fully, illuminated.

  White dwarf. A still-white-hot, old, dying star that has used up all of its nuclear fuel and has cast off its outer material.

  Zenith. The imaginary point directly above an observer.

  Zenith attraction. An effect in which meteor radiants appear to observers to be closer to the zenith than they really are.

  Zodiac (zodiacal signs). The 12 divisions, each 30 degrees long, into which the zodiacal band is divided in astrology. The divisions are named after the constellations that occupy the zodiacal band.

  Zodiacal band. The band of sky either side of the ecliptic, along which the Sun, Moon, and major planets appear to traverse the heavens.

  Zodiacal constellations. The constellations located along the zodiacal band.

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