Astronomical Glossary
Absolute magnitude
The apparent brightness (magnitude) a star would have if it were 32.6 light years (10 parsecs) from Earth. It is used to compare the true, intrinsic brightness of stars. The Sun has an absolute magnitude of +4.8.
Alfven Waves
Waves that travel through electrically conductive fluids or gases where a magnetic field is present such as the Sun’s plasma
Annular eclipse
A solar eclipse where the Moon passes directly in front of the Sun, but is too far from Earth to completely cover the Sun. A ring of sunlight (annulus) surrounds the Moon at the peak of the eclipse.
Antarctic Circle
The line of latitude on Earth’s surface that is 231/2° north of the South Pole. The Antarctic Circle marks the northernmost points in the Southern Hemisphere that experience the midnight Sun.
Antoniadi Scale
A scale used to estimate seeing conditions as devised by Eugene Antoniadi circa 1900 ranging from I for perfect seeing conditions to V - very poor.
Aperture
The diameter of a telescope’s clear aperture of its primary mirror or lens
Aphelion
The point in an object’s orbit where it is farthest from the Sun (Helios = sun). Earth is at aphelion each year on about July 3.
Apparent magnitude
A system used to compare the apparent brightness of celestial objects. The lower an object’s apparent magnitude, the brighter it is. A change in magnitude of 1 corresponds to a change in brightness by a factor of 2.5. Objects with a magnitude of less than 6 can be seen with the naked eye in good observing conditions.
Arc-minute
A unit of angular measure equal to one sixtieth of a degree. As an example, the Moon has an apparent diameter of about 30 arc-minutes.
Arc-second
A unit of angular measure equal to one sixtieth of one sixtieth of a degree, or one sixtieth of an arc-minute. As an example, the apparent diameter of Jupiter is about 45 arc-seconds.
Arctic Circle
The line of latitude on Earth’s surface that is 231/2 degrees south of the North Pole. The Arctic Circle marks the southernmost points in the Northern Hemisphere that experience the midnight Sun.
Asterism
A group of stars that people informally associate with each other to make a simple pattern, such as the Big Dipper and Square of Pegasus. The stars in an asterism can come from one or more official constellations.
Asteroid
One of the many thousands of chunks of rock or iron that orbit the Sun, also known as minor planets (an older term). Most orbit between Mars and Jupiter, where they formed, but some cross the orbit of Earth. Fragments of asteroids are called meteorites if they fall to the ground.
Astrological sign
One of 12 sections of the zodiac that are 30° long and that corresponds to the positions of the constellations as they were about 2,600 years ago when the astrological system was established. Do not confuse an astrological sign with an astronomical constellation with the same or similar name, as they only partially overlap.
Astronomical Unit
The mean distance from the Earth to the Sun referred to as 1AU that is equal to approximately 93 million miles
Autumnal equinox
The moment when the Sun crosses the ecliptic in a southward direction on or about September 22. In the Northern Hemisphere, it marks the first day of autumn. It is also the Sun’s position in the sky at that moment. In the sky, it is one of two intersection points of the ecliptic and celestial equator (the other being the vernal equinox).
Binary star
Two stars which orbit a common center of gravity. These stars often look like a single star to the naked eye. If more than two stars orbit a common center of gravity, it is called a multiple star.
Birth sign
See astrological sign.
Brown Dwarf
A small type of star that has too little mass to start normal stellar nuclear reactions.
Celestial equator
The projection of Earth’s equator into space; also a line in the sky midway between the North and South Celestial Poles. The celestial equator is the line of zero declination in the equatorial co-ordinate system.
Celestial meridian
The line of zero right ascension in the equatorial co-ordinate system.
Celestial sphere
The projection of Earth into space. One can imagine the stars to be drawn on the inside of this sphere.
Cell
The structure in a reflecting telescope that holds the primary mirror that has small pads in contact with the rear of the mirror. Small telescopes usually have three of these pads, a ‘three point’ cell. Larger telescopes sometimes have nine. The more pads there are the less likely the mirror is to flex.
Central Obstruction
This is the secondary mirror in a reflecting telescope. It blocks some of the light reaching the primary mirror, but not usually enough to degrade the final image.
Circumpolar
Those stars that are so far north (or so far south, in the Southern Hemisphere) they do not set at all as seen from a given latitude.
Comet
A body composed of ice and dust in orbit around the Sun.
Conjunction
The passing of one planet by another planet or by the Moon or Sun. Two planets are in conjunction when they have the same ecliptic longitude (or alternately the same right ascension).
Constellation
One of the 88 portions of the sky that are officially recognized by the International Astronomical Union. A constellation is an arbitrary area of the sky, and it includes everything within that area’s boundaries, regardless of distance from Earth.
Daylight Saving Time
The adjustment to the clock time that is put into effect during the summer to extend daylight one hour later in the evening.
Declination
The angular distance of an object north of (positive) or south of (negative) the celestial equator, expressed in degrees. It is the celestial equivalent of latitude on Earth’s surface. The declination
of the celestial equator is 0°; the declination of the North Celestial Pole is +90°, and the declination of the South Celestial Pole is -90°.
Double star
Two stars that appear near each other in the sky. Their apparent closeness may be due to chance alignment, with one star far beyond the other, or they may be in orbit around a common center of gravity, in which case they form a binary star.
Eclipse
The passage of one object in front of another (as the Moon passes in front of the Sun during an eclipse of the Sun), or the passage of one object through the shadow of another (as the Moon passes through the shadow of Earth during an eclipse of the Moon).
Eclipse season
The 38-day period when the Sun is near a node of the Moon’s orbit and one or more solar eclipses may happen.
Ecliptic
The Sun’s apparent annual path through the fixed stars; also the orbit of Earth if it could be seen in the sky. The 13 astronomical constellations that the ecliptic passes through are the astronomical constellations of the zodiac (12 in astrology).
Ecliptic co-ordinate system
The system of specifying positions in the sky that uses the ecliptic – the Sun’s apparent path around the sky – as the fundamental reference plane. Ecliptic co-ordinates are useful when specifying positions in the solar system and especially positions relative to the Sun.
Ecliptic latitude
The angular distance of an object above (positive) or below (negative) the ecliptic, expressed in degrees. The ecliptic latitude of the Sun is always zero.
Ecliptic longitude
The angular distance of an object, measured along the ecliptic, eastward from the vernal equinox, and expressed in degrees. The ecliptic longitude of the Sun is 0° when the Sun is on the vernal equinox, and it increases by very nearly 1° per day through the year.
Elliptical galaxy
One of the two major types of galaxies being apparently elliptical in shape, the other being a spiral galaxy.
Epoch
Particular date for which astronomical positions in a book or table are accurate. Most books give star positions that are valid for the J2000 (January 1, 2000) epoch.
Equation of time
The difference between true solar time (determined by the Sun’s position in the sky) and mean solar time (the time told by your watch). The two times can vary by as much as 16 minutes over the course of a year.
Equatorial co-ordinate system
A system of specifying positions in the sky that uses the celestial equator – the projection of Earth’s equator into space – as the fundamental reference plane.
Fast telescopes
This refers to short focal length telescopes or telescopes with low f/ratios. A common myth is that fast telescopes offer brighter images than slow ones, thereby allowing ‘faster’ exposures. Image brightness is only influenced by the telescope's aperture, which controls the telescope's light gathering ability.
Field of view
Angular width of sky that can be seen with an optical instrument. Field of view is measured in degrees, arc-minutes, and arc-seconds.
Figure
The final finish on a telescopes primary mirror directly affecting the quality of the image.
Focal length
This is the distance from a primary lens or mirror to the point where the image is brought to a focus. In a refractor this is just before the eyepiece and in a reflector is behind the secondary mirror so that the light being reflected by this mirror will place this point just before the eyepiece
Foucault pendulum
Pendulum that varies the direction of its swing as Earth rotates. Used to demonstrate that Earth rotates, not the sky.
Full Moon
The Moon when it lies directly opposite the Sun. The Moon is full two weeks after new Moon. The full Moon rises at sunset and sets at sunrise. Earth is between the full Moon and the Sun.
Galactic co-ordinate system
The system of specifying positions in the sky that uses the plane of the Milky Way as the fundamental reference plane.
Globular cluster
A huge spherical cluster of tens of thousands of stars. The stars of a cluster were born together and travel through space together. M13 and M22 are familiar examples.
Greatest eastern elongation
The greatest angular distance to the east of the Sun reached by Mercury or Venus. When a planet is at its eastern elongation, it sets after the Sun and is at its best visibility in the evening sky.
Heliocentric
Sun-centered
Heliopause
The point at which the solar wind meets the interstellar medium or solar wind from other stars.
Heliosphere
The space within the boundary of the heliopause containing the Sun and solar system.
Inferior conjunction
The passage of Mercury or Venus between Earth and the Sun. The outer planets cannot pass between Earth and the Sun and therefore cannot come to inferior conjunction.
Inferior planet
Mercury or Venus, so-called because their orbits are inside Earth’s orbit around the Sun and “inferior” to Earth in terms of distance from the Sun.
Infrared (IR)
Part of the electo-magnetic spectrum that has longer wavelengths than visible light.
Interstellar Gas
Gas that is present between the stars in space. Composed primarily of Hydrogen is it the raw material from which new stars can form.
Julian Day
The number of days (and fractions of days) that have elapsed since noon, Jan.1, 4713 BC (Greenwich Mean Time). It is used to simplify calculating the time interval between two events. For example, 900 P.M. P.S.T. on January 1, 2000, was Julian day 2,451,545.71.
Kelvin (K)
0 Kelvin is absolute zero - minus 273 °C
Latitude
The angular distance of an object north or south of Earth’s equator expressed in degrees. The latitude of the equator is 0°, London is 51° North, the North Pole is 90° North, and Lima, Peru is 12° South. Latitudes south of the equator are expressed as South or negative.
Light year
The distance light travels in one year. One light year equals 9,460,536,000,000 kilometers, or 5,878,507,000,000 miles.
Limiting magnitude
The magnitude of the dimmest object that can be seen through an optical instrument (including the eye). The limiting magnitude of an instrument will vary with light and sky conditions.
Light pollution
The brightening of the night sky due to artificial light. Light pollution makes it impossible to view many dim objects that can only be seen in a dark sky.
Locked rotation
Condition where a moon has the same period of rotation as its period of revolution around its parent body. This means that the moon always shows the same face to its parent planet. Our
Moon is in locked rotation around Earth.
Longitude
The angular distance of an object east or west of the Prime Meridian (the line of zero longitude which runs through Greenwich, England), expressed in degrees.
Luminosity
An expression of the true brightness of a star as compared to the Sun. The Sun’s luminosity is 1.0 by definition. Sirius has a luminosity of 23 and Rigel a luminosity of about 50,000.
Lunar eclipse
An eclipse of the Moon (lunar = moon), caused when the Moon moves partially or wholly into the shadow of Earth and grows dark for up to a few hours. Everyone on the side of Earth that is facing the Moon can see a lunar eclipse.
Magnitude
An expression of the brightness of a star (or other celestial object) as it appears from Earth, according to a system devised by Hipparchus. Also known as apparent magnitude, to distinguish from absolute magnitude. Larger numbers refer to fainter stars, and the brightest stars and planets have negative magnitudes. One magnitude difference is equal to a brightness difference of 2.5 times.
Massing
Close alignment of three or more planets (or two or more planets and the Moon), as seen from Earth. This occurs when all the bodies involved in the massing have similar ecliptic longitudes.
Meridian
The line in the sky that extends from the southern point on the horizon through the zenith overhead point) to the northern point on the horizon, bisecting the sky into an eastern and western half. Objects are at their highest when they cross the meridian; the Sun is on the meridian at local noon. It can also be a line on the surface of Earth (or another body) that extends from pole to pole.
Messier object
One of the 110 objects in the catalog compiled by Charles Messier. Most ‘Messier’ objects are galaxies, star clusters or nebulae.
Meteor
The visible flash of light produced when a meteorite falls through the atmosphere and bursts into flame because of friction with air molecules; also called a “shooting star” or “falling star”.
Meteorite
The solid particle, either stone or iron that falls through the atmosphere to produce a meteor. Most meteorites are fragments of asteroids. Science museums display meteorites that survived their falls.
Midnight Sun
The Sun when visible at midnight, which happens only in summer north of the Arctic Circle or south of the Antarctic Circle.
Milky Way
Our own galaxy.
Minor planet
See asteroid.
Month
The period of time it takes the Moon to orbit Earth (or a moon to orbit a planet). A sidereal month (271/3 days) is the time it takes the Moon to orbit Earth and return to the same position relative to the stars; a synodic month (291/2 days) is the time it takes the Moon to orbit Earth and return to the same position relative to the Sun and is the time between new Moon and new Moon.
Nadir
The point on the sky directly below the observer or below your feet if you could see through the Earth.
Nebula
A cloud of gas or dust in space, either between the stars or expelled by a star; nebula is Latin for cloud. There are many kinds of nebulae.
New Moon
The Moon when it lies in the same direction as the Sun and the beginning of a cycle of lunar phases. The new Moon rises and sets with the Sun. The Moon is between Earth and the Sun at new Moon.
Night vision
Enhanced ability to see objects in the dark. Night vision is ruined if the eyes are exposed to bright light.
Node
The point(s) in the sky where two orbits or paths cross. The nodes of the Moon’s orbit are the two places where the Moon’s orbit crosses the ecliptic.
North Celestial Pole
The sky’s north pole; the point in the sky directly above Earth’s north pole.
Obliquity of the ecliptic
The amount of the tilt of Earth’s axis (23.5°) which determines the angle the ecliptic makes with the celestial equator as they intersect in the sky.
Occultation
The disappearance (eclipse) of one object behind another, as a star or planet behind the Moon or a star behind a planet.
Ocular Lens
Otherwise known as an eyepiece, this is the final lens in an optical system that magnifies the image and brings it to a focus.
Open cluster
A diffuse association of a few dozen to a few thousand stars, all of which were born together and which travel through space together. The Pleiades in Taurus and Beehive Cluster in Cancer are familiar examples.
Opposition
The position of a planet when it is opposite the Sun in the sky. Only objects that orbit outside Earth’s orbit come into opposition; Mercury and Venus cannot.
Parallax
The apparent shift in position of an object when it is viewed from two different points. The parallax of a star is measured from opposite ends of Earth’s orbit, and for the nearest stars is less than one second of arc (one arc-second).
Parsec
The distance an object would have if its parallax were one second of arc (see parallax). One parsec equals 3.26163 light years or 30,856,780,000,000 kilometers.
Penumbra
Shadowed area in an eclipse where only part of the light source is blocked. Observers in the penumbral shadow of a solar eclipse see a partial eclipse.
Perihelion
The point in an object’s orbit when it is closest to the Sun (Helios = Sun). Earth is at perihelion each year on about Jan. 3.
Period
The time an object takes to complete a certain motion and return to its original state, e.g. period of revolution.
Photon
A quanta of light or packet of radiation. Light can be though of as taking two forms, waves or particles. It is these particles that are given the name.
Pickering Scale
A scale of seeing conditions named after the astronomer William Pickering. It ranges from Pickering 1 – very poor seeing conditions to Pickering 10 that is perfect conditions.
Pixel
The name is a play on ‘picture element’ and is the basic unit of an electronically stored or displayed image. Combined these give the viewer a smooth image.
Planetary nebula
A luminous cloud of gas expelled by an aging star that has become unstable. The name comes from a nebula’s superficial resemblance to the faint planets Uranus and Neptune as seen through a small telescope. The Ring Nebula M57 in Lyra is a familiar example.
Plasma
One of the four different phases of matter. A gas of highly charged particles that is electrically neutral.
Precession of the equinoxes
The slow wobbling of Earth’s axis in a 25,800-year cycle, caused by the gravitational attraction of the Moon on Earth’s equatorial bulge. Precession causes the vernal equinox (and all other points on the ecliptic) to regress westward along the ecliptic, slowly changing the equatorial co-ordinate grid.
Prime Meridian
The line of longitude that passes through Greenwich, England, and which is the zero line for expressing longitude on Earth’s surface.
Proper motion
The motion of the stars relative to each other, caused by their actual motion in different directions at different speeds through space.
Retrograde motion
The apparent “backwards” or westward motion of a superior planet against the background of the stars caused when the faster-moving Earth on an inside orbit passes that planet.
Revolution
The orbiting of one body around another body, as the Moon revolves around Earth and Earth revolves around the Sun.
Right ascension
In the equatorial co-ordinate system, the angular distance of an object eastward from the zero point (which is the vernal equinox), usually expressed in hours and minutes (which represents Earth’s rotation from the vernal equinox to the object). It is the celestial equivalent of longitude on Earth’s surface.
Ronchi grating
An optical device used to test the quality of a telescope’s optics. The image seen through a Ronchi grating form parallel lines which for a spherical mirror for example should appear straight.
Rotation
The spinning of a body on its own axis. Earth rotates once a day. See revolution, which is often confused with rotation.
Sidereal day
The time it takes Earth to rotate once relative to the stars, in 23 hours 56 minutes and 4 seconds, which is 4 minutes less than a solar day. (During one sidereal day the Sun moves 1° east along the ecliptic, and Earth has to rotate 4 additional minutes to complete one rotation relative to the Sun in one 24-hour solar day.)
Sidereal month
The time it takes the Moon to complete one orbit of Earth and return to the same position among the stars, on an average of 27.32166 days.
Sidereal year
The time it takes Earth to complete one orbit of the Sun relative to the stars, in 365 days 6 hours 9 minutes and 10 seconds. It is also the time it takes the Sun to appear to travel once around the sky relative to the stars.
Solar day
The time it takes Earth to spin once relative to the Sun, in exactly 24 hours (by definition).
Solar eclipse
An eclipse of the Sun by the Moon, when the Moon passes in front of the Sun. Solar eclipses can be partial, total, or annular. Only the few people in the narrow “path of totality” see a solar eclipse as total.
South Celestial Pole
The point in the sky directly above Earth’s south pole.
Space-time
The fabric of space as described in Einstein’s theory of relativity. Time is the fourth dimension. Einstein showed that gravity warps this fabric.
Spiral galaxy
The second major type of galaxy, characterized by a central bulge and a number of spiral arms extending from the bulge.
Standard time zone
one of the 24 sections of Earth, each about 15 degrees wide and extending from pole to pole, within which the time is the same. In practice, natural and political boundaries determine the edges of time zones.
Stellar Type
A classification of stars according to their properties. Based on temperature the sequence runs from the hottest to the coolest O, B, A, F, G, K, M. The Sun is a ‘G’ type star.
Summer solstice
The moment when the Sun reaches its greatest distance north of the celestial equator, on or about June 21. In the Northern Hemisphere this marks the first day of summer; in the Southern Hemisphere it marks the first day of winter.
Superior conjunction
The position of a planet when it is on the far side of the Sun (and in conjunction with the Sun).
Superior planet
The planets Mars through Pluto, so-called because their orbits are outside Earth’s orbit around the Sun and thus “superior” to Earth in terms of distance from the Sun.
Synodic month
The time it takes the Moon to complete one cycle of phases, such as from new Moon to new Moon, on an average of 29.53059 days.
Synodic period
This is the time between successive oppositions of a planet as viewed from Earth. This differs from the true orbital period due to the motion of the Earth about its own orbit.
Transit
The passage of one celestial body across the face of a second larger body.
Triple conjunction
Close alignment of a planet and a star at three distinct times, caused by the retrograde motion of the planet. The planet passes the star once in its forward motion, once more in its retrograde motion, and a third time when it resumes its forward motion.
Tropical year
The length of time it takes the Sun to circle the sky relative to the vernal equinox. The tropical year is identical to our standard “year”.
Tropic of Cancer
The line of latitude on Earth’s surface that is 231/2 degrees north of the equator. It marks the northernmost points in the Northern Hemisphere from which the Sun can appear directly overhead.
Tropic of Capricorn
The line of latitude on Earth’s surface that is 231/2 degrees south of the equator. It marks the southernmost points in the Southern Hemisphere from which the Sun can appear directly overhead
Umbra
Shadowed area in an eclipse where the light source is completely blocked. Observers in the umbral shadow of a solar eclipse experience a total eclipse.
Universal Time
In simplest terms, the time at the longitude of Greenwich, England. Universal Time (UT) is widely used in international publications as a standard time.
Variable star
A star whose light changes over time. Stars can vary in brightness for a variety of reasons, from eclipses by companions to instability of their interiors that causes the stars to swell and shrink.
Vernal equinox
The moment when the Sun crosses the ecliptic in a northward direction on or about March 21. In the Northern Hemisphere, it marks the first day of spring. It is also the Sun’s position in the sky at that moment. In the sky, it is one of two intersection points of the ecliptic and celestial equator (the other is the autumnal equinox).
Waning crescent
The phase of the Moon between third quarter and new Moon. Waning, means declining or fading.
Waning gibbous
Tthe phase of the Moon between full Moon and last quarter.
Waxing crescent
The phase of the Moon between new Moon and first quarter. Waxing means increasing.
Waxing gibbous
The phase of the Moon between first quarter and full.
Winter solstice
The moment when the Sun reaches its greatest distance south of the celestial equator, on or about December 22. In the Northern Hemisphere this marks the first day of winter; in the Southern Hemisphere it marks the first day of summer.
Zenith
The point in the sky directly above the observer; the top of the sky.
Zodiac
The band of constellations or signs that the Sun passes through as it moves around the sky. There are 12 signs of the astrological zodiac but 13 constellations of the astronomical zodiac.
Zodical light
A faint glow seen in the night sky which is due to sunlight reflecting off dust particles in our solar system and is best seen in the west after twilight and in the east before sunrise.
