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Glossary of Astronomy

Navigating by Astronomy
Glossary of Astronomy

Copyright © 2017 Travis N. Wood


Below is a glossary limited to the terms essential to understanding how to navigate in the backcountry using astronomy. In the end our concentration will be on just a few basic terms such as altitude and azimuth, and different modes of reckoning time such as daylight savings time, standard time, sidereal (star) time, and solar (sun) time. Together with those most basic terms we include a few other terms needed to explain the basics.

In the glossary below we have excluded many esoteric terms of a more elaborate knowledge of the field. But more extended glossaries are linked toward the bottom of the page.


Altitude--The angular distance of an object above a level horizon.

Analemma--Literally referring to the base of a "sundial," the analemma is a figure 8 pattern derived from the sun's position at the same time, such as noon, throughout the year. The average length of day over the course of a year is considered to be 24 hours. But because of the earth's tilt and its slightly elliptical orbit, some days are actually longer and some shorter than 24 hours. Thus the position of the sun at noon will vary throughout the year. If those positions are plotted upon a graph or photographed daily, the graph or composite photo will yield a figure 8 pattern crucial to calculating the exact time that the sun will be directly south of an observer's position. That time of "southing" or meridian transit may vary about 15 to 20 minutes before or after noon. At some times of year the earth's tilt is the stronger factor. At other times the earth's elliptical orbit is the stronger factor. Such graphs are easily available. (See our discussion in following pages on finding direction by the sun. Or for more information see this explanation of analemma.)

Angular Size-- The apparent size of an object measured as an angle. An index finger held at arm's length spans about 1° and a fist about 10°.

Angular distance-- The distance between two points or objects, measured as an angle.

Autumnal equinox--The time in autumn when the sun crosses the equator, thus creating days and nights of equal length, generally around September 22nd.

Azimuth--The angular distance along the horizon of a point or object, beginning at north. North is 0°, east is 90°, south is 180°, and west is 270°. A complete circle of 360° is again north at 0°.



Celestial--Relating to the sky or of the sky, sometimes termed "heavenly," as in a heavenly body or celestial body.

Celestial Coordinates--A system of grids for specifying positions of celestial objects and direction of those objects on the celestial sphere. The basics of the system place the celestial poles directly above Earth’s north and south poles. The celestial equator is directly above Earth’s equator. Parallels of latitude on earth project upward to declination (celestial latitude). Meridians of longitude on earth correspond to right ascension (celestial longitude) on the celestial grid. Right ascension is measured in hours (15° per hour) beginning at the location of the sun at the time of the spring equinox. Because of the earth's rotation and its revolution around the sun, much more must be said about right ascension in the following pages. (Also see right ascension below.)

Celestial dome or celestial sphere--An imaginary dome or sphere around the earth used to locate celestial objects within grid systems, without regard to distance from the earth. By way of this dome, the sun, moon, stars, planets and so on may be described by celestial coordinates.

Celestial equator--An imaginary line dividing the celestial dome into a northern and southern hemisphere. The celestial equator is located directly above and out from the earth's equator.

Celestial poles--The north and south poles of the celestial sphere, which are located directly above and out from the earth's north and south poles.

Celestial meridian--Speaking generally, the celestial meridian of an observer's position is the line of right ascension (celestial longitude) that passes from the north celestial pole through the zenith directly above the observer and on to the south celestial pole. Celestial objects above the southern horizon cross the observer's celestial meridian when those objects are at an altitude directly above 180° azimuth, or south. This phenomenon is also called meridian crossing, meridian transit, or "southing" of the celestial object.

Celestial sphere--(See celestial dome.)

Circumpolar Star--A star that appears to circle a celestial pole without ever dropping below the horizon as the earth rotates. Thus a circumpolar star never rises or sets. Which stars are circumpolar depends upon the latitude of the observer. In the Northern hemisphere, the further north the observer, the more stars will be circumpolar. The further south, the fewer stars. At a location of 44° north latitude (such as in Northern Wyoming), stars that are less than 44° angular distance from the North Star will be circumpolar.

Constellation--An area of the sky and its distinctive pattern of stars. To aid memory, each sector has been identified, since ancient times, with an animal or character of legend. The sky is thus organized into a mosaic of 88 officially recognized constellations.

Culmination--The moment when a celestial object crosses the observer's meridian and is thus at its highest altitude above the horizon. Also called meridian transit, meridian crossing, or southing.


Declination (Dec.)--The angular distance (in degrees) of an object in the sky north or south of the celestial equator. Declination is the celestial equivalent of latitude. The southing altitude of the same object can be calculated from the known latitude of the observer and the declination of the object.


Eccentricity--The measure of how a celestial object's orbit deviates from a perfect circle, thus forming an ellipse or oval orbit.

Ecliptic--The apparent yearly path of the sun through the sky. While other celestial grids are determined by the earth's rotation, the ecliptic depends upon the earth's revolution around the sun. Thus the ecliptic is tilted at 23.47° from the celestial equator. The sun's declination, following the ecliptic, varies throughout the year from 23.47° north to 23.47° south of the celestial equator. The moon and planets are also found within a few degrees of the ecliptic.

Ellipse--An ellipse is an oval shape. Orbits of the planets and other celestial bodies are eccentric and thus elliptical or oval in shape.

Elongation--As seen from earth, elongation is the distance in degrees between a planet or moon and the sun. While sun and planets appear to follow the ecliptic, the angular distance is measured along the celestial equator as the distance between celestial longitudes (meridians of right ascension). A planet near to the sun but with western elongation may appear in the morning before sunrise. A planet near to the sun but with eastern elongation may appear in the evening after sunset.

Equation of time--(See analemma.)

Equinox--Literally meaning equal night (and day). An equinox is one of two times during the year when length of night and day are equal. These are also the only two times of the year when the sun rises directly east and sets directly west. But despite some sources, we cannot assume the sun will be directly south at noon. That will depend upon the time zone, upon the observer's location within the time zone, and upon the sun's analemma. The dates of the equinoxes occur on or near March 21 (for the vernal or spring equinox) and September 22 (for the autumnal equinox). At these times the Sun crosses the celestial equator in its yearly path. Thus the sun is located in one of two places where the ecliptic intersects with the celestial equator. The equinoxes are taken as the beginning of the Spring and Autumn seasons.





Inclination--A) Inclination in astronomy generally refers to orbital inclination, that is, the tilt of a planet's orbital plane to the earth's orbital plane, which is the ecliptic. While we see the ecliptic as the sun's yearly path through the sky, viewed from the sun the ecliptic would correspond to the earth's yearly path through the sky. So by definition, the earth's orbital inclination to the ecliptic is 0°. Since the earth's orbit and thus the ecliptic closely match the plane of the solar system, the orbital inclination of other planets is generally within 3° of the ecliptic. Venus and Jupiter, two valuable bodies for navigation, follow orbits inclined 3.4° and 1.3° respectively. The orbital inclination of Mercury is by far the greatest at 7.0°. B) Orbital inclination may also refer to the tilt of moon or satellite orbits from the earth's equator. C) Less commonly, inclination may refer to axial inclination (also called axial tilt or obliquity), which is the tilt of a planet's rotation to its revolution around the sun. The axial inclination or tilt of the earth is 23.44°.




Lunation--Also called Synodic Lunar Month or simply lunar month, it is the interval of a complete cycle of moon phases, in particular, from one new moon to the next. The lunation is equal on average to 29.531 days in units of mean solar time. This is not to be confused with the moon's sidereal month, which is the average period it takes the moon to revolve around the Earth in reference to fixed stars, equal to 27.322 days. The synodic lunar month is over two days longer because of the earth's revolution around the sun.


Magnitude--A measure of brightness of a celestial body, especially stars and planets. Also called apparent magnitude, it follows a rather obscure scale where, on a logarithmic scale, negative numbers indicate brighter stars than positive numbers. So the brightest star has a magnitude of -1.4 (minus 1.4) and the faintest visible star has a magnitude of 6.

Mean Sun Time--Reckoning of time by 24-hour days of nighttime and daytime without incorporating the sun's analemma and equation of time to determine the sun's meridian transit.

Meridian crossing--The moment when a celestial object crosses the observer's meridian and is thus at its highest altitude above the horizon. also called meridian transit, culmination, or southing.

Meridian--The imaginary north-south line that passes directly from the north celestial pole, overhead through the observer's zenith, and on to the south celestial pole.


North Pole of the sky--The north celestial pole, that is, the north pole of the celestial sphere, which is located directly above and out from the earth's north pole.

North pole--Also known as the Geographic North Pole or Terrestrial North Pole, it is the point in the Northern Hemisphere where the Earth's axis of rotation meets its surface. It is also the northernmost point of earth and where meridians of longitude all meet. It is not to be confused with the Magnetic North Pole, toward which a compass will point. In the Northern Rockies a compass will currently (2010) point from 8° to 13° east of the Geographic North Pole. See map of magnetic declination.


Obliquity--Also known as Axial Tilt, obliquity is the angle between the plane of a celestial body's equator and the plane of its orbit. The obliquity or axial tilt of the earth is about 23.5°.

Orbit--The path of a celestial body in its periodic revolution around another celestial body. Or the process of following that path. The earth and other planets orbit the sun. The earth's moon and its satellites orbit the earth.


Parallel of declination--A circle of the celestial sphere parallel to the celestial equator. A Parallel of declination is identified by its angular distance from the celestial equator and to its corresponding earthly latitude above or below the earth's equator.

Planet--A large celestial body in orbit around our Sun or other star. Our solar system has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Currently Pluto is considered a dwarf planet. For purposes of navigation, the most important planets are Venus and Jupiter, which are brighter than the stars.

Prime meridian--Meridian of longitude 0 degrees, running from north to south through Greenwich, England. The prime meridian is internationally recognized as the reference point for measurements of longitude. Longitude lines up to 180° west of the prime meridian are considered "west longitude." Longitude lines up to 180° east of the prime meridian are considered "east longitude." The 180° line of longitude is the International Dateline (IDL), which has been subjected, like other time zones, to deviations for the convenience of Standard Time.



Retrograde motion--Appearing to move backward. Moving the reverse of what is considered the "normal" direction. If seen from the north celestial pole most solar system objects revolve around the sun counter-clockwise and rotate counter-clockwise. So those that revolve or rotate clockwise could be considered to have a retrograde orbit or rotation respectively. The term retrograde is also used to describe a planet's apparent backtracking to an observer on earth when the earth passes the other planet in their orbital paths. Finally, to an observer on earth, celestial objects appear to move from the eastern half of the sky to the western half through the course of a night. But because the moon is also slowly orbiting the earth from west to east, that component of its position throughout the night could be considered apparently retrograde to its movement from eastward to westward as the earth rotates.

Right Ascension (R.A.)--The angle of celestial longitude measured in hours eastward of the meridian where the sun's path, the ecliptic, crosses the celestial equator during the spring equinox in March. There are 24 hours in a complete 360° circle so that each hour of right ascension is equal to 15° celestial longitude. Declination and Right Ascension are the two primary coordinates for describing the location of a celestial body independent of the location of the observer on earth.

Rotation--The turning of a body as upon an axis or center line. Celestial bodies rotate. The earth's rotation accounts for the appearance of sun, moon, stars and planets moving from the eastern half of the sky to the western half.


Satellite--A natural or artificial body in orbit around a planet.

Sidereal Lunar Month--In contrast to Lunation or Synodic Lunar Month, the sidereal month is the average period it takes the moon to revolve around the Earth in reference to a fixed stars and is equal to 27.322 days in units of mean solar time. The sidereal lunar month does not follow the phases of the moon, which determine the synodic lunar month.

Sidereal Time--Also called star time, it is time measured relative to the stars rather than to the sun or civil time zones.

Sidereal--Of, or with respect to, the stars. Sidereal rotation is that measured relative to the stars rather than to the Sun.

Solar coordinates--Most simply, the position of the sun in the sky. For our interests of navigation, that position may be pin-pointed by either of two grid systems: first, if the position of the observer on earth is not known, by celestial longitude (right ascension) and celestial latitude (declination). Secondly, if the position of the observer is known, by azimuth and altitude. The coordinates are expressed in degrees of angular distance.

Solstice--One of two times each year when the sun is either farthest north or farthest south of the celestial equator, thus creating the longest or shortest day of the year respectively. Called the summer and winter solstices, they mark the beginning of summer and winter and generally fall on June 20th and December 21st. The word solstice is formed from the Latin words for "sun stands still," implying that between times the sun is ascending and descending in it's yearly path along the ecliptic, there are moments at the solstices when the sun, in effect, stands still.

Southing--Also called culmination, meridian transit, or meridian crossing, southing is the point at which a celestial object is directly south of an observer and at the highest altitude it will be in that rotation of the earth. Southing happens at the moment when the celestial object crosses the meridian of longitude at which the observer is located.

Spring equinox--(See equinox.)

Star--A ball of hot gas that emits its own light through nuclear fusion. While stars are gigantic in size, they appear to the observer on earth as pinpoints of light of various intensity. The sun is technically a star, and in ancient times planets were spoken of as stars, but these pages will generally exclude such celestial objects as sun, moon, planets and comets from consideration as stars. Such objects will receive consideration specific to their apparent behavior from earth. Unlike those other bodies, practically speaking, the stars are treated as fixed in position in the celestial sphere.

Stellar--Of or relating to star or stars. Astral, sidereal.

Sun Time-- Reckoning of time by 24-hour days of nighttime and daytime cycles together with the sun's analemma and equation of time to determine the sun's meridian transit.

Summer solstice--(See solstice.)

Synodic lunar month--(See lunation.) Also called simply lunar month or lunation, it is the interval of a complete cycle of moon phases, in particular, from one new moon to the next. The lunation is equal on average to 29.531 days in units of mean solar time. This is not to be confused with the moon's sidereal month, which is measured relative to the stars rather than to phases of the moon.


Terrestrial sphere--The planet earth.

Terrestrial--Of or pertaining to the earth; existing or originating on earth; earthly. Especially used in these pages with reference to grid systems on earth, such as terrestrial latitude and longitude.

Transit--The crossing of an observer's meridian; southing. The term is also used to describe the passage of a celestial body across the disk of another celestial body.

Twilight--The transition time of diminished light just after sunset or just before sunrise.


Universal Time (UT or UTC)--UTC is THE official civil time standard accepted throughout the civilized world for over a century. Because the time standard is maintained by numerous minutely synchronized atomic clocks, it is also called Coordinated Universal Time. Since the time standard is set for the Prime Meridian of longitude at Greenwich, England, it has also been called Greenwich Mean Time (GMT) or Zulu Time. But UTC is a standard and not a time zone. Thus it is not subject to official adjustments such as Daylight Saving Time. In contrast, Greenwich Mean Time, corresponding to the time zone around Greenwich, may be subject to Daylight Saving adjustments, depending upon official authorities in jurisdictions where Greenwich Mean Time is used. The hours of different time zones may be adjusted, depending upon their distance from Greenwich, but the seconds and minutes of the hour continue to match UTC as the only official time of those jurisdictions. In North America, the official time, accurate to the second, may be gotten by satellite, radio transmission, or Internet sources at Mountain Time, used in the Rocky Mountains, is 7 hours west of the Prime Meridian. During Daylight Time, the Mountain Time Zone sets official time as if it were 6 hours west of the Prime Meridian. Thus, if it is 0:00 hours UTC, it is 17:00 hours Mountain Standard Time (MST) or 18:00 hours Mountain Daylight Time (MDT.) Astronomers use Universal Time to describe when celestial events happen in a way that is independent of an observer’s time zone.



Waning--The decreasing illumination of the moon or other celestial body from its full phase to its new phase. The moon wanes from full to new.

Waxing-- The increasing illumination of the moon or other celestial body from its new phase to its full phase. The moon waxes from new to full.

Winter solstice--(See solstice.)




Zenith--The point in the sky that’s directly above the observer.


Astronomy Notes Glossary
Babylon Dictionary Glossary
CalTech Glossary
Dictionary Glossary
PlanetFacts Glossary
SeaSky Glossary
Sky and Telescope Glossary
Sky at Night Glossary
Skynet Glossary
Wikipedia Glossary

Copyright © 2017 Travis N. Wood

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