Celestial Mechanics Definitions List

As seen from Earth stars and deep sky objects seems to surround us, as Sun, Moon, and planets seem to move on the background of them. To report about this, to locate and to observe, astronomers built concepts, grids, and tools. Sky is mainly seen as a celestial sphere on which stars and deep sky objects are fictitiously seen affixed. This sphere has appropriate coordinate systems. Sun, Moon, and planets are journeying along this sphere, among stars. Here is a basic definitions list about this. Definitions are in alphabetical order

Celestial Meridian

Imaginary circle passing just overhead the observer. Starting and ending points are depending on the observer's location at Earth. In the northern hemisphere the celestial meridian begins on the northern horizon, passes through the north celestial pole, then overhead the observer (or zenith). The celestial meridian ends at the southern horizon. Things are reverted in the southern hemisphere. In the equatorial regions, the celestial meridian starts and ends at the northern and southern horizon respectively that is at either celestial poles. The celestial meridian defines where celestial objects are reaching their highest point in the sky relative to observer. Due to Earth's rotation, any celestial object is seen rising on the eastern horizon, reaching observer's meridian, and setting on the western horizon. Due to Earth's motion around the Sun, celestial objects are passing at observer's meridian 4 mn earlier each night, i.e. 2 hours earlier each month (this means that each day, Earth is progressing by 4 minutes of arc on its orbit, and 30° each month)

Celestial Sphere

The celestial sphere is the theoretical sphere on which stars seem affixed and onto which Sun, Moon, and planets' motions seem to take place. The celestial sphere somehow is the terrestrial globe's projection onto the sky. Hence it has a north and a south poles (north and south celestial poles are terrestrial north and south poles' continuation) and an equator (which is the terrestrial equator's projection). For the purpose of locating celestial objects on the celestial sphere the latter is subject to various coordinate systems. Most used by amateur astronomers is the "equatorial coordinate system" (see at Coordinate systems)

Conjunction

Particular position of a planet on its orbit, relative to Earth. Superior planets (that is planets beyond Earth) are at conjunction when they are on a line Earth-Sun, on the other side of the Sun relative to Earth. Inferior planets (that is planets orbiting between the Sun and the Earth) have two kind of conjunctions, inferior and superior. An inferior planet is at inferior conjunction when it's located on a line Sun-Earth, just between Sun and Earth; it's at superior conjunction when it's on a line Earth-Sun, on the other side of the Sun relative to Earth. Conjunctions are part of the yearly positions of planets relative to Earth. for more about such positions, see the tutorial "Planets Apparent Motion"

Coordinate Systems

For the purpose of locating celestial objects on the celestial sphere astronomers has deviced diverse spherical coordinate systems. These systems are mostly similar to Earth's latitude and longitude system. All of them have a "main axis", two poles, and a "main reference circle". Main axis, or polar axis, is the system's rotational axis. Two poles are where main axis intersects sphere. Main reference circle -or "great circle"- is the system's equator. It is perpendicular to the main axis. Equivalent of latitudes are measured each side of main reference circle. Equivalents of longitude are measured starting at a point defined on the main reference circle. Secondary circles (they are also said "main circles" too) -the first one beginning at this point of origin- are passing at system's both poles and are used to define objects longitude. For professional astronomical purposes, there are four coordinates systems: horizontal, equatorial, ecliptical, galactical. Amateurs mainly use the equatorial system and instinctively use the horizontal one. Ecliptical and galactical systems' main axis are ecliptic or galactic (Milky Way) north-south pole axis; their main reference circle are ecliptic or the Galaxy's plane respectively. Both have a coordinates units system: a 0°-360° system measuring "celestial longitudes" and "galactic longitudes"; a +90° / 0° / -90° system measuring "celestial latitudes" and "galactic latitudes"

Ecliptic

The ecliptic is Earth's orbit's plane. Projected on the celestial sphere, the ecliptic is a line along which Sun, Moon, and planets' apparent motions seem to occur. See too "Zodiac"

Celestial equator (white line) is Earth's equator projected on celestial sphere. Ecliptic is projected Earth's orbit plane (orange line). Zodiac (blue) is due to planets' orbits inclination relative to ecliptic yielding their apparent motion to vary by 8° either side of the latter. North is up

Equator (or Celestial Equator)

It's the projection of the terrestrial equator on the celestial sphere. Celestial equator is intersecting the ecliptic at two points. These points are named the vernal and the autumnal equinoxes (about equinoxes, see the tutorial "Seasons"; about the vernal equinox see below)

Equator (Terrestrial Equator)

On Earth, the equator is the line of latitude 0° which delineates the northern and southern hemispheres. At the vernal and autumnal astronomical equinoxes (that is mostly on the days of spring and of fall), Sun, at the equator, is directly overhead at noon

Equatorial Coordinate System

The equatorial coordinate system is one of the coordinates systems used about the celestial sphere. Simply said this system is a projection of Earth's coordinates on the celestial sphere. The main axis (or polar axis) of the equatorial system is the celestial sphere' axis. Hence its north and south poles are the north and south celestial poles. Its main reference circle is the celestial equator that is the terrestrial equator's projection on the celestial sphere. "Longitudes" of celestial objects are measured on the celestial equator, starting at the vernal equinox. The vernal equinox is the point where Sun is at the spring equinox (more simply on spring's day). Terms "right ascension" are used for celestial objects instead of "longitude". Right ascension is abbreviated "R.A.". Right ascension is measured eastwards using a time units system. Equator is divised in hour, minutes and seconds. This system appeared natural as celestial sphere' apparent rotation is taking one day. They are 24 R.A. circles (or "hour circles"), passing at the system's poles. First circle originates at vernal equinox. Each one R.A. hour is divided into 60 minutes. Each minute is divided into 60 seconds. Each R.A. hour equals 15° as the celestial sphere equals 360°. Each R.A. minute is 4 arcminutes. On the other hand, the "latitudes" of celestial objects are measured each side of the celestial equator, north, or south, using a +90° / 0° / -90° system. "Declination" is used instead of "latitude". Declination is abbreviated "dec.". The equatorial coordinate system is affixed to the celestial sphere. It rotates with it. Due to the precession of the equinoxes, Earth's equinoxes are slowly shifting. As vernal equinox is part of them and as it is the point of origin of the right ascension system the catalogs of stars have to be updated periodically. Shift is about 50" per year. Catalogs are modified about each 50 years. One consequence of this is that the celestial objects' equatorial coordinates are given for such or such vernal equinox position. Present reference is termed "J2000.0". Equatorial coordinate system used by professionals is technically a dynamical system based on the definition of its equator and its point of origin by reference to Earth's motion, complemented with a set of 1,535 bright stars (this set is said "Fifth Fundamental Catalogue (FK5)"). This system is in way of being improved at the International Astronomical Union (IAU) by a new one, the "International Celestial Reference System" (ICRS). ICRS is aiming at being independent from any Earth-linked dynamical reference. ICRS defines equator and equinox in reference to very accurate positions of 608 extragalactic radiosources. A subset of the Hipparcos Catalogue (said the Hipparcos Celestial Reference Frame, HCRF) serves as the FK5 of the new system. Due to how modifications of the axis of Earth's rotation (precession-nutation) are to be computed is still unclear, UAI's ICRS has not yet been implemented as of late 2002. ICRS, for accuracy requirements of no more than 0.1 arcseconds near epoch J2000.0 will be totally transparent with J2000.0-FK5 dynamical equator and equinox system

Horizon

The horizon is the line which, for a given location, is dividing the celestial sphere into a visible and an invisible celestial hemisphere

Horizontal Coordinate System

The horizontal coordinate system is one of the coordinates systems used about the celestial sphere. Simply said, the equator of this system is the observer's horizon. Main axis (polar axis) of the horizontal system is the observer's zenith-nadir axis. Main reference circle is the observer's horizon. It is called the "astronomical horizon". It features four main points: North, East, South, West. "Longitudes" are measured on the main reference circle, clockwise, starting North (which is the system's point of origin), using an angular units system (0-360°). "Azimuth" is used instead of "longitude". North is at 0°, East at 90°, South at 180°, West at 270°. Secondary circles (which are said "vertical circles") -first of them being located North- are passing by both poles and are helping defining objects' azimuths. "Latitudes" are measured either side of astronomical horizon in a +90° / 0° / -90° system. Latitude of the objects is termed "altitude". This system is fixed relative to the observer only which means that the celestial sphere is seen rotating beyond the horizontal system coordinates grid. When professionally used such a system needs an additional datum to be given: the time at the moment of the observation. The exact observer's location terrestrial coordinates are mandatory too. Amateur astronomers instinctively use horizontal system when they measure angles in the sky, e.g. when they try to figure out how high a celestial body like a planet or the Moon is above the horizon

Measurement of a Circle

A circle can be divided into 360 degrees of arc (°), each degree being divided into 60 arcminutes ('), each minutes being divided into 60 arcseconds of arc ("). The main reference circle of a coordinate system is such a circle

Nadir

It's the point of celestial sphere which at the opposite of the zenith, relative to the observer (that is the point beneath observer's feet, extending at the infinity)

Opposition

Particular position of a superior planet on its orbit, relative to Earth. Superior planets are planets which are located beyond Earth. Such a planet is at opposition when it's on a line Sun-Earth, beyond Earth. It's the moment when a superior planet is at its best visible for observers, rising at sunset and setting at sunrise. for more about such planets configurations, see the tutorial "Planets Apparent Motion"

Precession of the Equinoxes

The Earth's polar axis -the line which joins Earth's north and south pole- is tilted relative to Earth's orbit's plane (the ecliptic). This tilt (or inclination) is 23.5°. Due to the Sun and Moon's combined gravitational effects Earth polar axis is slowly moving clockwise, the way a spinning top's axis is slowly moving when the top is spinning. Such a motion is called the "precession of the equinoxes". The vernal and the autumnal equinoxes (the points where the ecliptic intersects the celestial equator) are slowly shifting westward each year due to the precession of the equinoxes. The shift is about 50" a year. It takes 26,000 years for the Earth's axis to complete a complete circle. This movement is explaining why Polaris has not always been the polar star nor will it always be. Nowaday, the Earth polar axis, projected on the celestial sphere, is pointing to the Polaris. 12,000 years ago it was to Vega. 5,000 years ago it was to a of the Dragon. Precession is inducing a change in celestial objects' equatorial coordinates, hence the equatorial coordinate system has to be re-defined about each 50 years

Tropics

Tropics are the lines of terrestrial latitude 23.5° North and South. Northern tropic is called the tropic of the Cancer, southern is called the tropic of the Capricorn. Tropics are defining an astronomical position, an astronomical event, and a climate zone. First, tropics are the northern and southernmost points where Sun is appearing directly overhead at least one day a year -which is occurring on the solstice day at noon. On this day only, Sun is overhead an observer there. Tropics are an astronomical event too. When Sun at the tropic is overhead an observer, it is summer solstice there, whatever the hemisphere (about solstices, see the tutorial "Seasons"). At last, this height of the Sun is defining a climate zone. This zone is termed "tropics". It is a warm climate zone either side of the terrestrial equator where Sun is always high in sky. Anecdotically, the polar circles -the Arctic Polar Circle, the Antarctic Polar circle- are located at a latitude, North or South, respectively, of 66° 33' 39". Those circles of latitude are defining beyond where the midnight Sun is seen during summer, and the arctic (or the antarctic) night in winter

Vernal Equinox

It's the point where the celestial equator is intersecting the ecliptic. The ecliptic is the plane of Earth's orbit. It appears like a line, on the celestial sphere. The Sun, Moon, and planets' apparent motion on the celestial sphere is about that line. The vernal equinox is where Sun is about March, 21st i.e. on spring day. It's also the point of origin of the right ascension in the equatorial coordinate system applied to the celestial sphere. The vernal equinox has the autumnal equinox as its counterpart. Both equinoxes are too part of the phenomenon of the seasons (see a tutorial about seasons)

As seen from Earth -which is orbiting from left to right- ecliptic (yellow line) and celestial equator (blue) have variable positions relative to each other along year. North is up

Zenith

The zenith is the point of the celestial sphere which is directly situated overhead the observer. The opposite point is the "nadir"

Zodiac

The zodiac is a 16° wide belt which marks the northern- and southernmost boundaries (8° either side of the ecliptic) where Sun, Moon, and planets' apparent paths are seen taking place. The zodiac is theoretically divided into 12 sections, each 30° wide. The zodiac originated for astrological purposes. These 30° wide sections are the zodiacal signs. These astrological signs were originally termed from the constellation found in each section. Astrological signs and constellations do not coincide anymore now, due to the precession of the equinoxes. They will again 24,000 years from now. The constellations which gave their names to the astrological signs of the zodiac are always found along the zodiac however. The 12 zodiacal constellations are, in this order: Aries, the Ram; Taurus, the Bull; Gemini, the Twins; Cancer, the Crab; Leo, the Lion; Virgo, the Virgin; Libra, the Scales; Scorpius, the Scorpion; Sagittarius, the Archer; Capricornus, the Goat; Aquarius, the Water Bearer; Pisces, the Fishes. Aries, the Ram is the first zodiacal constellation as it was there that the vernal equinox was when the zodiac was devised