Astronomy Terms and DefinitionsEarthOppositionWestern QuadratureEastern QuadratureInferior ConjunctionInner planets orbitEarths orbitOuter planets orbitConjunctionGreatest western elongationGreatest eastern elongationSunEASTWESTSuperior Conjunction

North Celestial PoleCelestial SphereThe Celestial Sphere is an imaginary sphere of infinite radius centred on the Earth, on which all celestial bodies are assumed to be projected. South Celestial PoleCelestial Equator

EclipticCelestial EquatorThe EclipticWhile in reality the Earth orbits around the Sun, it seems from the Earth that the Sun moves through the sky. The Ecliptic is the path the Sun traces through the sky during one year.

Vernal and Autumnal EquinoxCelestial Equator2004060- 60- 40- 2080- 80The EclipticThe Vernal Equinox is the point where the Sun crosses the Celestial Equator, moving from south to north. This occurs in March and marks the first day of spring in the northern hemisphere . The Autumnal Equinox occurs in September, when the Sun enters the Southern hemisphere.Vernal Equinox

Right Ascension0 hr2 hr1 hrRA is the celestial equivalent of terrestrial longitude. For RA, the zero point is known as the First Point of Aries, which is the place in the sky where the Sun crosses the celestial equator at the Vernal Equinox.RA is measured eastward from the equinox in hours, minutes, and seconds, with 24 hours being equivalent to a full circle. East

DeclinationCelestial EquatorDeclination is one of the two coordinates of the equatorial coordinate system, the other being Right Ascension. Dec is comparable to latitude, projected onto the celestial sphere, and is measured in degrees north and south of the celestial equator. Points north of the celestial equator have positive declinations, while those to the south have negative declinations.

Summer and Winter SolsticeCelestial Equator2004060- 60- 40- 2080- 80The Winter SolsticeThe Summer Solstice is the time in June when the Sun is over the Tropic of Cancer. At the winter Solstace in December, the sun is over the Tropic of Capricorn.The Summer Solstice

On the day of the solstice the Sun appears to have reached its highest (in summer) or lowest (in winter) annual altitude in the sky above the horizon at local solar noon.The day of the solstice is either the longest or shortest day of the year for any place outside of the tropics.Longest and shortest days

Zodiac The Zodiac denotes an annual cycle of twelve stations along the ecliptic, the apparent path of the sun across the heavens through the constellations that divide the ecliptic into twelve equal zones of celestial longitude EclipticCelestial EquatorZodiacalConstellations

Precession Precession refers to a change in the direction of the Earths axis, and its effects on astronomical observation. Because of the precession of the equinoxes, the vernal equinox moves through all the constellations of the Zodiac over the 26,000 year precession period. Presently the vernal equinox is in the constellation Pisces and is slowly approaching Aquarius. The North star Polaris is presently overhead the North Pole

Zenith The zenith is the direction pointing directly above a particular location . It is the highest point reached by a celestial body during its apparent orbit around a given point of observation.ZenithHorizonNadirThe Nadir is the point directly below the observerNadir

Meridian In the sky, a meridian is an imaginary great circle on the celestial sphere. It passes through the north point on the horizon, through the celestial pole, up to the Zenith directly overhead, through the south point on the horizon, and through the Nadir, and is perpendicular to the local horizon.Theupper meridianis the half above the horizon, thelower meridianis the half below it. ZenithNorthMeridianSouth

RegulusOccultation of Saturn and RegulusOccultationAn Occultation occurs when one moving celestial object moves in front of another

Circumpolar Stars PolarisNon Circumpolar StarCircumpolar StarHorizonA circumpolar star is a star that, as viewed from a given latitude on Earth, never sets below the horizon, due to its proximity to one of the celestial poles. Circumpolar stars are therefore visible from that location for the entire night on every night of the year

PolarisCulmination Upper CulminationLower CulminationMeridianCulmination is the time when a star or planet appears on an observer's meridian.During a sidereal day, an astronomical object will cross the meridian twice: once at its upper culmination, when it is at its highest point as seen from the earth, and once at its lower culmination, its lowest point. Often, culmination is used to mean upper culmination

Ellipse An ellipse is a flattened circle, obtained as the intersection of a cone with a plane. All planets, moons and comets move in elliptical orbits.

Eccentricity The shape of an ellipse can be expressed by a number called the eccentricity of the ellipse. The eccentricity is a number between 1 and 0, and determines how flattened the ellipse is. A value of 0 gives a perfect circle. Eccentricity = 0 (Circle)Eccentricity = 0.5Eccentricity = 0.9

Aphelion and Perihelion AphelionThe point on its orbit when the Earth is farthest from the sun.Occurs in JulyPerihelion

The point on its orbit when the Earth is closest to the sun,

Occurs in January

The Earths elliptical orbit around the Sun

Apogee and PerigeeApogeePerigeePerigeeis the closest point to the earth and it is in this stage that the moon appears larger.The Moon orbits the earth in am elliptical orbit. Apogee is the point when it is most distant from the Earth

ConjunctionEarthSunMars and Venus at conjunctionA conjunction occurs at the instant when the two bodies have the sameright ascension. In other words, they are crossing the same meridian and appear close together.Venus

JupiterVenusAppulse refers to the closest approach of two planets together in the sky, or of the Moon to a star orplanetas seen by an observer located on Earth.An appulse usually occurs around the time the planets are in conjunction.Appulse

EarthVenus at Superior ConjunctionSunSuperior ConjunctionA Superior Conjunction occurs when the Earth and the planet are on opposite sides of the Sun.

EarthSunVenus at inferior conjunctionInferior ConjunctionAn Inferior Conjunction occurs when the Earth and the planet are on the same side of the Sun. This occurs only with the interior planets, Mercury and Venus

EarthVenus and Mars at oppositionSunOppositionOpposition is a term used to indicate when one celestial body is on the opposite side of the sky when viewed from a particular place. Two planets are in opposition to each other when their ecliptic longitudes differ by 180.Venus

EarthMars at oppositionSunPlanets in OppositionA planet is said to be "in opposition" when it is in opposition to theSunas seen from the Earth. This occurs only insuperior planets, and is the best time to observe them.

At this point of itsorbitit is roughlyclosest to the Earth, making it appear bigger and brighter. The half of the planet visible from Earth is completely illuminated.

EarthVenus at maximum elongationSunMaximum ElongationeElongation is an astronomical term that refers to the angle between the Sun and a planet, as viewed from Earth.When an inferior planet is visible after sunset, it is near its greatest eastern elongation. When an inferior planet is visible before sunrise, it is near its greatest western elongation. The value of the greatest elongation for Mercury is 28; and for Venus 47. This value varies because the orbits of the planets are elliptical, rather than perfect circles.

A transit occurs when an apparently smaller body passes in front of an apparently larger one. Transits and OccultationsAn occultation occurs when an apparently larger body passes in front of an apparently smaller one In the combined case where the smaller body regularlytransitsthe larger object, an occultation is also termed a secondary eclipse.

EclipsesAn eclipse occurs when a body disappears or partially disappears from view, either by an occultation, as with asolar eclipse, or by passing into the shadow of another body, as with a lunar eclipse Solar eclipseLunar eclipse

Distant star overheadEarth points towards a distant star overheadSidereal DayEarth points towards same distant star overheadA Sidereal Day is the length of time which passes between a given "fixed" star in the sky crossing the meridian

1 Solar Day = 24 hrsEarth points towards Sun overheadEarth points towards Sun overhead againSolar DayThe length of time which passes between the Sun reaching its highest point in the sky two consecutive times

North Celestial PoleRotational AxisPerpendicular to OrbitAngle of TiltCelestial EquatorOrbit DirectionEclipticObliquityObliquity or axial tilt is the inclination angle of a planet's rotational axis in relation to its orbital plane . The Earth has an axial tilt of about 23.44 (23 26).

The axis is tilted in the same direction throughout a year; however, as the Earth orbits the Sun, the hemisphere tilted away from the Sun will gradually become tilted towards the Sun, and vice versa. This effect is the main cause of the seasons.SeasonsJuneSummer in the Northern hemisphereDecemberSummer in the Southern hemisphere

InclinationEclipticThe inclination of the orbit of aplanetis the angle between its orbital plane and the orbital plane of the Earth.

The equation of time is the difference, over the course of a year, between time as read from a sundial and a clock in an ideal situation.The sundial can be ahead (fast) by as much as 16 min 33 s (around November 3) or fall behind by as much as 14 min 6 s (around February 12). It is caused by irregularity in the path of the Sun across the sky, due to a combination of the obliquity of the Earth's rotation axis and the eccentricity of its orbit.The Equation of TimeSun Dial

If you are not confused then you were not paying attention !!!The End

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