Glossary

Altitude:

The angle subtended at the observer by the radius vector connecting a celestial object to an observer on the earth's surface, and the vector's projection onto the horizontal plane. Object's above/below the horizon have positive/negative altitudes.

Altitude Circle:

A great circle on the celestial sphere which passes through the local zenith at a given observation site on the earth's surface.

Anomaly:

Any deviation in an orbit from uniform circular motion which is concentric with the central body. Anomaly is also used as another word for angle.

Apocenter:

Point on a Keplerian orbit which is furthest from the central body. If the central body is the sun, then the apocenter is generally termed the aphelion. Likewise, if the central body is the earth, then the apocenter is termed the apogee.

Arctic Circles:

Two latitude circles on the earth's surface which are equidistant from the equator. Above the arctic circles, the sun never sets for part of the year, and never rises for part of the year.

Argument of Latitude:

Angle subtended at the central body by the radius vectors connecting the central body to the orbiting body, and the central body to the ascending node, in a Keplerian orbit.

Ascendent:

Point on ecliptic circle which is ascending at any given time on the eastern horizon.

Ascending Node:

Point on a Keplerian orbit at which the orbital plane crosses the ecliptic plane from south to north in the direction of motion of the orbiting body.

Autumnal Equinox:

The point at which the ecliptic circle crosses the celestial equator from north to south (in the direction of the sun's apparent motion along the ecliptic).

Azimuth:

Angle subtended at the observer by the projection of the vector connecting a celestial object to an observer on the earth's surface onto the horizontal plane, and the vector connecting the north compass point to the observer. Azimuth increases clockwise (i.e., from the north to the east) looking at the horizontal plane from above.

Celestial Axis:

An imaginary extension of the earth's axis of rotation which pierces the celestial sphere at the two celestial poles. The sphere's diurnal motion is about this axis.

Celestial Coordinates:

Angular coordinate system whose fundamental plane is the celestial plane, and whose poles are the celestial poles. The polar and azimuthal angles in this system are called declination and right ascension, respectively.

Celestial Equator:

The intersection of the imagined extension of the earth's equatorial plane with the celestial sphere.

Celestial Plane:

The plane containing the earth's equator.

Celestial Poles:

The two points at which the celestial axis pierces the celestial sphere. The north celestial pole lies to the north of the celestial plane, whereas the south celestial pole lies to the south. The celestial poles are the only two points on the celestial sphere whose positions are unaffected by diurnal motion.

Celestial Sphere:

An imaginary sphere of infinite radius which is concentric with the earth. All objects in the sky are thought of as attached to this sphere.

Compass Points:

At a given observation site on the earth's surface, the north, east, south, and west compass points lie on the local horizon due north, east, south, and west, respectively, of the observer.

Conjunction:

Two celestial objects are said to be in conjunction when they have the same ecliptic longitude. For an inferior planet in conjunction with the sun, the conjunction is said to be superior if the planet is further from the earth than the sun, and inferior if the sun is further from the earth than the planet.

Culmination:

A celestial object is said to culminate on a given day when it attains its maximum altitude in the sky.

Declination:

Angle subtended at the earth's center by the radius vector connecting a celestial object to the earth's center, and the vector's projection onto the celestial plane. Object's to the north/south of the celestial equator have positive/negative declinations.

Deferent:

Large circle centered on the sun about which the guide point rotates in a geocentric planetary orbit.

Deferential Latitude:

Ecliptic latitude a superior planet has by virtue of the inclination of its deferent.

Deferential Latitude Correction Factor:

Correction to the ecliptic latitude of an inferior planet due to the finite size of its deferent.

Diurnal Motion:

Daily rotation of the celestial sphere, and the objects attached to it, from east to west (looking south in the earth's northern hemisphere) about the celestial axis.

Eccentricity:

Measure of the displacement along the major axis of the central body from the geometric center in a Keplerian orbit.

Ecliptic Axis:

Normal to the ecliptic plane which passes through the center of the earth.

Ecliptic Circle:

Apparent path traced out by the sun on the celestial sphere during the course of a year.

Ecliptic Coordinates:

Angular coordinate system whose fundamental plane is the ecliptic plane, and whose poles are the ecliptic poles.

Ecliptic Latitude:

Angle subtended at the earth's center by the radius vector connecting a celestial object to the earth's center, and the vector's projection onto the ecliptic plane. Objects to the north/south of the ecliptic circle have positive/ecliptic latitudes.

Ecliptic Longitude:

Angle subtended at the earth's center by the projection of the vector connecting a celestial object to the earth's center onto the ecliptic plane, and the vector connecting the vernal equinox to the earth's center. Ecliptic longitude increases counter-clockwise (i.e., from the west to the east) looking at the ecliptic plane from the north.

Ecliptic Plane:

Plane containing the mean orbit of the earth about the sun.

Ecliptic Poles:

The two points at which the ecliptic axis pierces the celestial sphere. The north ecliptic pole lies to the north of the ecliptic plane, whereas the south ecliptic pole lies to the south.

Elongation:

Difference in ecliptic longitude between two celestial objects.

Epicycle:

Small circle, centered on the guide point, about which a planet rotates in a geocentric planetary orbit.

Epicyclic Anomaly:

Angle subtended between the radius vectors connecting the earth to the guide-point, and the guide-point to the planet, in a geocentric planetary orbit.

Epicyclic Latitude:

Ecliptic latitude an inferior planet has by virtue of the inclination of its epicycle.

Epicyclic Latitude Correction Factor:

Correction to the ecliptic latitude of a superior planet due to the finite size of its epicycle.

Epoch:

Standard time at which the orbital elements of an orbiting body in the solar system are specified.

Equant:

Point about which the orbiting body appears to rotate uniformly in a Keplerian orbit of low eccentricity. The equant is diagrammatically opposite the central body with respect to the geometric center of the orbit.

Equation of Center:

Difference between the true anomaly and the mean anomaly in a Keplerian orbit.

Equation of Epicycle:

Elongation of a planet from its guide-point in a geocentric planetary orbit.

Equation of Time:

Time interval between local noon and mean local noon.

Equinoxes:

The two opposite points on the ecliptic circle which the sun reaches on the days of the year that day and night are equally long.

Evection:

An anomaly of the moon's orbit about the earth which is associated with the perturbing influence of the sun.

Geocentric Planetary Orbit:

An orbit in which a planet rotates about a guide point in a small circle called an epicycle, and the guide point rotates about the earth in a large circle called a deferent.

Great Circle:

Circle on the surface of a sphere produced by the intersection of a plane which bisects the sphere.

Greatest Elongation:

Greatest elongation of an inferior planet from the sun. If the planet is to the east/west of the sun then the elongation is called the greatest eastern/western elongation.

Guide-Point:

Center of an epicycle in a geocentric planetary orbit.

Horizon:

Tangent plane to the earth's surface, at a given observation site, which divides the celestial sphere into visible and invisible hemispheres.

Horizontal Coordinates:

Angular coordinate system whose fundamental plane is the horizontal plane, and whose poles are the zenith and nadir.

Horizontal Plane:

Plane containing the local horizon.

Horoscope:

Point on the ecliptic circle which is ascending at a given time on the eastern horizon.

Inclination:

Maximum angle subtended between the plane of a Keplerian orbit and the ecliptic plane.

Inclination of Ecliptic:

Inclination of the ecliptic plane to the equatorial plane.

Inferior Planet:

A planet which is closer to the sun than the earth.

Julian Day Number:

Number ascribed to a particular day in a scheme in which days are numbered consecutively from January 1, 4713 BCE, which is designated day zero. Julian days start at 12:00 UT.

Keplerian Orbit:

Ellipse which is confocal with the central object. The radius vector connecting the central and orbiting bodies sweeps out equal areas in equal time intervals.

Local Mean Noon:

Instant in time at which the mean sun attains its upper transit.

Local Noon:

Instant in time at which the sun attains its upper transit.

Longitude of Ascending Node:

Angle subtended at the central body by the radius vectors connecting the central body to the ascending node, and the central body to the vernal equinox, in a Keplerian orbit.

Longitude of Pericenter:

Angle subtended at the central body by the radius vectors connecting the central body to the pericenter, and the central body to the vernal equinox, in a Keplerian orbit.

Major Axis:

Longest diameter which passes through the geometric center of a Keplerian orbit.

Major Radius:

Half the length of the major axis of a Keplerian orbit.

Mean Anomaly:

Angle which would be subtended at the central body by the radius vectors connecting the central body to the orbiting body, and the central body to the pericenter, in a Keplerian orbit, if the orbiting body were to rotate about the central body with a uniform angular velocity.

Mean Argument of Latitude:

Value the argument of latitude would have if the orbiting body in a Keplerian orbit were to rotate about the central body at a fixed angular velocity.

Mean Argument of Latitude at Epoch:

Value of the mean argument of latitude of a Keplerian orbit at the epoch.

Mean (Ecliptic) Longitude:

Value the ecliptic longitude would have if the orbiting body in a Keplerian orbit were to rotate about the central body at a fixed angular velocity.

Mean (Ecliptic) Longitude at Epoch:

Value of the mean longitude of a Keplerian orbit at the epoch.

Mean Solar Day:

Time interval between successive local mean noons.

Mean Solar Time:

Time calculated using the mean sun.

Mean Sun:

Fictitious body which travels around the celestial equator (from west to east looking south in the earth's northern hemisphere) at a uniform rate, and completes one orbit every tropical year.

Meridian Plane:

Plane passing through the zenith and the north and south compass points at a given observation site on the earth's surface.

Minor Axis:

The minor axis of a Keplerian orbit is the shortest diameter which passes through the geometric center.

Minor Radius:

The minor radius of a Keplerian orbit is half the length of the minor axis.

Nadir:

Point on the celestial sphere which is directly underfoot at a given observation site on the earth's surface.

Opposition:

Two celestial objects are said to be in opposition when their ecliptic longitudes differ by $180^\circ$.

Orbital Elements:

Eight quantities which completely specify a Keplerian orbit: i.e., major radius, eccentricity, rate of motion of mean longitude, rate of motion of mean anomaly, mean longitude at epoch, mean anomaly at epoch, inclination, rate of motion in mean argument of latitude, mean argument of latitude at epoch.

Parallactic Angle:

Angle subtended between the ecliptic circle and an altitude circle.

Parallax:

Apparent change in position of a nearby celestial object in the sky when it is viewed at different points on the earth's surface.

Pericenter:

Point on a Keplerian orbit which is closest to the central body. If the central body is the sun, then the pericenter is generally termed the perihelion. Likewise, if the central body is the earth, then the pericenter is termed the perigee.

Precession of Equinoxes:

A slow movement of the vernal equinox relative to the fixed stars which causes the ecliptic longitude of a fixed star to increase steadily at the rate of $50.3''$ per year.

Prograde Motion:

Motion of a superior planet in the sky in the same direction to that of its mean motion.

Radial Anomaly:

Difference between the length of the radius vector connecting the central body to the orbiting body, in a Keplerian orbit, and the major radius.

Rate of Motion in Mean Anomaly:

Time derivative of the mean anomaly of a Keplerian orbit.

Rate of Motion in Mean Argument of Latitude:

Time derivative of the mean argument of latitude of a Keplerian orbit.

Rate of Motion in Mean Longitude:

Time derivative of the mean longitude of a Keplerian orbit.

Retrograde Motion:

Motion of a superior planet in the sky in the opposite direction to that of its mean motion.

Right Ascension:

Angle subtended at the earth's center by the projection of the vector connecting a celestial body to the earth's center onto the celestial plane, and the vector connecting the vernal equinox to the earth's center. Right ascension increases counter-clockwise (i.e., from the west to the east) looking at the celestial plane from the north.

Seasons:

Spring is the time interval between the vernal equinox and the summer solstice, summer the interval between the summer solstice and the autumnal equinox, autumn the interval between the autumnal equinox and the winter solstice, and winter the interval between the winter solstice and the next spring equinox.

Sidereal Day:

Time interval between successive upper transits of a fixed star.

Sidereal Time:

Time calculated using the fixed stars.

Solar Day:

Time interval between successive local noons.

Solar Time:

Time calculated using the sun.

Solstices:

The two opposite points on the ecliptic circle which the sun reaches on the longest and shortest days of the year.

Station:

Point in the orbit of a superior planet at which it switches from prograde to retrograde motion, or vice versa. The former station is called a retrograde station, whereas the latter is called a prograde station.

Summer Solstice:

Most northerly point on the ecliptic circle.

Superior Planet:

A planet further from the sun than the earth.

Synodic Month:

Mean time interval between successive new moons.

Syzygy:

Conjunction or opposition of the sun and the moon.

Transit:

On a given day, and at a given observation site on the earth's surface, a celestial object is said to transit when it crosses the meridian plane. The object simultaneously attains either its highest or lowest altitude in the sky. The transit is called an upper/lower transit when the object attains its highest/lowest altitude.

True Anomaly:

Angle subtended at the central body by the radius vectors connecting the central body to the orbiting body, and the central body to the pericenter, in a Keplerian orbit.

Tropical Year:

Time interval between successive vernal equinoxes.

Tropics:

Two latitude circles on the earth's surface which are equidistant from the equator. Between the tropics the sun culminates both to the north and south of the zenith during the course of a year. Outside the tropics, the sun culminates either only to the north or only to the south of the zenith.

Universal Time:

Time defined such that mean local noon coincides with 12:00 UT every day at an observation site of terrestrial longitude $0^\circ$.

Vernal Equinox:

Point at which the ecliptic circle crosses the celestial equator from south to north (in the direction of the sun's apparent motion along the ecliptic).

Winter Solstice:

Most southerly point on the ecliptic circle.

Zenith:

Point on the celestial sphere which is directly overhead at a given observation site on the earth's surface.

Zodiac:

The signs of the zodiac are conventional names given to $30^\circ$ segments of the ecliptic circle.