Astronomical Geodesy

The branch of geodesy that uses observations of celestial bodies to determine Earth positions, orientation, and motion

Latitude and longitude

By measuring the transit of stars to establish orientation

Universal Time (UT)

The difference between the direction of gravity (plumb line) and the normal to the ellipsoid, measured by star observations

Sextants, theodolites, and zenith telescopes

Very Long Baseline Interferometry (VLBI)

Geodetic control networks, navigation, timekeeping, linking coordinate systems, and studying Earth’s rotation

The scientific study of the universe beyond the Earth, including celestial bodies’ positions, motion, composition, and evolution

The art and science of determining Earth positions and azimuths by astronomical observations

To understand celestial coordinate systems, time concepts, tidal studies, orientation of terrestrial networks, geoid determination, and boundary demarcation

Stars, Sun, Planets

Astronomic latitude (φ), astronomic longitude (λ), and orthometric height (H)

Angle between the gravity vertical (plumb line) and the equatorial plane

Angle between the Greenwich mean astronomic meridian and the local astronomic meridian

Height of a point above the geoid, measured along the plumb line

The angle between the astronomic normal and the ellipsoid normal at a point

An imaginary sphere with Earth (or the observer) at its center and stars considered infinitely distant on its surface

Point directly above the observer on the celestial sphere

Point directly below the observer on the celestial sphere

Intersections of Earth’s axis with the celestial sphere (North & South)

Vertical circle through the celestial poles

Vertical circle perpendicular to the celestial meridian

Great circle perpendicular to both the celestial meridian and the prime vertical

Altazimuth system

The horizon

Vertical circles

Angle from the horizon up to a celestial body

Complement of altitude (90° – altitude)

Angular distance along the horizon from the meridian to the vertical circle through a body

A small circle of constant altitude parallel to the horizon

Right ascension (RA) system

Celestial equator (primary), Equinoctial colure (secondary)

Angle north or south of the celestial equator to a star

Complement of declination (90° – δ)

Angle measured counterclockwise along the celestial equator from the First Point of Aries to the star’s hour circle

Celestial equator

The apparent annual path of the Sun around the celestial sphere

About 23.5°

The Sun crosses the equator (days and nights equal length, ~March 21)

The Sun’s declination reaches maximum or minimum; rate of change is zero (~June 21 & December 21)

Celestial equator (primary) and the hour circle through the zenith (secondary)

Angle measured westward along the equator from the meridian to the hour circle of the celestial body

Measured from observer’s meridian to the body’s meridian

Measured from the Greenwich meridian to the body’s meridian

Measured from the vernal equinox to the body’s meridian

Slow change in Earth’s axis orientation due to gravitational pull of Sun, Moon, and planets

Small periodic oscillations superimposed on precession

Apparent displacement of stars due to Earth’s motion around the Sun

Apparent shift in celestial body’s position due to observer not being at Earth’s center

Bending of light as it passes through Earth’s atmosphere