Geodetic Astronomy

Celestial Sphere

An imaginary surface whose center is the earth, where all celestial bodies are contained

360˚ 59.14' per 24 hours

The speed of rotation of the celestial sphere

celestial pole/s

the apparent axis of rotation of the earth; the points of intersection of the axis of rotation of the earth and the sphere

celestial equator

A great circle perpendicular to the celestial poles

Ephemeris

an

astronomic almanac containing tables giving the computed positions of the sun, the planets, and various stars for everyday of a given period

Equator

Observer's Reference Plane fr observing latitude

Primary Meridian

Observer's Reference Plane fr observing longitude

ALTITUDE (H or a)

the angular distance of the body from the horizon as measured along the vertical circle; complement of zenith distance

AZIMUTH (Z or A)

the angle, measured clockwise along the celestial horizon, from the north or south point to the vertical circle of the star

ZENITH DISTANCE (z)

the angular distance between the zenith and the celestial body measured along the vertical circle through the body.

HOUR ANGLE (t or h)

the angle, measured clockwise along the equatorial plane, from the zenith side of the celestial meridian (t = 0°) to the hour circle of the star

DECLINATION (δ or D)

the angular distance, north or south of the equator, measured along the hour circle from the equator to the body; complement of polar distance

POLAR DISTANCE (P)

the angular distance between the pole and the celestial body measured along the hour circle through the body

RIGHT ASCENSION (α)

the angle between the equinoctial colure and the hour circle of the star, measured from the vernal equinox τ, in the plane of the celestial equator

ECLIPTIC LATITUDE (β)

the angular distance, measured along the ecliptic meridian of the star, between the ecliptic and the direction of the star

ECLIPTIC LONGITUDE (λ)

the angle measured counterclockwise along the ecliptic, measured from the vernal equinox τ, to the ecliptic meridian of the star

ECLIPTIC POLAR DISTANCE

the complement of the ecliptic latitude

Celestial Horizon

the intersection of the observer's horizon/horizontal plane and the celestial sphere

Celestial Meridian / Observer's Celestial Meridian

the great circle passing through the zenith, nadir and the poles

Vertical Circles

great circles of the celestial sphere that passes through the zenith and the nadir

Prime Vertical Circle

vertical circle that is perpendicular to the observer's celestial meridian

Almucantars

parallels of altitude, intersection of any plane parallel to the celestial horizon and the celestial sphere

Zenith

the point of intersection (at the top) of the observer's vertical line and the celestial sphere

Nadir

the point of intersection (at the bottom) of the

observer's vertical line and the celestial sphere

North Point

the point of intersection of the celestial horizon and observer's celestial meridian nearest to the North Celestial Pole (NCP).

South Point

the point of intersection of the celestial horizon and observer's celestial meridian nearest to the South Celestial Pole (SCP)

East and West Point

points of intersection of the prime vertical circle and the celestial horizon

Hour Circle

any great circle in the celestial sphere that passes through the north and south celestial poles

6th Hour Circle

hour circle perpendicular to the meridian

Celestial Parallel

the intersection of any plane parallel to the celestial equator and the celestial sphere

Ecliptic

the intersection of the plane of the earth's orbit

around the sun and the celestial sphere

Vernal Equinox

the point of intersection of the celestial equator and the ecliptic (when the sun's declination is 0°) following the winter season

Autumnal Equinox

the point of intersection of the celestial equator and the ecliptic (when the sun's declination is 0°) following the summer season.

Winter/Summer Solstice

point on the ecliptic defined when the sun's declination is greatest

Equinoctial Colure

a great circle in the celestial sphere passing through the celestial poles and the equinoxes

Solstitial Colure

a great circle in the celestial sphere passing

through the celestial poles and the solstices

Plane of the Ecliptic

the plane containing the center of the sun, the barycenter of the earth-moon system, and the inertial heliocentric velocity vector of the earth-moon vector of the earth-moon system; plane of orbit of the earth around the sun

Ecliptic Poles

points of intersection of the line perpendicular

Ecliptic Parallel

intersection of any plane parallel to the ecliptic and the celestial sphere

Ecliptic Meridian

great circle in the celestial sphere that passes through the ecliptic poles

Obliquity of the Ecliptic (ε)

the acute angle between the plane of the ecliptic and the plane of the equator (23.5°)

PARALLACTIC ANGLE (p or γ)

the angle between the vertical circle and hour circle of the star

Rising/Setting

Instant when the star intersects the celestial horizon of the place (for equatorial stars only)

Prime Vertical Crossing

Instant when the star intersects the prime vertical circle

Culmination/Transit

Instant when the star intersects the celestial meridian

Elongation

Occurs when the hour circle and vertical circle of star are perpendicular to each other (for Type 1 stars only)

First-order (Precision) Instruments

Portable instruments which can provide standard deviations of less than 0.3"

Observatory (High-precision) Instruments

Permanently installed in laboratories; Used for repeated measurements of time and latitude

Second-order (Geodetic) instruments

Provides standard deviations of less than 1.0"

Lower order (Surveying) instruments

Used for property and engineering surveys

parallax

an error observed in vertical angle due to the fact that it is observed on the surface and not on the center of the earth

Index error

occurs when the instrument's initial vertical angle is not at the zero level, even if the telescope is set to be horizontally leveled

Refraction of the Ray

The angle of deviation of the ray from its direction at the surface of the earth