Celestial Navigation Cooney's Theory

Sidereal Day

The time the Earth takes to make 1 complete rotation: 23 Hours, 56 Minutes, 4 Seconds

Lower Transit

When a body crosses the lower branch of the observer's celestial meridian. Time of lower transit is Local Apparent Midnight.

Upper Transit

When a body crosses the upper branch of the observer's celestial meridian. Time of upper transit is Local Apparent Noon.

Quadrature

When a body observed from earth is at a right angle with the sun.

Civil Twilight

When the Sun is 0-6 degrees below the horizon, best time to observe stars and planets.

Solstice

When the sun is at the maximum latitude in a hemisphere. Winter Solstice (23.5 degrees S) is maximum south, Summer Solstice (23.5 degrees N) is maximum north.

Equinox

When the sun is directly over the equator. Happens twice a year. Vernal equinox, March 21. Autumnal Equinox, September 21.

Perihelion

the point in the earths orbit when it is closest to the sun

Greenwich Celestial Meridian

A celestial meridian using Greenwich as its zenith.

Geoidal Horizon

Assumes the observer's eye level is at surface of Earth.

EP

Elevated Pole

The true path of the Sun

Elliptic

Right Ascension (RA)

The compliment of SHA. (SHA + RA = 360)

Solar Day

24 Hours

Lunar Day

24 Hours 50 Minutes

Parallel of Latitude

A circle drawn around the globe parallel to the equator and at right angles to the meridians.

Parallel of Altitude

A circle on the celestial sphere, parallel with the horizon, containing all points of equal altitude.

Parallel of Declination

A circle passing through the celestial object and parallel to the celestial equator.

Celestial Meridian

A circle passing through the celestial poles and the zenith of a given place on the earth's surface.

Mean Solar Day

A day consisting of exactly 24 hours

Hour Circle

A great circle on the celestial sphere passing through the celestial poles. A line from the pole, through the celestial object, intersecting the celestial equator at right angles.

Precession

A slow motion of Earth's axis that traces out a cone. Causes smaller changes to apparent positions of Celestial Bodies over time.

Difference between celestial meridians and hour circles

Celestial Meridians are in reference to the Earth. Hour Circles are in reference to the Celestial Sphere, specifically the poles.

Three Sides of the navigational triangle

Co-Latitude (EP-Z), Polar Distance (EP to GP), Co-Altitude (Z-GP)

Diurnal Cycles

Concentric circles formed from the daily path of the celestial body

Height Observed (HO)

Corrected sextant measurement of a body

Geometric Horizon

Depends solely on the height of eye. Refraction is unaccounted for.

The apparent path of the Sun

Ecliptic

Three vertices of the navigational triangle.

Elevated Pole, Zenith of observer, GP of the star.

Another name for the Celestial Equator System

Equinoctial System

GP

Geographical Position

Vertical Circle

Great circles on the celestial sphere that pass through the Zenith and Nadir, perpendicular to the celestial horizon.

Ho Mo To/CGA

If HO > HC. Advance is towards the azimuth. If HC > HO, advance is away from the azimuth.

Three angles of the navigational triangle

LHA (Meridian Angle) (At the EP), Azimuth angle (At Zenith), Parallactic Angle (At GP)

Adjustable Sextant errors

Lack of perpendicularity of frame and index mirror, Lack of perpendicularity of frame and horizon glass (side error) Non-parallelism of index mirror and horizon glass Non-parallelism of the telescope with the frame (collimation error)

Magnitude

Measure of the brightness of a celestial body.

Height Apparent (HA)

Measured angle of a celestial body over the horizon after sextant corrections are applied

Height Sextant (HS)

Measured angle of a celestial body over the horizon obtained by a sextant reading.

Navigational Planets

Planets within the Solar System with enough magnitude to be used in navigation. Mars, Venus, Jupiter & Saturn

Non-adjustable sextant errors

Prismatic Error, Graduation Error, Centering Error

Nautical Twilight

Sun between 6 and 12 degrees below horizon

Height Calculated (HC)

Table value of a sextant measurement

Declination

The angle measured from the elevated pole along the hour circle to the body.

Latitude

The angle measured from the equator, along the meridian to the object.

Altitude

The angle measured from the horizon along the vertical circle to the body

Zenith Distance

The angular distance between the zenith and the celestial body measured along the vertical circle through the body. Complimented by Altitude.

Polar Distance

The angular distance measured from the elevated pole along the arc of an hour circle to the object.

Retrograde Motion

The apparent motion of the planets when they appear to move backwards due to a different sized orbit.

Diurnal Path

The daily path of a celestial body traced on the earth.

Azimuth

The horizontal angular distance from north measured eastward along the horizon to a point directly below a celestial body.

Line of Apsides

The line joining apehelion and perihelion

Celestial Horizon

The plane perpendicular to Zenith-Nadir axis that passes through the center of the Earth. Most measurements within the Horizon system come from the Celestial Horizon.

Sensible Horizon

The plane perpendicular to the Zenith-Nadir axis that passes through the observers eye.

Apehelion

The point in the earth's orbit when it is furthest from the sun.

Zenith

The point on the celestial meridian directly above an object or observer.

Nadir

The point on the celestial meridian directly below the object or observer.

First Point of Aries

The point on the celestial sphere where the path of the sun crosses the celestial equator from south to north in March, marking the zero point of right ascension. Used as the reference point for stars (SHA). Also called the "vernal equinox."

Celestial Equator (CE)

The projection of the earth's equator onto the celestial sphere. Also called the Equinoctial

Precession

The result of gravitational forces exerted by the sun and moon on E's equatorial bulge

Meridian Angle (t)

Angular distance, measured 0˚ to 180˚ East or West of the observer's celestial meridian. (Find t using LHA. If LHA > 180, 360-LHA = t. If LHA < 180, LHA=t)

Local Hour Angle (LHA)

Angular distance measured 0˚ -360˚ west of the local celestial meridian

Greenwich Hour Angle (GHA)

Angular distance measured from 0˚ to 360˚ west of the Greenwich meridian

Sidereal Hour Angle (SHA)

Angular distance, measured 0˚ -360˚ west of the vernal equinox. We use this to determine the position of stars and planets

Circumpolar

A star or celestial body that never sets below the observers horizon.

The Prime Vertical Circle

A vertical circle that passes through the east and west celestial poles of a horizon, perpendicular to the observer's celestial horizon.

The Principle Vertical Circle

A vertical circle that passes through the north and south celestial poles of the horizon.

Nutation

An irregularity in precession caused by the gravitational effects of other celestial bodies.