Cal Maritime - Celestial Navigation - Pearson
Zone Time (ZT)
The time of the zone your in - adjusted from GMT according to your zone description.
•Upper Limb (UL)
The upper edge of the diameter of the celestial body
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere.
•Systems of Coordinates
There are several systems of coordinates (Lat and long being one of them) that we use for celestial navigation. By conceiving of them as
Time Tick
This is a radio signal indicating the precise time GMT - it is used to determine chronometer error. At sea, it is possible to get an accurate update of the time over a high-frequency receiver. Time ticks are available over the radio from the National Institute of Standards and Technology (NIST). The NIST time tick broadcast station WWV is in Ft. Collins, Colo. WWV broadcasts continuously on 2.5, 5, 10, 15 and 20 Mhz for much of the North Atlantic, and from WWVH in Hawaii on 2.5, 5, 10 and 15 Mhz.
Celestial Horizon
This is above the visible horizon. it is often measured as 2/3 the diameter of the celestial body being measured above the visible horizon. This difference has to do with the line of visibility and angular relationships with celestial bodies.
Chronometer Error
This is the amount by which chronometer time differs from GMT. It is measured as "behind" or "ahead".
Chronometer Time (CT)
This is the time displayed on the chronometer - it may vary slightly from GMT due to chronometer error.
Little Dipper
Ursa Minor
Azimuth Circle
What is used to measure the bearing and azimuths of terrestrial and celestial objects.
Local Apparent Noon (LAN)
When the sun is directly over head. This is often at odds with the local mean time, which may fall before or after the sun passes directly over head.
•Horizontal Parallax (HP)
a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.
Local Celestial Meridian
a great circle which passes through both the zenith and the celestial poles
Big Dipper
a group of seven bright stars in the constellation Ursa Major familiar shape within the constellation Ursa Major which contains the pointer stars
Mean Sun
an imaginary object that moves at a uniform rate along the celestial equator. This concept allows for the 24 hour day and assumes that 1200 is always the moment of Meridian Passage.
sextant
an instrument with a graduated arc of 60° and a sighting mechanism, used for measuring the angular distances between objects and especially for taking altitudes in navigation.
Time Zone
any of the 24 longitudinal areas of the world within which the same time is used - marked in 15 degrees of longitude.
Local Mean Time (LMT)
is a form of solar time that corrects the variations of local apparent time, forming a uniform time scale at a specific longitude. This measurement of time was used for everyday use during the 19th century before time zones were introduced beginning in the late 19th century; it still has some uses in astronomy and navigation This seems to be somewhere between ZT and the local apparent time.
Polaris (North Star)
star at the zenith of the North Pole
Meridian Angle (t)
the angle, measured eastward or westward through 180°, between the celestial meridian of an observer and the hour circle of a celestial body
Zenith Distance
the angular distance between the zenith and the celestial body measured along the vertical circle through the body. Also called the co-altitude and is derived by the equation 90 degrees - altitude.
Geoidal Horizon
the circle of the celestial sphere formed by the intersection of the celestial sphere and a plane through a point on the sea level surface of the earth, and perpendicular to the zenith-nadir line.
•Sensible Horizon
the circle of the celestial sphere formed by the intersection of the celestial sphere and a plane through any point, such as the eye of an observer, and perpendicular to the zenith‐nadir line.
Compass Error
the combined effects of both variation and deviation, expressed as an angle east or west of true north.
Equation of Time
the difference between apparent solar time and mean solar time
•Lower Limb (LL)
the lower edge of the diameter of a celestial body.
Coaltitude
90 degrees - altitude In celestial navigation, the Co-Altitude is used to calculate the distance of the observer from a point on the earth directly beneath the sun (or other celestial body), called the geographical position or GP.
Codeclination
90 degrees - declination The sun's declination is the distance from the equator to the sun. The distance from the sun to the pole is 90° minus the declination, or codeclination.
Astronomy
Astronomy is the study of the celestial universe (sun, planets, moons, stars, space, etc.)
Apparent Sun
The Sun as we see it in the sky.
Upper Branch
The half of the meridian which passes through its place or zenith
•Observed Altitude (Ho)
This is the measurement used for celestial calculations. It is the Apparent Altitude adjusted for the altitude correction (a result of paralax)
Visible Horizon
where the land and sky meet. When the sun's center is on the visible horizon we say that the sun is on the visible horizon. This must be corrected to get celestial horizon.
Earth's axis of rotation
23.5 degrees to the plane of the orbit around the sun
Local Apparent Time (LAT)
24 hour time cycle based on the sun as it is seen in the sky. If, for example, the sun is directly overhead, the local apparent time is noon. If we followed this measure of time, the hours would be shorter or longer based on the season.
Colatitude
90 degrees - (observer's latitude) The distance from the zenith to the north celestial pole is 90° minus the latitude, and we call this the colatitude (because it is the compliment of latitude — two numbers added together that result in 90 are called complimentary numbers).
Celestial Horizon
A great circle located 90 degrees from the zenith and the Nadir, the Celestial Horizon drawn on the celestial sphere which is tangent to the Earth at the observer's position
Hour Circle
A line from the pole, through the celestial object, intersecting the celestial equator at right angles. A great circle on the celestial sphere passing through the celestial poles.
Declination
A measure of how far north or south an object is from the celestial equator - it is the celestial analog to latitude
Zone Description (ZD)
A numeric value designating how many hours would have to be added to or subtracted from your local time in order to determine GMT.
Azimuth
Bearing of a celestial body at any time other than rising or setting. the direction of a celestial object from the observer, expressed as the angular distance from the north or south point of the horizon to the point at which a vertical circle passing through the object intersects the horizon.
Amplitude
Bearing of a celestial body while rising or setting.
Refraction
Bending of light
Equinoctial
Celestial Equator. The primary great circle of the celestial sphere, everywhere 90 degrees from the celestial poles; the intersection of the extended plane of the equator and the celestial sphere.
•Index Error
Error inherent in your sextant - measured as "on the arc" or "off the arc"
Celestial Triangle
Essentially another name for the navigational triangle, but specified as projected on the celestial sphere, the celestial triangle is an astronomical triangle on the surface of the celestial sphere whose vertices are the zenith, an elevated pole, and a given celestial body
GP (sub point)
Geographical position - The equivalent spot on Earth to where a celestial body appears on the celestial sphere (the GHA and Dec should correlate to terrestrial latitude and longitude).
GHA
Greenwich Hour Angle Like longitude is measured from 0 degrees (which runs through Greenwich England and marks Greenwich Mean Time) the longitude on the celestial sphere is measured from this 0 degree mark, through a 360 degree arc westward, called the Greenwich Hour Angle. Any physical longitude will correspond to a specific Greenwich Hour Angle (though we must be aware, since terrestrial longitude is measured in 180 degrees both east and west.
•Latitude by Polaris
If we are able to determine the altitude of polaris (by determining LAN, and accounting for time of day and longitude), then we can also determine the latitude of polaris.
•Semi-diameter (SD)
In the most basic sense, this is a half circle.
LHA
Local Hour Angle It is the angular distance measured westward of the observer's celestial meridian and up to 360 degrees to the position of the celestial body.
Nautical or Navigational Astronomy
Nautical or Navigational Astronomy is a simplified version of astronomy which deals with applicable celestial bodies for navigational purposes. This process is based on a few false assumptions to expedite computation (to be discussed later).
Elevated Pole
Pole above you on the celestial sphere or the pole nearest to your position on the planet.
Depressed Pole
Pole beneath you or furthest from your position on the planet.
Sight Reduction
Process of solving celestial observations - Publication 229 is used for sight reduction.
SHA
Sidereal Hour Angle This is an angular distance measured west of Aries. We use this to determine the position of stars and planets. T This is because we can tabulate Aries predictable rotation, and instead of creating an encyclopedia of tabulations for each of the other celestial bodies, we simply are provided a relation of that body to Aries.
Meridian Passage (MER PASS)
Similar to local apparent noon (which is the sun's meridian passage), but this concept can apply to any heavenly body. If the body is directly overhead, it is crossing your own meridian, and hence the designation "meridian passage".
Terrestrial Triangle
Terrestrial Triangle - the navigational triangle as projected onto Earth with the corresponding coordinates expressed in latitude and longitude.
Altitude
The Altitude of a celestial body is its angular distance above the Horizon.
False Assumptions of Nautical Astronomy:
The Earth is a perfect sphere, stationary at the center of the universe. Celestial bodies travel from east to west. Although many celestial bodies are so far away, they can be considered fixed on the inside of the imaginary celestial sphere.
Ecliptic
The Ecliptic is the path of the apparent sun around the Earth in one year.
Apparent Altitude (Ha)
The Sextant Altitude, corrected for index error and dip.
Variation
The angle between the magnetic and geographic meridians at any place, expressed in degrees east or west to indicate the direction of magnetic north from true north
Deviation
The angle between the magnetic meridian and the axis of a compass card, expressed in degrees east or west to indicate the direction in which the northern end of the compass card is offset from magnetic north
Sextant Altitude (Hs)
The angle measured from the visible horizon to the celestial body.
Celestial Navigation
The art and science of using observations of the celestial bodies (Sun, Moon, certain visible planets and brighter stars) for navigation purposes.
Azimuth (Zn)
The azimuth is the angle between a celestial body (sun, moon) and the North, measured clockwise around the observer's horizon. It determines the direction of the celestial body. For example, a celestial body due North has an azimuth of 0º, one due East 90º, one due South 180º and one due West 270º.
•Index Correction
The correction made to the Sextant Altitude, in order to account for index error.
Nadir
The nadir is an imaginary point directly "below" a particular location, on the imaginary celestial sphere.
Lower Branch
The other half of a meridian, 180 away from the upper branch
First Point of Aries
The point on the celestial sphere where the path of the sun (the ecliptic) crosses the celestial equator from south to north in March, marking the zero point of right ascension. Also called the "vernal equinox."
Prime Celestial Meridian
The prime meridian projected onto the celestial sphere. Selected, through historical circumstances, as running through Greenwich, the prime meridian is the starting mark for the 360 degrees of longitude, and is also a foundational mark on the celestial meridian, measured as the Greenwich Hour Angle.
Navigational Triangle
A spherical triangle made up of great circles that we use to solve celestial problems and observations. The navigational triangle or PZX triangle is a term used in Astro-Navigation, or Celestial Navigation, and its solution can give you your position anywhere (more or less) on the globe. P is the Celestial Pole (North or South). Z is the point on the observer's meridian, where a line from the centre of the earth (treated in this case as a 'sphere') projected outwards through the observer and onto the Celestial Sphere points to his Zenith. X is the point where center of celestial body lies on celestial sphere.
Alidade
A telescope on a bearing circle used to measure the visible horizon. the astrolabe's movable arm, called the alidade, and measure the angle of height of the object above the horizon, thereby determining the ship's position.
Time Diagram
A time diagram is a diagram on the plane of the celestial equator or equinoctial in which the celestial equator appears as a circle, and celestial meridians and hour circles as radial lines. It is used to facilitate solution of time problems and other problems involving arcs of the celestial equator or angles at the pole, by indicating relations between various quantities involved. Conventionally, the relationships are given as viewed from a point over the south pole ( Ps ) westward direction being counterclockwise.
Chronometer
A time piece with a near constant rate. On a ship, it is set to Greenwich Mean Time and is not adjusted for time zones. An instrument for measuring time very accurately
•Computed Altitude (Hc)
Altitude determined by computation, table, mechanical computer, or graphics, particularly such an altitude of the center of a celestial body measured as an arc on a vertical circle of the celestial sphere from the celestial horizon. Also known as calculated altitude. The difference between Ho and Hc will be useful in determining fixes.
Chronometer Correction
An error ahead of GMT is marked with (-) and an error behind GMT is marked (+) This can be compared to the east and west designations for the different time zones that are either behind or ahead of GMT. The amount that must be added algebraically to the chronometer time to obtain the correct time. Chronometer correction is numerically equal to the chronometer error, but of opposite sign.
Dip
An error as a result of the observers position above the earth. The result of Dip is that the observed altitude will be greater than the actual altitude of a celestial body.
Celestial Meridian
An imaginary line that starts at the North point on the horizon, goes to the zenith, and from the zenith to the south point of the horizon.
Right Ascension (RA)
Analogous to longitude, but on the celestial sphere; the angular distance measured eastward from Aries
Azimuth Angle (Z)
Angle of the celestial body measured from your celestial meridian.
