Astro Navigation
Definitions:-
Latitude and Longitude is the way we define a location on the Earths surface.
LatitudeIs measured in degrees N & S of the Equator. The Equator is Zero and the Poles 90 degrees N&S, the equidistant lines joining places of equal Latitude are known as parallels. The Equator is the only Great Circle.
LongitudeIs Measured East and West of the Prime Meridian (The Greenwich Meridian), Greenwich is zero, Easterly and Westerly Longitude terminate at 180 E or W; sometimes known as the Date Line. Places joined by the same Longitude are said to be on the same Meridian.
Observers ZenithThe point at which a straight line joining the centre of the earth to the observer's position meets the celestial sphere. To a particular observer it is the highest point of the celestial sphere.
The Celestial SphereIn astronomy and navigation, the celestial sphere is an imaginarysphere of arbitrarily large radius, concentric with Earth. All objects in the observer's sky can be thought of as projected upon the inside surface of the celestial sphere, as if it were the underside of a dome or a hemispherical screen. The celestial sphere is a practical tool for spherical astronomy, allowing observers to plot positions of objects in the sky when their distances are unknown or unimportant.
Right ascensionRight ascension is essentially the longitude on a star chart where you may look to find something in the sky among the stars. Facing north from the northern hemisphere, when stars rise, they ascend (rise) on the right and move from east to west, hence the term right ascension. Right ascension is the equivalent of longitude in astronomy measured eastward from the first point of Aries (astronomical prime meridian). It is generally measured in terms of time, but can also be expressed in degrees. Right Ascension is not used for Navigation; however it is in common use amongst Astronomers.
Hour AngleThe same as Right ascension but measured Westward, the sun Moon and planets are tabulated in the Nautical Almanac as a GHA(Greenwich Hour Angle) – 0 at Greenwich all the way round to 360 back at Greenwich. The Longitude of Celestial Co-ordinates.
Local Hour AngleIs the GHA + / - the Longitude of the Observer. This is commonly abbreviated to LHA.
(+ For Easterly Longitude – for Westerly Longitude)
(If the sum exceeds 360 simply deduct 360 to obtain the LHA)
Plane of the Rational HorizonThe small circle of the celestial sphere whose plane is tangent to the earth at the position of a given observer, or the plane of such a circle (In Astronomy it’s known as the sensible horizon). One of the finest tools for determining the location of a heavenly body, if you can draw this for your location you will understand what and approximately when and where a given body is visible.
The EclipticThe ecliptic is the apparent path of the Sun on the celestial sphere, and is the basis for the ecliptic coordinate system. It also refers to the plane of this path, which is coplanar with the orbit of Earth around the Sun (and hence the apparent orbit of the Sun around Earth). The path of the Sun is not normally noticeable from Earth's surface because Earth rotates, carrying the observer through the cycles of sunrise and sunset, obscuring the apparent motion of the Sun with respect to the stars.
First Point of AiresThe First Point of Aries is the location of the vernal equinox, and is named for the constellation of Aries. It is one of the two points on the celestial sphere at which the celestial equator meets the ecliptic plane, the other being the First Point of Libra, located exactly 180° from it. Over its year-long journey through the constellations, the Sun crosses the celestial equator from south to north at the First Point of Aries, and from north to south at the First Point of Libra. The First Point of Aries is considered to be the celestial "prime meridian" from which right ascensions are calculated.
DeclinationDeclination in astronomy is comparable to geographic latitude, projected onto the celestial sphere, and hour angle is likewise comparable to longitude. Points north of the celestial equator have positive declinations, while those south have negative declinations. Any units of angular measure can be used for declination, but it is customarily measured in the degrees ( ° ), minutes ( ' ), and seconds ( " ). In the Nautical Almanac - & + are given as N & S.
Zenith DistanceThe angular distance of a celestial object from the zenith measured by the arc of a vertical circle intercepted between the object and the zenith : the complement of the altitude.(90 – Altitude).
AltitudeThe angular elevation of a celestial object above the horizon of a heavenly body. In Celestial Navigation it is measured using a sextant. The raw sextant angle is inaccurate and must be corrected before using the result for navigating or defining an accurate position on the surface of Earth.