is similar to a parallel of latitude on the earth.
A point on the celestial sphere may be identified at the intersection of its parallel of declination and its hour circle.
The parallel of declination is identified by the declination.
测者真地平圈： A plane perpendicular to the true vertical is a horizontal plane, and its intersection with the celestial sphere is a horizon. It is the celestial horizon if the plane passes through the center of the earth, the geoidal horizon if it is tangent to the earth, and the sensible horizon if it passes through the eye of the
observer at A. Since the radius of the earth is considered negligible with respect to that of the celestial sphere, these horizons become superimposed, and most measurements are referred only to the
30
celestial horizon.
This is sometimes called the rational horizon.
测者真地平圈与测者子午圈相交两点——北、南； 测者真地平圈与天赤道也相交两点——东、西； 天球区域的划分：
北天半球和南天半球——以天赤道为基准； 东天半球和西天半球——以测者子午圆为基准； 上天半球和夏天半球——以测者真地平圈为基准； 高极与测者纬度之间的关系：
One of these poles (having the same name as the latitude) is above the horizon and is called the elevated pole. The other, called the depressed pole, is below the horizon.
在上天半球的天极叫做高极。在北半球，天北极是高极；在南半球，天南极是高极。
从测者真地平圈到高极的弧距，称为高极的高度，所以，高极的高度=测者的纬度
2.3.4 Two Equatorial Coordinates
Two basic methods of locating the hour circle are in use. The first method of locating the hour circle is to indicate its angular distance west of a celestial meridian. If the Greenwich celestial meridian is used as the reference, the angular distance is called Greenwich hour angle (GHA) (格林时角), and if the
meridian of the observer, it is called local hour angle (LHA)(地方时角).
这种方法称作第一赤道坐标系，由天体地方时角和天体赤纬来决定。基准大圆是天赤道和测者子午圆，辅助圆是天体时
31
圆。
天体赤纬(declination)+天体极距(pole distance)=90?
Second, the angular distance west of a reference hour circle through a point on the celestial sphere, called the vernal equinox or first point of Aries(白羊座), is called sidereal hour angle(SHA) (恒星时角).
这种方法称作第二赤道坐标系，由天体赤经和天体赤纬来决定。基准大圆是天赤道和春分点时圆，辅助圆是天体时圆。
Another Coordinate, Horizontal Coordinate(地平坐标系) 天体方位圆：
天体高度+天体顶距=90? 2.3.5 The Navigational Triangle
A triangle formed by arcs of great circles of a sphere is called a spherical triangle. A spherical triangle on the celestial sphere is called a celestial triangle. The spherical triangle of particular significance to navigators is called the navigational triangle, formed by arcs of a celestial meridian, an hour circle, and a
vertical circle. Its vertices(至高点) are the elevated pole, the zenith, and a point on the celestial sphere (usually a celestial body).
The terrestrial counterpart is also called a navigational
triangle, being formed by arcs of two meridians and the great circle connecting two places on the earth, one on each meridian. The vertices are the two places and a pole.
32
2.4 Radio navigation
Radio navigation is the process of determining a ship‘s position by applying radio wave propagation, using the parameter (course, distance, etc) determined by its transmitters on a known position and receivers on a vessel.
Electromagnetic spectrum.
2.4.1 Classify of Radio navigation:
1. Short range radio navigation- DME, VOR, TACAN, etc. 2. Intermediate range radio navigation- Loran-A, Loran-D, etc. 3. Long range radio navigation- Loran-C, etc.
4. Exceed long-range radio navigation-Omega, etc.
33
2.4.2 Advantages and Disadvantages of Radio Navigation Advantages
1.no influence for radio navigation with weather and range of visibility. 2. Fast measure 3. Easy to realize 4. Better precision 5. Better reliability Disadvantages:
1. Easy to be disturbed by nature factor such as magnetic storm, macula, etc.
2. Easy to be destroyed by enemy.
3. Complex system (Transmitter, receiver, etc). 4. Can not be used under water.
2.4.3 Loran-C navigation and positioning system Components of Loran-C
34