What is the Horizon?
When "we stand in open country and look around, we tjuickly note, that the earth's surface appears as a round plane, in whose centre we ace -ourselves. This circle of vision is called the Apparent Horizon. Every thing seems to be bordered by the bowl of heaven, which appears to rest -upon in. The True Horizon is a plane, parallel to che one just explained, buc running through the center of the earth. Thus, this true horison is always a greatest circle of the heavenly bowl proper, arid divides that bowl into two equal parrs.
Let us assume we put into the place of the observer, or into the centre of the horizon a vertical line. We then obtain with this imaginary line the Axis of the Horizon. To obtain the idea of an axis, we extend the line on both sides through the earth up to the heaven. Where chis line touches the heaven we have the Poles of the Horizon-, above, the North Pole, called the Zenith, below, rhe South Pole, called Nadir. Circles drawn through these two points, whose plane passes through the centre of the earth, arc called Vertical Circles. (See Fig. 2 NZSZ') on page 10.
The distance o£ a Star E from the horizon or the angl<? that is formed by a line drawn from the Observer towards the Star, is called the Altitude of the Star (EMH).
The angle which complements this altitude to a right angle (co W), is called the Zenith Diitance (EMZ). All points of equal altitude lie on one and the same Altitude Circle, such as EFC. These altitude circles, since the sky appears like a bowl, become smaller, the more we imigine them removed from the horizon. A point in the horizon has an altitude of ,0*, whereas the point of the Zenith has an altitude of 90*. All other polnrs lie between 0" and 90'.
The horizon is divided into four parts, the four points of the compass. When we look at noon -towards the Suh and imagine through this Sun a semi-circle standing vertically on the horizon, the point that cuts this horizon is Midday or the south point; opposite of this point is the Midnight point or the north, point, towards the left, 90* away from che former, is Morning or che East point and opposite to it. Evening or the West point. In these latter points the Sun rises and sets on March 21st and September 23rd each year.
The arcs between these points are, of course, still further divided. We obtain NE, SE, W and SW. Between the sub-division points we still have smaller parts, but rhey are only used for navigation. They have no practical value for our specialized work. They would furnish us a 16th resp. 32nd division of the circle for which we have no use and which division would be even detrimental to us in our operations. Forget them!
When vc consider the axis of our earth to extend infinitely, calling ir the World Axis, it meets the north and south pole of the heaven, or the World. Poles. (NS) Fig. 3, page 10.
The plane of the Earth's Equator extended outward towards the heaven describes on it the Heaven's Equator (Fig. 3 AQ).
Therefore, the latter is an imaginary circle, placed on the inside of the apparent heavenly bowl, whose position is fixed by extending the plane of the Earth's Equator into the heaven.ly sphere. On Ibuch 21st or 22nd also on September 23rd of each year ive find the Sun in this Heaven Equator and on these days the Sun's path indicates the position of the Heavenly Equator.
The circles -which pass through the World's Poles, erected vertically to the Heavenly Equator, are called Declination or Horn Grdes and the. planes which iie parallel to the individual parallel circles oi the earth, going through the starry heaven, define on them the Parallel and Day Circles of the Stars.
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