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its highest point (on the meridian) in the sky at the instant of apparent noon, and the measured altitude by sextant de- termines this point. At sea the naviga- tor has his chronometer . regulated. to mean time of the meridian of Greenwich, his reckoning point for time and _ longi- tude. If a computation at sea requires the navigator to know the mean time at ship he turns his longitude into time and subtracts it from chronometer (Green- wich) time if in west longitude, and adds it if in east longitude. If he desires the apparent time all that is necessary is to find the equation of time from the Nautical Almanac for his date and ap- ply it to his mean time. If he should desire the mean time from his apparent time he can find it by the reverse ope- ration. The equation of time is itself a portion of mean time. Since the earth rotates on its axis at Zar¢hA a uniform rate of speed throughout the year, the interval between two transits of the same spot on the earth over some imaginary but stationary point in the sky would then be always the same, and an exact measure of time. The sidereal day conforms to the two transits mentioned and the correct measure of time is, there- fore, sidereal time. The word sidereal means--of or belonging to the stars. Si- dereal time is divided into twenty-four hours the same as mean and apparent solar time, but its hours do not conform to mean solar or apparent solar hours. A sidereal day is the period in which the earth performs one complete revolu- tion round its axis, and, setting frac- tions on one side, is equal to 23h. 56m. 4s. of mean time, as measured by our clocks or watches; so that the common expression, that the earth rotates on its axis once in 24 hours, is incorrect, un- less sidereal hours are either specified or understood. The reason that it is called sidereal or star time is because the stars according to this time come to the meridian at exactly the same time, that is, if a star is on your meridian at a certain hour of sidereal time, 24 hours -- thereafter; it will .be back on your meridian; therefore, the stars keep pace with the rotation of the earth, and their revolu- tion in the heavens may be taken as perfectly regular. the stars are regular and not so with the sun? It is because the stars are at such inconceivable distances from the earth that the earth's annual motion in space around the sun is quite impercepti- ble when compared with them. Unlike the sun, the non-coincidence of the equator with the ecliptic, and the unequal motion of the sun in the latter, due to the eccentricity of the earth's orbit, the earth's erratic motion can have no such influence on the stars as it has with the sun. A sidereal day is shorter thana 'Now, 'why is it that ~ "TAE Marin\e. REVIEW mean solar day by 3m. 56s.; consequent- ly, the stars come to the meridian of any place nearly four minutes of mean clock time earlier on each succeeding day. The sidereal year does not commence the first of January like the solar year, and neither does the sidereal day begin Diagram showing the earth in the four important positions in the sun. This is shown by the outside circle and figures. moves and not the earth, the orbit becomes the figure. in its ecliptic or annual path around the earth. in its ecliptic as the earth does in its orbit. 27 nox when its declination changes from south to north; or, as sailors say, when the stn crosses the line bound north. This point, like the mean sun, is purely an imaginary one, as nothing exists to mark its place, nor would it be any par- ticular advantage were it otherwise; moreover, the point itself is liable to a \ ji its orbit or annual path around If we assume that it is the sun that ecliptic, as shown by the central 'Jnder this condition the sun instead of the earth is shown in four important positions The sun at these times occupies the same position If we could view the earth from the sun we would ind it traveling round us as shown by the outer ellipse, but as we view the sun from the earth, we find the sun apparentiy describing the path known as the ecliptic. and the sun's ecliptic are one and the same thing. Therefore, the earth's orbit In the one case we must assume the sun to be stationary while the earth revolves around it, and in the other case we must assume that the earth does not revolve, but it is the sun that moves. 1e, _ It is for this reason that the sn in the ecliptic for Dec. 21 conforms to the position of the earth in its orbit on June 21. The earth's orbit is its real revolution around the sun, while the ecliptic is the apparent path described by the sun around the earth during the same tiiire. The equinoctial is a great circle in the sky like the equator is a great circle on the earth, These two circles correspond and coincide with each other, for what the one is to the earth the other is to the heavens. The earth's orbit or the sun's ecliptic is another great circle in the heavens just like that of the equinoctial or celestial equator. But owing to the inclination of the earth's axis to the plane cf its orbit, or the plane of the ecliptic, which is all the same thing, site. 'these two great circles do not coincide with each other, but cross at two points diametrically oppo- All great circles bisect each other; they must, since they have the same center. noctial and ecliptic have the same center, the center of the earth being'the point or vertex. The equi- Bear in mind that the inclination of the earth's axis is always the same with the plane of the ecliptic (23% degrees). same (23% degrees). This 23% So, too, does the angle between the equinoctial and ecliptic remain always the' degrees represents the greatest distance these two circles are separated from each othei, hence, the limits of the sun's declinations, and the real cause of declination. the equator, hence the angie there must be zero. The two points where these great circles bisect each other must occur directly over When the earth in its orbit or the sun in its ecliptic occupies either of these positions, the sun must be directly over the equator, and at that time his declination must be zero. These intersecting points are also called the equinoxes on account of when the sun occupies either one of them, the days and nights are of equal length all over the world. When the sun occupies' the position at the intersection of the equinoctial and ecliptic (the vernal or Maich equinox), the sun, earth, and the First Point of Aries, are all in the same true line. It is at this time that the sidereal year begins. The vernal equinox remains fixed at this position of intersection, but the sun moves away from it as it travels round in its ecliptic. As the earth rotates and brings this point successively over the same meridian, . accounts for the beginning and ending of the sidereal day. When the sun reaches its position in the ecliptic for June 21, or thereabouts, the sun and this point have separated 90 degrees, or 6 hours, or the sun's apparent right ascension is 6 hours. Six. months later they will be 180 degrees, or 12 hours apart, on Dec. 21 18 hours, and on March 21, 24 hours apart. with the solar day, nor is sidereal noon solar noon, mean or apparent. « Sidereal time begins (that is, a sidereal clock points to oh. om. os.) when the first point of Aries is on the meridian of the observer, and is counted straight through 24 hours till the same point re- turns again. It is now necessary to know what is meant by the first point of Aries. It is that point in the heavens which the sun's center occupies at the March equi- certain slow movement westward, ow- ing to precession. gS Aries (Latin for Ram) is the constel- lation known as the Ram, and is one of the signs of the zodiac. It used to contain the first point of Aries, but owing to the precession of the equinoxes, this imagin- ary point has now retrograded into the constellation Pisces, but, nevertheless, its original position is made to answer the same thing in imagination, so that it in no wise affects the calculations of sider-