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28 . eal time and the right ascensions of the stars, tke sun and the planets. Since the imaginary first point of Aries corresponds to the vernal or spring equi- 'nox, it will be more simple and there- fore more convenient to mention it as the vernal equinox instead of the first point of Aries. Roughly speaking, sider- eal time is measured by the daily motion of the stars; but to be precise, it is measured by the daily motion of that point in the equator from which the true right ascensions of the stars are counted, but it amounts to the same thing. The point in question is the vernal equinox, and its hour angle is always the sidereal time. Astronomical clocks are usually regulated to sidereal time, and are then called sidereal clocks. Therefore. the sidereal year begins at the instant that the sun occupies the ver- nal equinox in March. A sidereal day is the interval between two successive transits of the vernal equinox over the same meridian--equal to one absolute ro- tation of the earth. It is 3m. 55.9s. of mean solar time shorter than the mean solar day, the tropical year of 365.24 so- lar days, being divided into 366.24. sider- eal days, each comprising 24 sidereal | hours. The sidereal hours are counted from 0 to 24, commencing with the in- stant: of the passage of the true vernal equinox over the upper. meridian, and ending with itsreturn to the same meri- dian. About March 23 of each year the sidereal clock agrees with the mean time or ordinary clock, and the former gains on the latter 3m. 56.5s. of sidereal time per day, so that the end of a year. it will, have gained an entire day, and will again agree with the mean time clock. Sidereal noon is the. instant that you have the vernal equinox on your meridian, This may occur at any time of the day or night ac- cording to the mean time clock, but with a clock regulated to sidereal 'time it would occur at the same in- stant of time. which would be 24 o'clock, but with our mean solar time it would take place about 4 minutes earlier every day. Now, let us get the correct idea of what Right Ascension is. Right ascen- sion is nothing more than sidereal time. Imagine you have a clock regulated to sidereal time. Since sidereal time is reck- oned through 24 hours from west to east, the dial of such a clock is divided into 24 equal parts instead of 12 as the ordinary clock shows. We will start with the beginning of. the sidereal year, say, that is, when the sun and vernal equinox are in one, start the sidereal clock, which had previously been set to oh. om. os. to going. Now, it is plain to be seen that when the earth has made one rotation and brought you and your meridian back to the same point of this - TAE Marine REVIEW Another view of the ecliptic, showing the points of intersection with the equator and the two tropics. In consequence of the inclination of the earth's axis to the plane of the orbit, the plane of the ecliptic is inclined to the plane of the equator at an angle of 23% degrees, and as the sun moves along in the ecliptic it travels vertically over that portion of the earth's surface lying 23% degrees north and south of the equator. Hence, these parallels of latitude on the earth are taken" as the limits of the hottest or 'torrid'? zone. The parallel on the north is called the tropic of Cancer, and the one on the south the tropic of Capricorn. They are called tropics (turnings), because over them the sun appears to turn and retrace his course toward the equator. From the various positions of the sun in the ecliptic the cause and principle of the sun's declination become matters of simplicity. Starting from either of the equinoxes, where the declina- tion is zero, it is plain to he seen that as the sun moves forward in the ecliptic that it gets farther and farther from the equator, from where it is measured. When it reaches either of the tropies it has reacned its extreme limits, 2344 degrees, the same amount that the earth's axis is inclined from a perpendicular line. When the sun has moved along in the- ecliptic so that it is vertically over the tropic of Cancer at nocn, summer commences in the northern and winter in the southern hemi- spheres. The declination is then 23% degrees north. It is north from about March 21 to about Sept. 21. When the sun has moved along in the ecliptic till it is vertically overhead for those in the latitude of 23% degrees south, it is then. over the tropic of Capricorn, and has reached its extreme southern limits. It is the beginning of summer in the southern hemisphere and the begin- ning of winter in the nozthern hemisphere. The sun's declination increases from zero to 23% degrees and from 23% degrees to zero. It is twice zero in the year and also twice 2314 degrees during the year, once north and once south. To find the positions of stars we map out the sky by the same system of lines that we use on the surface of the earth 'n finding or locating places or points situated thereon. The terrestial equator is taken as the initial curve. The plane of the terrestrial equator being produced to infinity, marks on the apparent dome of the heavens the position of the celestial equator or equinoctial, and this divides the heavens into two hemispheres, the same as with the earth. The equinoctial and the meridians of the sky are divided, like the terrestrial equator, into degrees, minutes and seconds. The zero, or starting point, of the equinoctial is the point of the vernal equinox. This division is based on the daily rotaticn of the earth on its axis, or on the apparent motion of'the starry heavens in the contrary direction, the stars seem to us to move around the pole in the same direc- tion as the sun does, namely, from east to west, at the rate of 15'degrees each hour. The space between these degrees grows smaller and the apparent motion slower as we approach. the pole, which appears at rest. Polaris, or the pole star, being very near the pole, its motion is but slightly pereeeyDie and it appears to the eye as the steady point around which the whole heavenly vault urns. : By the orbital motion of the earth round the sun our position with respect to the stars is changed proportionately less than would be that of a grain of dust placed in the middle of the dome of St. Paul's, if it were made to whirl round in a circle having a diameter of 0.01 of an inch. The stars really move in their orbits at the rate of 15 degrees an hour, if sidereal hours are employed. The sun would do the same thing were its axis not inclined to the plane of its orbit, but were perpendicular instead. When the sun is at either of the equinoxes (directly over the equator, twice in the year) it performs this feat according to sidereal time. Although the speed of the earth's rotation is less as we recede from the equator and nothing at the poles, the stars near the pole move through the same number of degrees as do those on or near the equator, but since the distance between degrees is much greater in low latitudes, the stars near the equator must travel that much faster because they have that much farther to go. Just as a rotating wheel, a point on ihe tire moves faster than a point on the hub; but, notwithstanding this, when the point on the tire has completed a revolution, so has the point on the hub. As the circum- ference of the earth is about 25,000 miles, and the earth completes one rotation in 24 hours, it fol- lows that a. point on the equator moves at a speed of over 1,000 miles an hour, or through 15 degrees of longitude. Tle speed of rotation on the parallel of 60 degrees (which is just one- half of that of the equator) is about 500 miles in an hour, the space between the degrees being just half of what they are on the equator. On the parallel of 60 in one hour the earth rotates through the 15 degrees just the same, and this is true for any parallel. It is the space between the dégrees that makes the difference.