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AE Marine Review As measured on the earth the distance around the orbit of Polaris is a little better than 360 nautical miles, since a degree of longitude on the parallel of 89 degrees is a trifle better than one nautical mile. Polaris completes this journey in 24 hours or with the rotation of the earth; in other words, Polaris has passed over the 360 degrees of the circumference. The star Dubhe is about 28 degrees from the pole or over the parallel of 62 degrees north latitude, and since a degree of longitude on this parailel is equal to 32.5 nautical miles, Dubhe must travel 11,700 miles to complete the circuit of its orbit in the 24 hours. Polaris and Dubhe pass over or through the same number of degrees of longitude, in the same length of time, but not through the same space. The space between the degrees of longitude' on the equator is 60 nautical miles, on the parallel of 62 degrees 32.8 miles, and on the parallel of 89 degrees about one mile. A parallel of latitude cn the earth's surface equal to a star's declination is the same thing 'as that star's orbit of revolution round the pole; hence, a parallel of latitude and a parallel of declina- tion are practically one aid the same thing. : In the above diagram. if we draw a line from the true pole to any given day on the outer circumference, where 'it cuts the ecliptic, indicates the position of the sun for that day. The outer circumference may be divided into the number of days of the- year, or thereabouts. The sun's and any star's right ascension, both in time and degrees, may be found in the same manner, viz.: by drawing 1 line from the pole passing through the star to the outer circumference. With the sun a line drawn from the pole to the day of the month on the circumference will give it, though roughly. The inhabitants of the equator behold both the northern and southern poles in the centers of their respective hemispheres. and all the stars of the heavens rise and set to them in the center of each daily rotation of the earth on its axis. . At either terrestrial pole the corresponding celestial pole stands in the zeniti, and one and the same hemisphere is always visible the whole year through. : On the equator, with a seen, but that is all. Since the position of the imaginary pole remains unchanged the whole year through for every point of the earth, the shadow-pin of the sun' dial is always directed to this point. It will be seen from the foregoing diagram, and knowing that the earth rotates on its axis constantly, and at the same time is on its way around the sun and that not more nor less than one-half of the earth can be in the sun's light at a time, that the days and nights are always of equal length (12 hours) at the equator, but at all other places they are of unequal length, except- good clear water horizon, Polaris, when above the true pole, can be ing when the earth is at the equinoxes. In looking for different stars or for the various constellations or groups in the sky, it is best to begin with the Pole Star and Big Dipper. be explained in another paper. equinox, the mean sun will be 3m. 56.5s. behind it, or that many minutes and sec- onds east of your meridian, because mean solar hours are that much longer than sidereal hours, consequently it takes FOg 27 Mole uUeIpliz{ys 27 uo JE2G 2fog Pole Star Ux t eo ® Pole} 1 Z \ s fos . : x * | Gy Wot ny 3 ! \ R Oy 4 - # ON ro \ ; ¥ Si! \ 9 S| 3 \ | g a \ a ds | q Ye" Guar 1 ' gre | by Ww \ h . s 4 S o . x 1a Ml) Thubere 4 Aw. e * Roe AON 4 1 / > S Q ! \ oe aA iN | ve / Q a ! rey be o ' ; 3 hor ea fs ae ~K * 5 oe 7 / 4 x Pole Star on the Meridian above the Pole -- THE BIG AND LITTLE DIPPERS. that much longer to bring the mean sun back to your meridian. At the second rotation of the earth this mean sun, will be twice 3m. 56.5s. behind the point in the vernal equinox, and so on by the same amount day after day, till the end of the sidereal year, when they are again together; thus the vernal equinox has gained a whole day on the mean sun. Now, this separation of the vernal equi- nox from that of the mean sun, is what is known as sidereal time or right as- cension of mean sun. When we say that the right ascension of the mean sun is 2 How to find the other useful navigational stars will hrs. 20 mins. it means that the vernal equinox is that number of hours and minutes from the mean sun, _ that is, ahead or west of it; or in other words, when you have the vernal equinox on your meridian, or when the sidereal clock shows oh. om. os., the sun is 2h. 20m. from your meridian, that is, east of your meridian. Now, what about the position of the real sun during all this time? We know that the imaginary mean sun is supposed to keep as near the true sun as is con- sistent with perfect uniformity of mo- tion, but it is sometimes before and sometimes behind the latter, the greatest difference amounting to rather more than a quarter..of .an hour:. Sinee: the: true sun has a varying motion due to the obliquity of the ecliptic and the unequal motion of the earth in its orbit, the in- tervals between successive returns of the sun to the same meridian cannot be equal. For this same reason the vernal equinox and real sun do not separate from each other by the same amount at each rotation of the earth as do the vernal equinox and mean sun, but their difference is not much. Some days they separate at the rate of 3m. 30s. and other days as much as 4m. 25s. \These are their extremes, the least and the most, so that between these times it those times. The mean or average of these times. for one year will equal 24 hours or one day, the same as with the mean sun. real sun is from the vernal equinox at any day is called the sun's apparent right ascension. The difference between the right ascension of the mean sun and the right ascension of the apparent sun is equal to the equation of time, the same as the difference between mean and ap- parent times, or the difference in time that the real sun is separated from the is between The distance in time that the . 29 imaginary sun. Like the equation of time sometimes the right ascension of mean sun is ahead of the right ascension of the apparent sun, and then other times it is behind it. Unlike the stars the right ascension of the sun is different each day owing to the variable motion of the earth in its orbit.. The stars are such vast distances from the earth that these irregularities are unnoticeable, hence the reason that a star's right ascension. like its declina- tion, is practically always the same. When we say that a star has a right as- cension of 11 hours it means that the» star :n question will come to your meridian at .11 o'clock by sidereal time, not solar time, but sidereal time. There- fore right ascension of the meridian and sidereal time are one and the same thing when used with the stars. Now, how would we determine the right ascension of any star if we did not have recourse to the Nautical Alma- nac, which contains all such data? -We could do it this way: When any star is on the meridian, bearing either true north or true' south of you, note the exact time by sidereal clock, and' what- ever the clock says is the right ascension of that particular star, so that any time thereafter it is necessary to know the time that this star will be on the merid- ian it will only be necessary to refer to this. sidereal time. . Now, the pole star,.. or Polaris, has a right ascension of th- 25m. 8s. so that when the sidereal clock shows th. 25m. 8s. Polaris will: be on the meridian above the pole. The star Dubhe has a right ascension of 10h. 57m. 56s., so too, when the sidereal. clock shows this hour Dubhe-will be on the meridian above the pole, and just so with all the other stars. Now, bear in mind that the sidereal day does not correspond to a-definite hour of - solar mean time, but in the course of a year runs through all the hours of the ordinary day by which the affairs of life are regulated. To test this roughly mul- tiply 365 days by 4 minutes, and the re- sult will be approximately 24 hours. Hence, 366 sidereal days are equal to 365 mean solar days. For confirmation of. this, look in the Nautical Almanac: for . March, and you will find that the sun's right ascension is oh. om. Os. when on the equator. The "Sidereal. Time,' in the last column of Page II for the month, . would: also be oh. om. os. were it not for mean time. Thence, onward, month -- by month, it steadily increases, until in September, when the sun is again cross-. | ing the line, but this time in the oppo- site direction, (the autumnal equinox) the "Sidereal Time" has grown to 12> hours, and continues to grow till it has -attained the maximum of 24 hours in the following 'March. The consequence is that those stars