122 FORMS AND DIVISIONS OF TIME. That bright eternal beacon, by whose ray QUESTIONS.-1. What is said of the first division constellations? 2. Why do not the constellations & zodiac coincide? 3. What is the present difference 4. At what rate does the change take place? 5. D tion of the polar star. LESSON 56. Forms and Divisions of As the form of the year is various tions, so is its beginning. The Jev tions of the East, had a civil year, the new moon in September; and which commenced from the new Persians begin their year in the June; the Chinese, and most o begin it with the first moon in the new moon that follows the lo America, the civil or legal year twenty-fifth of March, and the - und sixhundred s regular - an easy urtes keep www-shutter, - it be obchimney en if the and fifty-six the night Two seconds e machine goes s evidently not stantaneous, au a second at ummencement of conomical month? 6. Solar day? real days are all of January. But since the alterationen the lengths place in 1752, the civil year in space through one of the signs stantly travels through the and sixty-five days five o solar month is found b Contain thirty days, t sua said to be slow of has sean time? 13. Vhas follows in conWhat is an easy was seep true time? the eccentricity clearly illustratas been recomse statement of PLANETARY SYSTEM. 125 LESSON 57. The Planetary System. IR star of eve, thy lucid ray Directs my thoughts to realms on high; Great is the theme, though weak the lay, For my heart whispers God is nigh.' The Sun, vicegerent of his power, Mercury and Venus first appear, Then Mars is seen of fiery hue; Mars soon his revolution makes, In twice twelve months the sun surrounds; Jupiter, greater limit takes, And twelve long years declare his bounds. With ring of light, see Saturn slow, 122 FORMS AND DIVISIONS OF TIME. That bright eternal beacon, by whose ray The voyager of time should shape his heedful way. QUESTIONS.-1. What is said of the first division of the stars into constellations? 2. Why do not the constellations and signs of the zodiac coincide? 3. What is the present difference between them? 4. At what rate does the change take place? 5. Describe tne situation of the polar star. LESSON 56. Forms and Divisions of Time. As the form of the year is various among different nations, so is its beginning. The Jews, like most other nations of the East, had a civil year, which commenced with the new moon in September; and an ecclesiastical year, which commenced from the new moon in March. The Persians begin their year in the month answering to our June; the Chinese, and most of the inhabitants of India, begin it with the first moon in March; and the Greeks with the new moon that follows the longest day. In England and America, the civil or legal year formerly commenced on the twenty-fifth of March, and the historical year on the first of January. But since the alteration of the style, which took place in 1752, the civil year in both countries has likewise begun on the first of January. The principal division of the year is into parts called months, which are either astronomical or civil. An astronomical or natural month is that which is measured exactly by the motion of the Earth or Moon, and is accordingly either lunar or solar. A lunar month is the time the moon takes to revolve round the earth, which she performs in twenty-seven days, seven hours, and forty-three minutes. A solar month is that space of time in which the earth runs through one of the signs of the zodiac; as the earth constantly travels through the twelve signs in three hundred and sixty-five days five hours and forty-nine minutes, each solar month is found by dividing this number by twelve, to contain thirty days, ten hours, and twenty-nine minutes. EQUATION OF TIME. 123 Civil months are those which are framed to serve the uses of life, and approach nearly to the quantity of astronomical months either lunar or solar; being made, with the exception of February, to consist of thirty and thirty-one days. To the days of a week, the Pagans gave the names of the sun, moon, and planets; and for the first two days and last day of our weeks, those names are still retained. A natural or solar day is the time which the sun takes in passing from the meridian of any place till it comes round to the same meridian again; or it is the time from noon to noon. A sidereal day is the time in which the earth revolves once about its axis. The rotation of the earth is the most equable and uniform motion in nature, and is completed in twenty-three hours, fifty-six minutes, and four seconds, for any meridian on the earth will revolve from a fixed star, to that star again in this time. Sidereal days, therefore, are all of the same length; but solar or natural days are not. The mean length of a solar day is twenty-four hours, but it is sometimes a little more, and sometimes less. The reason of the difference between the solar and sidereal day is, that as the earth advances almost a degree eastward in its orbit, in the same time that it turns eastward round its axis, it must make more than a complete rotation before it can come into the same position with the sun that it had the day before; in the same way, as when both the hands of a watch or clock set off together, as at twelve o'clock, for instance, the minute hand must travel more than a whole circle before it will overtake the hour hand, that is, before they will be in the same relative position again. It is on this account that the sidereal days. are found to be, on an average, shorter than the solar ones by three minutes and fifty-six seconds. As a clock is intended to measure exactly twenty-four hours, it is evident that, when a solar day consists of more than twenty-four hours, it will not be noon by the sun till it is past noon by the clock; in which case the sun is said to be slow of the clock. But when a solar day consists of less than twenty-four hours, it will be noon by the sun before it is noon by the clock; and the sun is then said to be fast of the clock. Time measured by a clock is called equal or mean time, and that measured by the apparent motion of the sun in the heavens, or by a sun-dial, is called apparent time. The adjustment of the difference of time, as shown by a 124 EQUATION OF TIME. well-regulated clock and a true sun-dial is called the equation of time. Since the stars are found to gain three minutes and fifty-six seconds upon the sun every day, amounting in a year to one diurnal revolution, it follows that, in three hundred and sixty-five days as measured by the sun, there are three hundred and sixty-six days as measured by the stars. This regular return of the fixed stars to the meridian affords an easy method of determining whether our clocks and watches keep true time. For if through a small hole in a window-shutter, or in a thin plate of metal fixed for that purpose, it be observed at what time any star disappears behind a chimney or the corner of a building at a small distance; then if the star disappears the next night three minutes and fifty-six seconds sooner by the clock or watch than it did the night before, on the second night seven minutes fifty-two seconds sooner, and so on, it is a certain sign that the machine goes right; but if it does not observe this rule, it is evidently not accurate, and as the disappearing of a star is instantaneous, we may depend upon this information to half a second at most. QUESTIONS.-1. What is said of the form and commencement of the year among different nations? 2. What is an astronomical month? 3. Lunar month? 4. Solar month? 5. Civil month? 6. Solar day? 7. Sidereal day? 8. How does it appear that sidereal days are all of the same length? 9. Why is there a difference between the lengths of a solar and sidereal day? 10. When is the sun said to be slow of the clock? 11. Fast of the clock? 12. What is mean time? 13. Apparent time? 14. Equation of time? 15. What follows in consequence of the stars gaining upon the sun ? 16. What is an easy method of determining whether clocks and watches keep true time? [NOTE. The inequality of solar days, as caused by the eccentricity of the earth's orbit, and the obliquity of the ecliptic, is clearly illustrated in Wilkins' Elements of Astronomy: the work has been recommended as containing a judicious selection and concise statement of the leading facts and principles of the science.] |