Big Dipper always appear as now? Name three bright stars near the first meridian. (Ans. a Andromedæ, y Pegasi, and § Cassiopeiæ.) How many degrees of longitude correspond to an hour of time? At what rate is Sirius receding from the earth? How has this motion been discovered? (See page 261.) 239. DOUBLE STARS, ETC.-Does any star appear double to the naked eye? How many have been found by the use of the telescope? What is an optical double star? Are all double stars of this class? Describe the revolution of a binary system. What other combinations have been discovered? What are their periods? Orbits? Mass? Are these companion stars as close to each other as they seem? colors ever What is the 241. Name some prominent colored stars. Do their change? Which colors would indicate the hottest star? probable effect in a system having colored suns? 242. What are variable stars? Describe the changes of Algol. Of Mira. What is the cause? 243. What are temporary stars? Describe the one seen in Cassiopeia. The one in Corona Borealis, in 1866? What are lost stars? Can you give any explanation of this phenomenon? Of what did the star of 1866 consist? Are these stars destroyed? Is the process of creation now complete? 245. What are star clusters? Name several. Is such a grouping a mere optical effect? Are they probably as closely compacted as they seem to be? 246. What are nebula? How do they differ from clusters? Is it probable that all nebula will be resolved into clusters? What is the general belief concerning nebula ? What has spectrum analysis proved some of the nebula to be? Where are they most abundant? What can you say about their distances? Into how many classes, for convenience, are they divided? Describe and illustrate the elliptic nebulæ. What is said of the distance of the great nebula in Andromeda ? Αρκτον θ ̓ ἣν καὶ ἄμαξαν ἐπίκλησιν καλέουσιν, Ητ ̓ αὐτοῦ στρέφεται, καί τ' Ωρίωνα δοκεύει, Οἴη δ' ἄμμορός ἐστι λοέτρῶν Ωκεανοῖο, ILIAD, Xviii. 487-9, and ODYS. V. 273-5. *Αρκτοι κυανέου πεφυλαγμένοι Ωκεανοῖο. η ARATUS, PHENOM. 48. "Arctos oceani metuentes æquore tingui." VIRG. GEORG. i. 246. In order to understand the meaning of the expressions πεφυλαγμένοι 'Ωκεανοίo, and "æquoris expertes," as used by a Greek or Italian, we should remember that the north polar distance of Ursa Majoris is 39° 56′ 48′′; and since the latitude of Athens is 37° 58', and that of Naples 40° 50', an inhabitant of the former city would see this star descend below the northern herizon for a small portion of its course; and an inhabitant of Naples would see it sink within 3' of the horizon, so as just to move along its northern edge. The number of stars it contains? Describe the annular nebulæ. What is said of the "ring universe" in Lyra? Its diameter? Describe the spiral nebula in Canes Venatici. Describe the planetary nebulæ. What is said of the number and size of these "island universes"? Describe the fantastic appearance of the irregular nebulæ. 251. What are nebulous stars? What is their structure? What are variable nebulæ? Give instances. What is said of double nebulæ? Is anything definite known with regard to them? What are the Magellanic clouds? 253. Describe the Milky-way. What can you say of the number of stars in the Galaxy? Are the stars uniformly distributed ? 254. What is Herschel's theory of the constitution of the universe? If this theory be true, what is our sun? 255. Give an account of the Nebular hypothesis. Saturn's rings? May they ultimately disappear? What is said of 259. What is spectrum analysis? Name the three kinds of spectra. What colored rays will a flame absorb?* Describe the spectroscope. What are Fraunhofer's lines? What is known of the constitution of the sun? What proof have we that iron exists in the sun? What elements have been found in the sun? What proof have we that the The power which gases possess of cutting out the particular lines which belong to the color that each emits has been beautifully illustrated by Prof. Newcomb. He says: "Suppose nature should loan us an immense collection of many millions of gold pieces, out of which we were to select those which would serve us for money, and return her the remainder. The English rummage through the pile, and pick out all the pieces which are of the proper weight for sovereigns and half-sovereigns; the French pick out those which will make five, ten, twenty, or fifty-franc pieces; the Americans the one, five, ten and twenty dollar pieces, and so on. After all the suitable pieces are thus selected, let the remaining mass be spread out on the ground according to the respective weights of the pieces, the smallest pieces being placed in a row, the next in weight in an adjoining row, and so on. We shall then find a number of rows missing: one which the French have taken out for five-franc pieces; close to it another which the Americans have taken for dollars; afterwards a row which have gone for half-sovereigns, and so on. By thus arranging the pieces, one would be able to tell what nations had culled over the pile, if he only knew of what weight each one made its coins. The gaps in the places where the sovereigns and half-sovereigns belonged would indicate the English, that in the dollars and eagles the Americans, and so on. If, now, we reflect how utterly hopeless it would appear, from the mere examination of the miscellaneous pile of pieces which had been left, to ascertain what people had been selecting coins from it, and how easy the problem would appear when once some genius should make the proposed arrangement of the pieces in rows, we shall see in what the fundamental idea of spectrum analysis consists. The formation of the spectrum is the separation and arrangement of the light which comes from an object on the same system by which we have supposed the gold pieces to be arranged. The gaps we see in the spectrum tell the tale of the atmosphere through which the light has passed us; in the case of the coins they would tell what nations had sorted over the pile."-Newcomb's Astronomy, p. 228. stars are suns? What can you say of the similarity existing between the stars and our earth? What has been discovered with regard to the constitution of the Nebulæ ? Of their relative brightness? How has the proper motion of the stars been shown? 263. TIME.-What two methods of measuring time? What is a sidereal day? What are astronomical clocks? Tell how they are used. Why do astronomers use sidereal time? What is a solar day? What causes the difference between a sidereal and a solar day? To how much time is a degree of space equal? Which is taken as the unit, the solar or the sidereal day? How long is a solar day? A sidereal day? A solar day equals how many sidereal hours? A sidereal day equals how many solar hours? Describe mean solar time. What is apparent noon? Mean noon? The equation of time? When is this greatest? When least? When do mean and apparent time coincide? Can a watch keep apparent time? How may apparent time be kept? How can it be changed into mean time? Tell how to erect a sun-dial. When will a sidereal and a mean-time clock coincide? A mean-time clock and the sun-dial? How did the ancients measure time, before the invention of clocks and watches?* State the two reasons why the solar days are of unequal length. What is the civil day? Who invented the present division? Describe the customs of various nations. What *"The ancients used clepsydræ and sun-dials, to measure time. The clepsydræ, in its simplest form, resembled the hour-glass, water being used instead of sand, and the flow of time being measured by the flow of the water. After the era of Archimedes, clepsydræ of the most elaborate construction were common; but while they were in use, the days, both winter and summer, were divided into twelve hours from sunrise to sunset, and consequently the hours in winter were shorter than the hours in summer; the clepsydra, therefore, was almost useless except for measuring intervals of time, unless different ones were employed at different seasons of the year. The sun-dial was a great improvement upon the clepsydræ; but at night and in cloudy weather it could not be used, of course, and the rising, culmination, and setting of the various constellations were the only means available for roughly telling the time during the night. Indeed, Euripides, who lived 480-407 B. C., makes the Chorus in one of his tragedies ask the time in this form : and the answer is: 'What is the star now passing?' 'The Pleiades show themselves in the east ; It is also on record that as late as A. D. 1108 the sacristan of the Abbey of Cluny consulted the stars when he wished to know if the time had arrived to summon the monks to their midnight prayers; and in other cases, a monk remained awake, and to measure the lapse of time repeated certain psalms, experience having taught him in the day, by the aid of the sun-dial, how many psalms could be said in an hour. When the proper number of psalms had been said, the monks were awakened.”— Lockyer. is the origin of the names of the days?* What is the sidereal year? The mean solar year? What causes the difference? What is the anomalistic year? How did the ancients find the length of the year? What error did they make? What was the result? Give an account of the Julian calendar. The Gregorian calendar. What is the meaning of the terms O. S. and N. S.? What country now uses O. S.? When was the change adopted in England? How was it received? How could a child be eight years old before a return of its birthday? When do the Jews begin their year? Why does our year begin January Ist? Show how the earth is our timepiece. What influence has Jupiter's moons on the cotton trade? CELESTIAL MEASUREMENTS.-These problems are to be used throughout the study. They require no questions but the formal statement of the problem requiring solution. *It is said that the Egyptians named the seven days from the seven celestial bodies then known. The order was continued by the Romans. Tuesday they called Dies Martis; Wednesday, Dies Mercurii; Thursday, Dies Jovis; Friday, Dies Veneris. In the Saxon mythology, Tius, Woden, Thor, and Friga are equivalent to Mars, Mercury, Jupiter, and Venus. Hence we see the origin of our English names. + "As an illustration of the effect of the change of style, we may instance the case of Washington. He was born February 11, 1732, before the change of style. Inasmuch as 1752 began on the 25th of March and ended on the 31st of December, he had no birth-day in that year; hence, he was 20 years old on the 22nd of February, 1753, new style. Because anniversaries are always determined according to the civil calendar, the birth-day of Washington is properly celebrated on the 22nd of February, and not on the 23d, as some have contended, on account of the day dropped in the year 1800."-Peck's Astronomy, p. 216. "In England, from the 14th century till the change of style in 1752, the legal and the ecclesiastical year began March 25. After the change was adopted in 1752, events which had occurred in January, February, and before March of the old legal year, would, according to the new arrangement, be reckoned in the next subsequent year. Thus the revolution of 1688 occurred in February of that legal year, or, as we should now say, in February, 1689; and it was, at one time, customary to write the date thus: February, 1688."-Appleton's Cyclopædia, article on Calendar. GUIDE TO THE CONSTELLATIONS. THE following is a description of the appearance of the heavens on or about the first day of each month in the year. January. (7 P. M.)—In the North, Cassiopeia and Perseus are above Polaris, Cepheus and Draco west, Ursa Minor is below, and Ursa Major below and to the east. In the East, Cancer is just rising, Canis Minor (Procyon) has just risen. Along the Ecliptic, Gemini is well up, then Taurus, Aries-reaching to the meridian, next Pisces; Aquarius (letter Y) and Capricornus are just setting. In the Southeast, Orion and the Hare are well up. In the South, Cetus swims his huge bulk far to the east and west. In the Southwest, is Piscis Australis (Fomalhaut). North of the Ecliptic, the Triangles are nearly in the zenith, Perseus is just east, below is Auriga, Andromeda lies just west of the meridian, and Pegasus is midway; Delphinus (the Dolphin, Job's Coffin), Aquila (Altair), and Lyra (Vega) are fast sinking to the western horizon; while, along the Milky Way, blazes the brilliant cross of Cygnus. February. (7 P. M.)—In the North, Ursa Major lies east of Polaris, Ursa Minor and Draco are below, Cepheus is west, Cassiopeia above and to the west. In the East, Regulus and Cor Hydræ are just rising. Along the Ecliptic, Leo (Regulus, the sickle) just rising, Cancer well up, Gemini midway, Taurus on the meridian, Aries (the scalene triangle) past, Pisces next, and, lastly, Aquarius just setting. In the Southeast, Canis Minor, Canis Major (Sirius), and Orion are conspicuous. In the Southwest, Cetus covers nearly the whole sky. North of the Ecliptic, Perseus is on the meridian, while Auriga is a little east of it; west of Perseus is Andromeda, while the Great Square of Pegasus is fast approaching the horizon. In the Northwest, Cygnus is setting. March. (7 P. M.)-In the North, Ursa Major lies east of Polaris, Draco and Ursa Minor are below, Cepheus is below and to the west, and Cassiopeia west. In the East, Cor Caroli is well up, toward the northeast, and Coma Berenices is rising. Along the Ecliptic, Leo is fully risen, next Cancer, Gemini reaches to the meridian, Taurus is past, Aries midway, and, lastly, Pisces is just beginning to set. In the Southeast, Cor Hydræ, Canis Minor, and Canis Major are conspicuous. In the South, |