PERPETUAL CALENDAR. The first seven letters of the alphabet, in numerical order, have long been used in almanacs as Dominical letters, or Sunday letters, to indicate what day of the week the year begins on. The days on which consecutive years begin are in the reverse order of the Sunday letters, A, B, C, D, E, F, G, and these show the days of the week for January 1 as follows: A, Sunday; B, Saturday; C, Friday; D, Thursday; E, Wednesday; F, Tuesday, and G, Monday. If the year is a leap year, there must be two letters for it; the left hand letter must be used for January and February only, and the other letter used for the other ten months. A table of Dominical letters affords a very simple and ready method of finding the day of the week, of any day of any month, in any year. Table 1 gives the Dominical letters for the Gregorian style, now in general use, beginning in the Catholic states of Europe in 1582. This was not adopted by England until 1752, and by Russia not until 1901. The Julian Calendar, or old style, as now designated, was founded by Julius Cæsar, and began with the year 45 B. C. It assumed the civil year to be just 365 days long, which was 11 minutes too much, and every fourth year was of 366 days. In 1582 this error amounted to 10 days, and October 5 was counted the 15th. The history of the world from 45 B. C. to 1582, over 1,600 years, is very largely recorded in the old style dates. That these dates may be put into weekdays for the convenience of students of history, Table 2 is given above. There are no dates in this style prior to 45 B. C. Table 3 can be used in both old and new style. In this table the Sundays in any month are under the Dominical letter for the year given, and In the vertical column opposite to the month given, and, once knowing the Sundays, the ether days are mentally obtained. Example: On what day of the week did Columbus discover America (San Salvador), October 12, 1492? This being an old style date, Table 2 must be used. Under the Centurial 1400, and at the right of 92 are the letters AG (a leap year). In Table 3, under G, the 14th of October is found to be Sunday, and hence the 12th was Friday, the day sought. When the given year has two letters, the first applies only to January and February. Therefore G is used in this case. Example: On what day of the week was Abraham Lincoln shot, April 14, 1865? This is a new style date, and Table 1 must be used. Under the Centurial year 1800, and opposite 65 is A, and in Table 3, under A, the 16th of April is found to be Sunday, hence the 14th was Friday, the day wanted, and by a strange coincidence, it was "Good Friday" of church days. DAY CALCULATING TABLE. For ascertaining number of days between any two days within two years. 1 2 3 368 399 427 488 366 397 425 579 610 640 519 549 580 611 641 Explanation.-Ascertain by Table I the number of days from the first of the two days and from Table II the number of days from the second, and deduct the former from the latter. If between the two dates a February of twenty-nine days should occur add one to each number of days after February 28. Example. How many days are there between October 20 and June 15? Against October 20 you will find 293; against June 15, in Table II, 531. 531-293-238, the answer. 1 1 TURNING MOSLEM INTO CHRISTIAN TIME. The following ready, though sufficiently accurate, methods are useful: First-Deduct 3 per cent from the Moslem years, add to the answer 622. Thus, taking the year of the Hegira, 1328: Three per cent to be deducted gives 40; 40 from 1328 leaves 1213; then add (22, and that makes 1910. Second-Multiply 970,224 by the year of the Hegira, cut off six decimals from the product and add (21.5774. The sum will be the year of the Christian Era, and the day of the year will be found by multiplying the decimal figures by 365. The result nay sometimes differ a day from the truth, as the intercalery days do not occur simultaneously; but as the day of the week can always be accurately obtained from cther tables the error, if any, can be readily adjusted. Example: Required, the date on which the year 1362 of the Hegira begins: 970,224×1,362=1,321.445088 (after cutting off six decimals). To this add 621.5774-1,943.0225. The decimal multiplied by 365 gives 8. Thus the date is the 8th day, or Eth day of January, of the year 1943. From other tables it is known that January 8, 1943, falls on a Friday. FACTS ABOUT THE SUN. Distance and Size.-The sun's mean distance from the earth (always reckoned from centre to centre) is 92,885,000 miles, with a variation between January 1 and July 3 cf 3,100,000 miles, owing to the ellipticity of the earth's orbit. The diameter of the sun or the length of the line passing through its centre from one side to the other is $67,000 miles. If a railroad were laid round the sun a train moving seventy miles an hour would take five years, without intermission, to make the journey. A Journey around the earth at the same rate would take only a fortnight. Weight. The sun weighs 330,000 times as much as the earth. But, bulk for bulk, thirteen million earths would go to make up one sun. It follows from this that the matter composing the sun must on an average be about one-fortieth as dense as the matter composing the earth, or less than one-seventh as dense as water; so it may be fairly concluded that the sun is a gaseous body. Heat. It has been calculated that the heat thrown on a square mile exposed at noon under the equator would melt in an hour 26,000 tons of ice. This amount has to be multiplied fifty million times to arrive at the quantity of heat received by the earth's surface during a single hour. Yet this cnormous supply is even less than one two-thousand-millionth part of what the sun pours forth in all directions in space. There are various theories to account for the enormous store of heat and light in the sun. The theory now generally accepted by physicists is that the gradual contraction of the solar orb in cooling is the chief source of the sun's apparent inexhaustible energy. It has been calculated that at the present rate of expenditure of heat the sun's diameter would contract four miles in a century, and in a few millions of years it may become as dense as the earth. The sun is composed of very much the same materials as the earth, except that they are at a much higher temperature. About forty of the seventy terrestrial elements have been identified by the spectroscope as existing in the vapors around the sun. Astronomers think that they find traces there of very few substances not already known. Even these may be discovered on the earth some day. Motion.-The sun revolves on its axis in about twenty-two days: but apparently It takes two days longer than this, owing to our own globe's motion in its orbit. This period of revolution has been computed from observation of sunspots, it having been noted that a certain spot, vanishing at one edge of the solar disk-or "disappearing 'round the corner," as one may say-returns to the same position in the period named. The other real motion of the sun is the almost imperceptible one through space, in which "grand march" the members of the solar system accompany him-earth, moon, planets, planetoids and some comets. Whence this movement is directed is still a subject of discussion FACTS ABOUT THE MOON. Distance and Velocity.-The greatest distance of the moon from earth's centre is 252,600 miles, its least distance from earth's centre 221,700 miles, its least distance from earth's surface 217,740 miles. It travels in its orbit with a velocity of 3,334 feet a second, and its equatorial velocity is 10 miles an hour. Sidereal and Synodic Revolutions of the Moon.-When the moon, after passing a star, completes a revolution so as to come back to the same star, it is said to make a sidereal revolution. The average time required for this is 27 days 7 hours 48 minutes and 11.46 seconds, but owing to her motion in common with the earth around the sun the mean duration of the lunar month, that is the time between successive new moons is a little longer, i. e., 29 days 12 hours 44 minutes and 2.87 seconds, which is called the synodic revolution. Nodes.-The shape of the moon's orbit is approximately an ellipse whose two axes are nearly of equal length, but an ellipse will represent its orbit for a very short time. Owing to the inclination of the earth's equator to the ecliptic, the moon is sometimes seen, at the full, coursing along a circle which passes near the zenith In these latitudes and sometimes, in the same phase, along an arc low down in the southern sky, but there is a residual effect, which is due to the inclination of the moon's orbit to the plane of the ecliptic, amounting to 5° 8', so that during one-half of her orbit she is south of the sun's annual path and during the remaining half north of it. The points where she crosses the ecliptic are known as her nodes. Rotation. The moon's rotation on its axis agrees in period with its revolution around the earth, so that we have always the same side presented to our view. Occasionally, however, we see a little around one or the other edge. This phenomenon is known as libration in longitude. Phases. The moon's changes in shape from a crescent to a full disc are due conjointly to the globular form of the moon, its motion and the fact that it does not shine by its native light, but simply reflects the solar rays. The illuminated (or convex) edge of its figure is always turned toward the sun. When right opposite the sun it appears as full, and sometimes is so situated as to be partially obscured by the earth's shadow. When it is near the sun in the sky it appears as a thin crescent, turning almost entirely its dark side to the earth. Sometimes, at new moon, it comes between us and the sun, obscuring his disc either in a partial or total eclipse. At either half moon the moon is said to be in its quadrature, or in the "first" or "last quarter." At new and full moon it is said to be in syzygy (Greek syn, "together"; zygon, "yoke"). These changes result from the constant darkness of one side of the moon, and constant brightness of the other, the crescent being larger or smaller as, from the moon's change of position, more or less of the bright side is seen from the earth. Eclipses. Whenever the earth gets between the moon and the sun, cutting off the light of the latter, a so-called eclipse of the moon takes place. An eclipse of the moon occurs only at full moon. During a lunar cycle there will be, on the average, twenty-nine eclipses of the moon. The Harvest Moon.-If the plane of the moon's orbit coincided with that of the earth's equator, the moon would rise about fifty minutes later each day, but owing to the inclination between these planes to one another this retardation is quite different at different times. This retardation may be reduced to nothing when in the northern latitude full moon occurs near the autumnal equinox, so that for several nights the full moon rises about the same time soon after sunset. At or about the time of harvest in the northern temperate zone the sun in its annual course is approaching the celestial equator, which it crosses from north to south on September 22. On that date it sets close to the exact western point of the horizon. If it happens to be also full moon, the moon rises that evening as the sun sets, or close to the exact eastern point of the horizon. Thus it begins to give light at sunset, and continues to do so until sunrise, when it sets opposite the sun, just as the latter rises. This arrangement holds good without any great change for several days, so that there is practically no darkness, especially if the weather is fine. The full moon which thus illumines the autumn night is called the harvest moon. The hunter's moon is the next full moon after the harvest moon; the same phenomenon, less marked, occurs. Tides.-The chief cause of the tides is the attraction of the moon, which, affecting most strongly the side of the earth nearest to it, draws or heaps up the waters in the parts of the earth successively turned toward it. At the same time the moon attracts the bulk of the earth, and, as it were, pulls the earth away from the water on the surface furthest from it, so that here also the water is raised, although not quite so much as on the nearer side. The waters being thus heaped up at the same time in these two parts of the earth, and the waters situated half way between them being thus necessarily depressed, two high and low tides occur in the period of a little more than one revolution of the earth on its axis. When the sun and moon are in conjunction or opposition, at times of new and full moon, their tidal waves will be superposed crest upon crest, and the effect will be what is called "spring tide'; when they are in quadrature the lunar tide will be partially neutralized by the solar tide, and the result will be a "neap tide." Size, Volume, Mass, Density.-The moon's diameter is 2,163 miles, a little more than a quarter of the earth's. Its surface is therefore 0.074 of the earth's, or, in square miles, about 14,657,402. The earth taken as a unit, the moon's density is 0.63; mass, 1/so, volume 1/50: that is to say, it would require the materials of 80 moons to form our globe; the earth is 50 times larger than the moon and its density is in the ratio of 10 to 16. Its smaller size and mass cause gravity to be only % of the terrestrial attraction; the same exertion which would lift a given weight here would raise a weight six times as great there, and a body instead of falling 16 feet in the first second would fall only 2% feet. Light, Temperature.-Like the earth, the moon has no light of its own, but receives all from the sun, and its day-the interval from sunrise to sunrise-is a month. At full moon it sends to us about 1-600,000 part of the light given by the midday sun. Physical Conditions, as Seen Through the Telescope. The surface of the moon is totally unlike that of our earth. All the details are hard, cold and glaring in their delineations. All are marked in white and black or in various shades of yellowish gray. Nothing like mist, cloud or water has ever been seen. The so-called seas on the moon are simply portions of the surface darker in color than the and very much broken up by craters and mountain ranges. Nor is there any evidence average of an atmosphere. Observation of the stars suddenly occulted by the moon, as well as the spectroscope, confirms this, and if there be even an attenuated atmosphere it cannot have more than 1/200 of the surface density of our own. there is no vegetation, no life. The mountain ranges, called the lunar Alps, Appenines, In consequence Cordilleras, etc., range from 20,000 feet in height downward; the lunar rills, clefts or cracks in the surface pass often right through mountains and valleys, sometimes for a distance of 300 miles, their breadth being relatively so small as to give them the appearance of true cracks. The whole aspect suggests volcanic action on lunar surface in remote ages, but nothing like an active volcano has ever been seen. the FACTS ABOUT THE EARTH. The total area of the earth is about 197,000,000 square miles, and its total population 1,626,000,000. The area of the water of the earth is about 145,000,000 square miles. The area of the land of the earth is about 52,000,000 square miles. 'The figures of population, excepting those for the United States, are taken from the Year Book of the Bureau des Longitudes. Including all islands in the Eastern Indian and Southern Pacific oceans. *Including population in the Dutch East Indies. The largest states, comprising parent country and colonies or possessions, are: According to the number of inhabitants, the countries range as follows: British Empire and Colonies....403,000,000 China 50,000,000 Austria-Hungary .350,000,000 Netherlands 44,000,000 38,000,000 United States. 98,000,000 [Italy 36,000,000 France 81,000,000 Belgium and the Congo. 27,000,000 German Empire. 78,000,000 [Spain 20,000,000 Japan and Corea. 62,000,000 The longest rivers in the world are: In Europe, Volga, about 2,200 miles; in Asia, Yenisei, about 2,700-3,000 miles, and Yang-tse-Kiang, about 3,000; in Africa, Nile, about 3,240 miles; in North America, Mississippi and Missouri, 4,300 miles; in South America, Amazon and Beni, 4,000 miles; in Australia, Darling, more than 2,345 miles. The largest lake in the world is Lake Superior. It covers an area of 31,200 square miles and has a mean depth of about 475 feet. The greatest cataract in the world, surpassing by far Niagara and Zambezi Falls, is on the Ignazu River, which partly separates Brazil from Argentina, one thousand miles by boat from the nearest settlement. The precipice over which the river plunges is 210 feet high, that of Niagara being 167 feet. The cataract is 13,123 feet wide, or about two and a half times as wide as Niagara. It is estimated that 100,000,000 tons of water passes over Niagara in an hour; a like estimate gives the Falls of Ignazu 140,000,000 tons. The oldest city in the world is Damascus, in Syria. The exact date of the founding of this city, once so famous for its manufacture of silks, jewelry and blades, is |