The Jewish year generally contains 354 days, or 12 lunations of the Moon, but, in a cycle of 19 years, an intercalary month (Veadar) is 7 times introduced, for the purpose of rendering the average duration of the year quite or nearly correct. The Mahometan Era dates from the flight of Mahomet to Medina, July 16th, A. D. 662. The Mahometan year is purely lunar; it consists of 12 synodical periods of the Moon, or of 354 days, 19 times in a cycle of 30 years, and 11 times of 355 days. The average length of this year is therefore 354 days, which differs only thirty-three seconds from the truth; a degree of exactness that only could have been attained by a long series of observations. But as no allowance is made for the excess of 11 days in the length of a tropical year over the time of 12 revolutions of the Moon, it is obvious that in about 33 years, the above months will correspond to every season and every part of the Gregorian year. HEIGHT OF THE GREATEST OR SPRING TIDES IN 1844. Computed by the formula of Laplace (Mécanique Céleste, Vol. II. pp. 289 Paris ed., and [2858] Bowd. ed.) The unit of altitude at any place, is the height at that place of that tide which arrives about a day and a half after the time of New or Full Moon, when the Sun and Moon at the moment of conjunction or opposition are at their mean distance from the Earth, and in the plane of the celestial equator. This unit of altitude, which must be derived from observation for each place, multiplied by the quantities in the above table, gives the height of the spring tides at that place during the present year. By the above table it appears, that the highest tides of 1844 will be those of March 5, April 4, May 3, September 13, October 12, and November 11. The actual rise of the tide, however, depends so much upon the strength and direction of the wind, that it not unfrequently happens that a tide, which would, independently of these, have been small, is higher than another, otherwise much greater. But when a tide, which arrives when the Sun and Moon are in a favorable position for producing a great elevation, is still further increased by a very strong wind, the rise of the water will be uncommonly great, sufficient perhaps to cause damage. The formula, from which these tides were computed, is, however, strictly true only for Brest and its vicinity, and must be regarded as a very uncertain approximation for the coast of the United States. For tables exhibiting the rise of the tide, and the differences between the times of high water at many places on the American coast and at Boston, see the American Almanac for 1840, pages 7, 8, and 9. DARKNESS OF THE NIGHTS DURING THE YEAR 1844. For Boston, New York, Philadelphia, Washington, &c. The number of hours at the top of the page denotes the average time for the month from the end of evening twilight to the beginning of morning twilight. The dots in the table denote the hours of entire darkness, when there is neither sun, moon, nor twilight, and their disposition denotes the hours before or after midnight. 13th day. h. m. Boston, 5 48 m 6 20a 5 48 m 6 24a5 48 m Twilight begins and ends. Mean time. 7th day. 25th day. Begins. Ends. Begins. Ends. Begins. Ends. Begins. Ends. 19th day. h. m. h.m. h. m. h. m. h. m. 6 29a 5 47 m N. York, 5 46 6 22 15 46 7 Su. 7 30 4 43 7 25 4 48 7 19 4 54 7 3 5 10 6 53 5 15 44 25 49 19 45 25 19 50 46 25 51 19 47 24 52 18 48 24 53 18 59 49 24 54 18 5 h. m.h. m. h. m. h. m. h. m. h. m.h. m. 3 3 3 3 3 667722 8 9 3 11 53 3 12 53 2 14 58 2 14 15 58 14 Su. 7 28 4 50 7 23 4 55 7 17 5 1 7 2 5 16 6 58 5 21 21 Su. 7 23 4 59 7 19 5 4 7 14 5 9 7 15 21 6 56 5 26 28 Su. 7 19 5 8 7 14 5 12 7 9 5 16 6 58 5 28 6 53 5 33 888856 26 59 54 27 66668 888 33 33 17 57 23 7 53 5 33 3 53 23 9 16 6 56 5 16 57 24 10 20 80 6 20 57 56 56 55 28 |