before sunrise. When the line on which this 18° is measured is shortest then the twilight will be the shortest, and vice versa. This causes the shortest twilights to occur in the winter months in northern latitudes, and longest in summer. By an examination of the chart we find the measure of twilight, darkness and daylight of the 24 hours to be: Jan. 1, Length of day....... .8h. 54m. Length of twilight......1h. 45m. June 21, Length of day..15h. 34m. The remainder of the space will more than make the 24 hours, which is due to refraction of light, which causes an increase in the apparent length of the day, as also of duration of the twilights. This apparent error is greatest in the winter months, as the Sun then shines through a denser and deeper atmospheric envelope. THE HORIZON AT SEA LEVEL. Distances of objects visible at varying elevations from one foot to one thou *DISTANCE FROM THE EARTH IN LIGHT YEARS OF CERTAIN DEGREES OF TEMPERATURE AT WHICH CERTAIN SUBSTANCES FREEZE, FUSE AND BOIL. EXPLANATION. -The term "Light Year" is the time required for light to travel from these bodies to the earth. Their actual distance, in miles, being too great for use or comprehension, the distance, in miles, may be found by multiplying the velocity of light (183,000 miles per second) by the number of seconds in a year. and that product by the light years. Thus: Alpha Centauri = 3.6×183,000×365×24× 60X60 miles distant. Note. The time for the phenomena given in the table above is for the geocentric event, and as seen from the earth's surface the error may be as much as 21m., more or less; but the given time is near enough for all purposes and places, Char les sets. ton. H.M.S. H. MH. MH. MH. MH. MH. MH.MH. MH. MH. MH.MH. MH. MH. MH. MH. MH. MH.M 1 1 Tu 22W ..12 3 24 7 37 431 619 7 80 4 38 6 32 ev 257 25 443 6 37 832 7 19 449 6 41 7 3 5 5 6 54 8 43 12 3 53 7 37 4 32 7 19 730 4 39 7 30 1 4 7 25 4 44 7 34 9 5 7 19 4 50 738 73 5 6 7 49 9 20 33 Th .. 12 4 21 7 37 4 33 8 23 7 30 4 40 8 31 1 43 7 25 4 45 8 34 9 44 7 19 451 8 37 73 5 7 8 46 10 00 4 4 Fr ..12 4 48 737 4 34 9 34 7 30 4 41 9 40 2 27 7 25 4 46 9 42 10 26 7 19 452 944 7 3 5 7 9 50 10 42 5 5 Sa .. 12 5 16 7 37 4 35 10 34 7 30 4 42 10 37 3 11 7 25 4 47 10 38 11 12 7 19 4 52 10 39 7 3 5 8 10 43 11 27 66S ... 12 5 42 7 37 4 36 11 43 7 29 4 43 11 43 4 2 7 25 4 48 11 43 ev 3 7 19 4 53 11 43 7 4 5 9 11 43 ev 16 7 7 Mo 12 6 9 737 437 mo. 729 444 mo. 457 7 25 449 mo. 12 58 719 454 mo. 745 10 mo. 112 88 Tu 12 6 35 7 36 438 12 52 7 29 4 45 12 49 5 56 7 24 4 50 12 48 159 7 19 455 12 47 74 5 11 12 44 2 12 99W .. 12 7 0 736 439 2 5 7 29 446 159 656 7 24 451 157 3 8 7 19 456 155 74 5 12 1493 16 10/10 Th ..12 7 25 736 440 3 21 7 29 447 3 11 7 58 7 24 452 38 421 719 457 3 5 7 4 5 12 2 56 4 22 11 11 Fr ..12 750 736 441 438 7 29 448 4 26 858 7 24 453 422 5 30 7 19 458 4 18 7 3 5 13 4 6 5 26 12 12 Sa 12 8 14 735 4425 53 728 450 5 39 957 7 23 454 5 35 6 33 7 18 459 530 735 14 5 16 6 26 18 13 S 12 837 735 444.659 728451 6 46 10 52 7 23 455 6 41 731 7 18 5 0 636 7 3 5 15 6 217 21 14/14 Mo 12 90734 445 sets 7 28 4 52 sets 11 447 23 456 sets 826718 5 1 sets 7 3 5 16 sets 8 13 15 15 Tu 16 16 W 1717 Th 18 18 Fr 19 19 Sa .. 12 9 22 734 446 635 727 453 6 46 mo. 722 457 649 9 19 718 5 2 653 73 5 17 7 4 94 |