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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.

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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. June 21, Length of day........15h. 34m. Length of twilight......1h. 45m. Length of twilight.... 2n. 30m. 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

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*DISTANCE FROM THE EARTH IN LIGHT YEARS OF CERTAIN

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*EXPLANATION.-The term "Light Year" is the time required for light to travel from these bodies to the earth. Their actual distance, in milos, 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 3.6X183,000X365X24X 60X60 miles distant.

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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.

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