280 GEOGRAPHY AND ASTRONOMY. Thus May 10th' in the morning, if the altitude be near 48 de, green, the azimuth from the south will be 60, and the hour 26 minutes past 9.- Here Note, That to find the sun's hour or azimuth by his altitude, you should never seek it too near noon, because then the altitude alters so very little for two hours together. ProblemXXIII. " When the sun is due east or west in summer, how to find the hour, and his altitude." Rectify as before ; then bring the quadrant to cut the east or west point of thehorizon, and turn theglobe till the sun's place in the ecliptic meet the edge of the quadrant. Thus the quad- rant will shew the altitude, and the index will point to the hour; thus May10th in the afternoon, the sun will be due west at about 56 minutes past 4; and its altitude will be near 26 degrees. This is called the vertical altitude by some writers. Thus if the place and day be known, and either'the hour, the azimuth, or the altitude be given, you may easily find the other two. ProblemXXIV. " To find the degree of the depression of the suri below the horjzon, or its azimuth at any given hour of the night." Observe the place of the sun, suppose May 21st in the first degree of Gemini, then seek his opposite place in the ecliptic at half a year's distance, (viz.) the first degree of Sagittary on the 23d of November; this being done, seek the altitudes, the azimuths, and the hours just as you please for that day, and they will shew you what are the sun's depression, azimuths, and hours on the 21st of May at night.* Problem XXV. " To find how long the twilight continues in any given place and given day, suppose the 21st of May at London, both at morning and evening." The way to answer this question is to .seek how many hours or minutes it will be after sun set, before the sun be deprest 18 degrees below the horizon in that placeon the 21st of May. And so before sun rise for the morning twilight. This is best per- formed by seeking how long it will be after sun rise or before sun set on the 23d of November, that the sun will have 18 degrees of altitude, which is done by the foregoing problem. Note, That from the '26th of May to the 18th of July at London, there is no dark night, but constant twilight. For during this space the sun is never deprest above 18 degrees be- low the horizon. >K Bote, The reason why we use the opposite part of the globe, to find the degrees of depression of the sun, is because the wooden horizon is so thick that we cannot conveniently see, observe, or compute the distance of de- pression from the upper edge of it, which edge is the true representative of the real horizon.
RkJQdWJsaXNoZXIy OTcyMjk=