Hence, 27'. 12′′,1 : 1°. 5′. 56′′,7 :: 1h. : 2h. 25'. 28" the interval from the beginning to the time of the true conjunction; consequently 21k. 6. 47" (beg.) +2h. 25'. 28" 23h. 32'. 15" for the time of the conjunction at the Observatory of Trinity College, Dublin. = Hence, 27'. 12′′,1 : 1°. 2′. 42',4::1 hour: 2h. 18'. 18",9 the interval from the beginning to the time of the true conjunction; consequently 21h. 39'. 21" (beg.) +2h. 18'. 18",9 23h. 57. 39",9 the time of the true conjunction at Greenwich. Dr. BRINKLEY observes, that in an occultation, or eclipse of the sun, when the calculation is made for the difference of longitudes to be deduced from the beginnings or endings at two places, it will be sufficient to use the equatorial parallax to the nearest second, and not to regard the inflexion and irradiation of light; but when the difference of longitudes is to be deduced from the beginning at one place and the ending at the other, these circumstances ought to be strictly attended to. To find the Longitude by a Time-keeper. 756. Let the Time-keeper be well regulated, and set to the meridian of Greenwich; then if it neither gain nor lose, it will always show the time at Greenwich. Hence, to find the longitude of any other place, find the mean time, either by the sun's altitude or that of a fixed star by Art. 92, or 106, and observe, at the instant of taking the altitude, the time by the watch; and the difference of these times, converted into degrees, at the rate of 15° for an hour, gives the longitude from Greenwich. If, for example, the time by the watch when the altitude was taken, was 6h. 19', and the mean time deduced from that altitude was 9h. 23', the difference 3h. 4' converted into degrees gives 46° the longitude of the place east from Greenwich, because the time at the place of observation is forwarder than that at Greenwich. Thus the longitude could. be very readily determined, if you could depend upon the watch. But as a watch will always gain or lose, before it is sent out its gaining or losing every day for some time, a month for instance, is observed; this is called the rate of going of the watch, and from thence the mean rate of going is thus found. 757. Suppose, for instance, I examine the rate of a watch for 30 days; on |
