. A new treatise on the practice of navigation at sea : containing all the details necessary to enable the mariner to become a good practical navigator. . test atthe Horizon. Again, to an observer at the surface, the Moon would appear at a, but to an observer at thecentre of the Earth she would be at t. b«r true place in the Heavens. The difference between these twoplaces is called her Parallax in Altitude. The Sun and Planets being at a greater distance from the Earthhare only a very small parallax. S and P represent the Parallax of the Sun and Planet. When a bodyu in the Horizon its Parallax
. A new treatise on the practice of navigation at sea : containing all the details necessary to enable the mariner to become a good practical navigator. . test atthe Horizon. Again, to an observer at the surface, the Moon would appear at a, but to an observer at thecentre of the Earth she would be at t. b«r true place in the Heavens. The difference between these twoplaces is called her Parallax in Altitude. The Sun and Planets being at a greater distance from the Earthhare only a very small parallax. S and P represent the Parallax of the Sun and Planet. When a bodyu in the Horizon its Parallax is greatest The Suns Parallax is only 9, while the Moons Parallax isabove 1* sometimes. But when a body is in the Zf-nitb Mh Parallax is 0, because it is seen in the sameline from the centre as from the surface as at Z. The Suns Parallax in Altitude is given in Tabls VI. The Moons Horizontal Parallax, which is in perpetual change, and the Parallax of the Planet*, ar*fiven in the Nautical Almanac. M NAUTICAL ASTRONOMYDIAGRAM, Showing the Manner of Measuring the Altitudes of the Heavenly Bodies ai Sea, and the Correction ftp Semi-diameter. Fio. This figure repreient* the different methods of observing th^ Altitudes of the Sun and Moon by bringinfIheir upper or lower limbs in contact with the Horizon. No. 1 is an Altitude of the Suns lower limb brought in contact with the Horizon. This it the usualmethod practised at Sea, being the most simple and correct mode of doing it. His semi-diameter addedgiTea his observed Central Altitude. No. S ii an Altitude of the Suns upper limb brought in contact with the Horizon. This is only resortedto in the event of the lower limb being hidden by clouds. His semi-diameter subtracted gives hia observeaCentral Altitude. No. 3 is an Altitude of the middle of the Sun brought down to the Horizon. This kind of observationu only used when his limbs are so ill-defined, in consequence of the sky being overcast, as in the case whenhe shines through
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectnavigat, bookyear1902
