« PreviousContinue »
The Ecliptic is that great circle, in which the annual revolution of the Earth round the Sun is
performed. It is so named, because Eclipses cannot happen but when the moon is in or near that circle. The inclination of the Ecliptic and Equinoctial is at present about 23° 28', and by comparing ancient with modern observations, the obliquity of the Ecliptic is found to be diminishing which diminution, in the present century, is about half a second yearly.
The Ecliptic, like all other great circles of the sphere, is divided into 360°; and is further divided into twelve equal parts, called Signs: each Sign, therefore, contains 300. The names and characters of these Signs are as follows: Aries, gr Cancer, Libra, Capricornus, in Taurus, o Leo, 86 Scorpio, m Aquarius, Gemini, o Virgo, me Sagittarius, $ Pisces, ,
Since the Ecliptic and Equinoctial are great circles, they, therefore, bisect each other in two points, which are called the Equinoctial Points. The Sun is in one of these points in March, and in the other in September ; hence, the first is called the Vernal, and the other the Autumnal Equinox-and that sign which begins at the Vernal Equinox is called Aries. Those points of the Ecliptic, which are equidistant from the equinoctial points, are call
. ed the Solstitial Points ; the first the summer, and the second the winter solstice. That great circle which passes through the equinoctial points and the poles of the earth, is called the Equinoctial Colure : and the great circle which passes through the solstitial points and the poles of the earth, is called the Solstitial Colure.
When the Sun enters Aries, it is in the Equinoctial ; and, therefore, has no declination. From thence it moves forward in the Ecliptic, according to the order of the signs, and advances towards the north pole, by a kind of retarded motion, till it enters Cancer, and is then most distant from the Equinoctial ; and moving forward in the Ecliptic, the Sun apparently recedes from the north pole with an accelerated motion till it enters Libra, and being again in the Equinoctial, has no declination; the Sun moving through the signs Libra, Scorpio, and Sagittarius, enters Capricorn ; and then its south declination is greatest, and is, therefore, most distant from the north pole ; and moving forward through the signs Capricorn, Aquarius, and Pisces, again enters Aries : Hence, a period of the seasons is completed, and this period is called a Solar Year.
The signs Aries, Taurus, Gemini, Cancer, Leo, and Virgo, are called Northern Signs, because they are contained in that part of the Ecliptic which is between the Equinoctial and North Pole; and, therefore, while the Sun is in these signs, its declination is north: the other six signs are called Sonthern Signs. The signs in the first and fourth quarters of the Ecliptic are called Ascending Signs : because, while the Sun is in these signs, it approaches the north pole-and, therefore, in the northern, temperate, and frigid zones, the Sun's meridian altitude daily increases ; or, which is the same, the Sun ascends to a greater height above the horizon every day. The signs in the second and third quarters of the Ecliptic are called Descending Signs.
The Tropics are circles parallel to the Equinoctial, whose distance therefrom, is equal to the obli
quity of the Ecliptic. The Northern Tropie touches the Eclip:ic at the beginning of Cancer, and is, therefore, called the Iropic of Cancer ; and the Southern Tropic touches the Ecliptic at the beginning of Capricorn, and is hence called the Tropic of Capricorn.
Circles about the poles of the Equinoctial, and passing through the poles of the Ecliptic, are call. ed Polar Circles; the distance, therefore, of each Polar Circle from its respective Pole, is equal to the inclination of the Ecliptic. and Equinoctial. That Circle which circumscribes the North Pole is called the Artic, or North Polar Circle ; and that towards the South Pole, the Antartic, or South Polar Circle.
That semicircle which passes through a star, or any given point of the heavens, and the Poles of the Ecliptic, is called a Circle of Latitude.
The Reduced Place of a Star is that point of the Ecliptic, which is intersected by the circle of latitude passing through that star.
The Latitude of a Star is that portion of the circle of latitude contained between the Star and its reduced place and is either north or south, according as the Star is between the Ecliptic and the north or south pole thereof.
The Longitude of a Star is that portion of the Ecliptic, contained between the Vernal Equinox and the reduced place of the Star.
Description of the Instruments, reguisite in Astronomical
T is generally allowed that we are indebted to John Hadley, Esq. for the invention, or at least for the first publicaccount of that admirable instrument, commonly called Hadley's Quadrant, whoin the year 1731, first communicated its principles to the Royal Society, which were by them published soon after in their Philosophical Transactions; before this period, the Cross Staffand Davis's Quadrant were the only instruments used for measuring altitudes at sea, both very imperfect and liable to considerable error in rough weather; the superior excellence however of Hadley's Quadrant, soon obtained its general use among seamen, and the many improvements this instrument has received from ingenious men at various times, has rendered it so correct, that it is no'v applied, with the greatest success, to the important purposes of ascertaining both the latitude and longitude at sea, or land.
The Octant or Frame, is generally made of ebony, or other hard wood, and consists of an arch firmly attached to two radii, or bars, which are strengthened and bound by the two braces, in order to prevent it from warping.
The Arch, or Limb, although only the eighth part of a circle, is on account of the double reflection, divided into 90 degrees, numbered 0, 10, 20, 30, &c. from theright towards the left; these are subdivided into 3 parts, containing each 20 minutes, which are again subdivided into single minutes, by means of a scale at the end of the Index. The arch extending from 0 towards the right hand is called the arch of excess.
The Index is a flat brass bar, that turns on the centre of the instrument; at the lower end of the Index there is an oblong opening: to one side of this opening a Nonius scale is fixed to subdivide the divisions of the arch ; at the bottom, or end of the index, there is a piece of brass which bends under the arch, carrying
a spring to make the Nonius scale lie close to the divisions ; it is also furnished with a screw to fix the Index in any desired position.
Some instruments have an adjusting or tangentscrew, fitted to the Index, that it may be moved more slowly, and with greater regularity and accuracy than by ti e hand; it is proper, however, to observe, that the Index must be previously fixed near its right position by the above mentioned screw, before the adjusting serew is put in motion.
The Nonius is a scale fixed to the end of the Index for the purpose, as before observed, of dividing the subdivisions on the Arch into Minutes ;
it sometimes contains a space of 7 degrees, or 21 subdivisions of the limb, and is divided into 20 equal parts; hence each division on the Nonius will be one-twentieth part greater, that is, one minute longer than the divisions on the Arch ; consequent