## An elementary treatise on astronomy |

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aberration according altitude angle angular annual apparent Astronomical axis body called cause centre CHAP circle circumstance clock computed consequently correction corresponding declination deduced described determined difference direction Earth ecliptic effect epoch equal equation error exactly explained expression fact Figure fixed former formula given greater greatest Greenwich half happen Hence horizon increase inequality instance interval known latitude latter least less longitude lunar mean measure meridian method minutes Moon Moon's motion move nearly node north polar distance nutation object obliquity observations obtained orbit parallax passing period planet pole position preceding precession present proper quadrant quantity radius reason reduced refraction represent respectively right ascension round seen sidereal solar spectator star star's Sun's supposed Tables telescope term theory transit true variation Venus wire zenith distance

### Popular passages

Page 373 - Observer' at a salary of 100£ per annum, his duty being 'forthwith to apply himself with the most exact care and diligence to the rectifying the tables of the motions of the heavens and the places of the fixed stars, so as to find out the so much desired longitude of places for the perfecting the art of navigation.

Page 235 - ... south when it came about six in the morning. " Though I have since discovered that the maxima in most of these stars do not happen exactly when they come to my instrument at those hours, yet not being able at that time to prove the contrary, and supposing that they did, I endeavoured to find out what proportion the greatest alterations of declination in different stars bore to each other ; it being very evident that they did not all change their declinations equally.

Page 446 - Astronomers, with a view of obtaining a convenient and uniform measure of time, have recourse to a mean solar day, the length of which is equal to the mean or average of all the apparent solar days in a year. An imaginary Sun, called the mean Sun, is conceived to move uniformly in the Equator with the real Sun's mean motion in Right Ascension...

Page 10 - M', M", &c. and E may, for small portions near those points, be held as parallel. Nn is the line of the nodes, that is, the intersection of the plane of the Moon's orbit with the plane of the ecliptic, or the plane of the Earth's orbit round the Sun.

Page xxvii - We must, however, be content, at present, to take for granted the truth of the hypothesis of the earth's rotation. If it continues to explain simply and satisfactorily other astronomical phenomena than those already noted, the probability of its being a true hypothesis will go on increasing. We shall never indeed arrive at a term when we shall be able to pronounce it absolutely proved to be true. The nature of the subject excludes such a possibility.

Page 42 - Let AD represent a telescope fixed, as it is represented in the figure, to an horizontal axis formed of two cones. The two small ends of these cones are ground into two perfectly equal cylinders : which cylindrical ends are called Pivots. These pivots rest on two angular bearings, in form like the upper part of a Y, and denominated Y's. The Y's are placed in two dove-tailed brass * We may, for the above purposes, use the Sun and observe his equal altitudes and azimuths. As we cannot pretend to bisect...

Page 28 - In this case so that these instruments can be read off by the aid of their verniers to an accuracy of 10 seconds. The verniers occupy on the limbs spaces equal to 9° 50'.

Page 409 - I) apside or aphelion A. The condition given, is the time elapsed from the planet's quitting A ; the result sought is the place P ; to be determined either by finding the value of the angle AEP, or by cutting off, from the whole ellipse, an area AEP bearing the same proportion to the area of the ellipse which the given time bears to the periodic time. There are some technical terms used in this problem which we will now explain. Let a circle AMB be described on AB as its diameter, and suppose a point...

Page 261 - I am of opinion, that if it were 1" I should have perceived it, in the great number of observations that I made, especially of y Draconis; which agreeing with the hypothesis (without allowing any thing for parallax) nearly as well when the sun was in conjunction with, as in opposition to, this star, it seems very probable that the parallax of it is not so great as one single second; and consequently that it is above 400,000 times farther from us than the sun.

Page 232 - ... in its situation. By the middle of April it appeared to be returning back again towards the north; and about the beginning of June, it passed at the same distance from the zenith as it had done in December, when it...