ASTRONOMY. 



[DEFINITION'S. 



competent judge*, deserves to be ranked among the most 

 brilliant triumph* of the human mind. 



In tln> til-Id of telescopic research, the labour* of Sir 

 \V. Ht-r-fc-lifl deserve particular nn-nt ion. His discovery 

 of the planet Uranus and its satellites, as well as his 

 investigation on Saturn's rings bin theory of tii.- motion 

 of the solar system in space and it* direction, which 

 hare been verified recently by several independent re- 

 searches, are sufficient monuments of his skill and 

 industry. 



In France, the cultivation of practical astronomy in 

 the eighteenth century was well sustained by the cele- 

 brated astronomers Lalande and La Caille, the latter 

 having observed the stars of the southern hemisphere 

 about the epoch 1700 ; the publication and reduction o 

 which, have since been undertaken by the British Asso- 

 ciation. He is also known as having determined the 

 constant of lunar parallax, by corresponding muridiona 

 observations at the Cape of Good Hope and at European 

 stations. Besides these important undertakings, he was 

 an energetic calculator of ephemerides, and useful in- 

 vestigations on refraction, the figu-y of the earth, <fcc. 



Lalande is known as an industrious and indefatigable 

 observer. His immense catalogue of stars bears suf- 

 ficient evidence of his unremitting industry for many 

 yean. The catalogue of stars formed from his observa- 

 tions, have been reduced and published by the British 

 Association. 



In Sicily, the labours of Piazzi have been principally 

 occupied in an extensive catalogue of stars, the epoch ol 

 which is the beginning of the present century, and whicli 

 is now one of the standard works of the day. His zeal 

 was rewarded by the discovery of Ceres among the 

 asteroids, in 1801. The discoveries of these bodies, 

 during the last ten years, are too numerous to be men- 

 tioned here. 



The progressive improvements of instruments during 

 the last century, have been rewarded by increased accu- 

 racy of results. At Greenwich, the mural arc of Flam- 

 steed is now replaced by the magnificent transit circle ; 

 and the aid of galvanism has been brought to bear on an 

 essential and delicate element of time viz., that of 

 right ascensions. All these circumstances produce a cor- 

 responding improvement in theory. Successive compari- 

 sons of theory and observation engage the attention of 

 our analysts ; and to this we are in a great measure 

 indebted for the important discoveries of the various 

 inequalities, which have enriched the science of astro- 

 nomy. 



ASTRONOMICAL DEFINITIONS. 

 Having thus shown how the investigating disposition 

 of man has been occupied in its endeavour to obtain 

 a more correct and perfect knowledge of the universe, 

 we proceed to explain the laws which have been estab- 

 lished. 



Astronomy is usually divided into three parts : 

 I 1. SPHERICAL ASTRONOMY, which teaches the knowledge 

 of the various points and circles of the celestial sphere, 

 the constellations, the position of the stars with respect 

 to these points and circles, and the phenomena occurring 

 in the sphere of the heavens. 2. THEORETICAL ASTRO- 

 MOMY, which enables us to determine, from observation, 

 the path of the heavenly bodies. 3. PHYSICAL ASTRO- 

 HOMY, which gives the laws by which the heavenly bodies 

 are regulated, teaches how their motions are to be calcu- 

 lated according to the rules of mechanics, and combines 

 all that is known of their physical characters. Without 

 the- formality of so dividing our subject, which our space 

 does not permit, we shall endeavour to combine all the 

 useful and practical portions of the subject. 



In order to facilitate the study of the heavens, artificial 

 representations have been made, similar to those of the 

 surface of the earth. These are called celestial globes, 

 on which the stars are depicted in their natural positions, 

 the observer being supposed to bo in the centre, viewing 

 them in the concave surface. 



To represent the apparent diurnal motion of the 



heavenly bodies, the celestial globe must be turned from 

 east to west. 



Circlet. To designate with precision the situation of 

 the sun, moon, and stars, imaginary circles have been 

 considered as drawn in the heavens, most of which 

 respond to, and are in the same plane with similar circles 

 supposed to bo drawn for similar purposes on the surface 

 of the earth. If a line be drawn on the sphere (Fig. 1) 

 of the earth PQ, the plane cuts the surface of the sphere, 

 and forms a great circle E E, which is the celestial equa- 

 tor, the sphere being divided in this circle into two 

 hemispheres, in which one of the poles forms a <vntr.il 

 position ; these form the northern and southern he- 

 mispheres. 



A plane is that which has surface, but not thickness. 

 The plane of a circle is that imaginary surface which the 

 circle bounds. 



The axis of the earth, P Q, is an imaginary lino pass- 

 ing through its centre, north and south, about which its 

 diurnal revolution is performed ; the poles of the earth 

 are the two extremities of the axis, where it is supposed 

 to cut the surface. The axis of the heavens is the 

 earth's axis, produced both ways to the concave of the 

 sky ; the poles of the heavens are two imaginary points 

 exactly above the terrestrial poles. 



Great circles are those which divide the globe into two 

 equal parts, as the equator, the ecliptic, and the colures. 

 RR, SS, and TT (Fig. 1) 

 are small circles which divide 

 the globe into two unequal 

 parts, as the tropics, polar 

 circles, and parallels of lati- 

 tude. 



Every circle is supposed 

 to be divided into 360 equal 

 parts or degrees. A degree 

 is further subdivided into 

 CO equal parts or minutes ; 

 and a minute into CO se- 

 conds. Degrees are marked , 

 minutes', seconds*. The 

 space included by a degree of a great circle in the 

 heavens, is equal to nearly twice the appareut diameter 

 of the sun or moon, when considerably above the 

 horizon. 



The equator of the earth, E E (Fig. 1), is an imaginary 

 great circle passing round the globe, east and west, 

 everywhere equidistant from the poles, dividing it into 

 northern and southern hemispheres. The equator of 

 the heavens, or the equinoctial, is the plane of the ter- 

 restrial equator extended to the concave surface of the 

 heavens, and called the equinoctial, because, when the 

 sun appears in it, the days and nights are equal all over 

 the world. 



The Ecliptic is the via gnlis, or sun's path, the great 

 circle which he appears annually to descnbe among the 

 fixed stars ; though, more properly, it is the track which 

 the earth actually describes among the stars, as viewed 

 from the sun. The ecliptic is so called, because solar 

 and lunar eclipses only can happen when the moon is in 

 or very near this circle. It cuts the equinoctial ob- 

 liquely at two opposite points, making an angle with it 

 of 23 J, which is called the obliqttity of the ecliptic. 

 One half lies on the north side of the equinoctial ; the 

 other half on the south side. The points of crossing are 

 the equinoctial points. A zone or girdle extending 8 

 on each side of the ecliptic, or 16 in breadth, is the 

 zodiac, in which are the orbits of all the planets, with 

 ;he exception of some of the asteroids. The ecliptics 

 and zodiac are divided into twelve equal parts, called 

 signs, each containing 30. Their names, with the days 

 on which the sun enters them, are as follows : 



Northern signs, being north of the equinoctial. 



Spring Sign*. 

 Aries, the Ram, March 21. 

 I'aurus, the Bull, April 1!>. 



. the Twin*. M..v '.' > 



nttr Signs. 



r, the Crab, Juno 21. 

 Leo, the Lion, July 23. 

 Virgo, the Virgin, Aug. 22. 



