LESSONS IN ASTRONOMY. 



absence of them was adduced M an argument against the truth 

 ..I tliii Copernican system ; Galileo, howeTer, on turning hi* 

 telescope to tho pkinut, at once discovered the fact of their 

 existence. 



The period that elapses from one inferior conjunction to 

 another, or that occupied in going through this cycle of changes, 

 is 584 days, and thin is called it* tynodic period. The time, 

 however, it occupies in oompluthitf it* circuit round the HUH, in 

 only U24 days and 17 hours. At tint sight these result* appear 

 inconsistent, but the apparent 

 discrepancy vanishes when wo 

 recollect that tho earth is itself 

 in rapid motion ; BO that by tho 

 time that Venua has completed 

 a revolution round tho HUH, the 

 earth has travelled round a 

 large portion of her orbit, and 

 Venus haa to overtake her bo- 

 fore another conjunction can 

 occur. 



The distance of Venus from 

 the sun is about 66,130,000 

 miles, and its orbit is nearly 

 irrulur, so that its distance 

 varies but slightly. When seen 

 through a good telescope, this 

 planet is a very beautiful ob- 

 ject, especially when near ita 



inferior conjunction, so aa to appear in the form of a crescent, 

 but the brilliancy with which it shines ia so great, that no dis- 

 tinct markings can be made out on ita surface. The inner edee 

 is, however, considerably indented, indicating the presence of 

 inequalities ; from this some observers have calculated that the 

 height of its mountains must be much greater than those of 

 any on the earth. The appearance of its horns seems also to 

 indicate that it possesses an atmosphere. 



Its period of rotation on its axis is not very different from 

 that of the Earth, being 23 hours 21 J minutes, and its axis has 

 an inclination of 73^. Its actual diameter, likewise, closely 

 approaches that of the Earth, being 7,510 miles; so 

 that the planet which is nearest to us is found in many 

 important reapecta to resemble ua very closely, and 

 analogy leads us to infer that, in many other particu- 

 lars it may be a counterpart. No satellite has indeed 

 been as yet 

 discovered ac- 

 companying it. 

 Several obser- 

 vers assert 

 that they have 

 eeen one, but 

 continued ob- 

 eervations, 

 however, by 

 our ablest as- 

 tronomers 

 have failed to 

 corroborate 

 the statement. 



Tho transits 



of Venus have already been referred to, as being of great 

 importance in measuring the sun's distance. The transit 

 of 1882 followed closely on that of 1874, but none will 

 occur again till June 4, 2004. 



THE EARTH. 



The next planet to Venua is tho Earth, which we have already 

 considered as the station of all our observations : wo shall treat 

 of it now as one of the planets revolving around the sun. Its 

 mean distance from that orb has already been given as 91,430,000 

 miles, and it completes ita revolution in this orbit in 365 days, 

 5 hours, 48 minutes, 49 seconds. This period is called a solar or 

 tropical year, and ia reckoned from the time of the sun's pausing 

 the equinoctial point till it again reaches the same spot. Tho 

 sidereal year is reckoned from the time of the sun passing any 

 fixed star till its return to it, and is 20 minutes 21 seconds 

 longer than the solar, the reason of the difference being the re- 

 trograde motion of the equinoctial point, which travels, as it 

 were, to meet the sun, so that he comes to it before ho has quito 



completed hi* circuit. These two periods may be given 



thu* : 



D. . K. B. 



Solar Year 866 5 48 40 



MS 10 



At an early date the year was taken to consist of 865 days. 

 As, however, the solar year is very nearly 3654 day", the date 

 of the equinox soon became wrong: to remedy thi*. Julio* Cesar 

 introduced an additional day into February in every fourth yew, 



thus making that year contain 

 860 days. This arrangement 

 was known a* the Julian Style, 

 and continued in use till nearly 

 the end of the sixteenth cen- 

 tury, but as the year is a few 

 minutes shorter than 365J 

 days, the equinoxes had by thi* 

 time fallen back as much as 

 ton days. Pope Gregory XIII. 

 corrected this error by ordering 

 ten days to bo left oat of the 

 year 1582, and then he modi- 

 fied the Julian Stylo by the fol- 

 lowing rule: Every year divi 

 sible by 4 was to contain 366 

 days ; the even hundreds, how- 



20. ever, unless divisible by 400, 



were to be considered as or- 



dinary years of 365 days : thus 1800 and 1900 are ordinary 

 years, while 2000 will be a leap year. By this means the error 

 is very nearly eliminated. This alteration, which is known as 

 the Gregorian Calendar, was not adopted in England till 1752, 

 and eleven days had then to be struck out of that year to 

 correct tho error, which hod increased one day in the 170 years. 

 In addition to its movement round the sun, the earth rotates 

 on ita own axis, thereby producing the changes of day and night. 

 The interval in which thia rotation is completed, aa ascertained 

 by observing the passage of any star across the meridian on 

 two successive days, is called a sidereal day. It is, in fact, the 

 time occupied by the heavens in making one apparent 

 revolution. In this we have an invariable measure ; it 

 ia, therefore, frequently adopted in the observatory ; 

 but for practical purpoaea of every-day life it would 

 not answer well, aa it ia 3 minutes 55'91 seconds 



shorter than 

 that deter- 

 mined by the 

 sun, and thus 

 clocks regu- 

 lated by it 

 would gain 

 that amount 

 on the sun 

 every day. 



The day, 

 therefore, in 

 ordinary use is 

 that reckoned 

 by the move- 

 ments of the 



sun, and is known as the solar day, being tho interval 

 which elapses between two following meridian passages 

 of tho sun. As, however, the distance of tho earth 

 from the sun varies in different parts of its orbit, and 

 its diurnal rate of motion varies in like manner, this 

 period is not uniform ; ita mean length is therefore ascertained 

 and taken as the natural or mean solar day. Our clocks are 

 all regulated so as to indicate mean solar time, and hence they 

 are sometimes faster than the sun, and sometimes slower. The 

 greatest discrepancies are about February 10th, when the clock 

 ia fifteen minutes faster than true solar time, as indicated by a 

 auii- lial, and October 27th, when it ia sixteen minutea slower. 



Tho axis of the earth is inclined to the piano of its orbit at 

 an angle of 66 32', and hence, as it rotates, the son shines 

 alternately over each pole, thereby producing the varying lengths 

 of the day and night, and also the changes of the seasons. Thia 

 will bo understood by reference to Fig. 21. When tho earth ia 

 in the position A, the sun just shines up to each polo, and one 



