MERCUHY. 95 



The sun is accompanied by a phenomenon called the zo- 

 diacal light. It is a beam of light of a triangular form, visi- 

 ble a little after sunset and before sunrise, with the base 

 towards the sun. It is most clear about the beginning of 

 March in the evening, and in September in the morning, 

 but in the torrid zone it is constantly seen. It is generally 

 supposed to proceed from the sun's atmosphere. 



Questions.— 1. What is the figure of the sun? 2. Describe the 

 motions of the sun. 3. How i.s it made evident that the sun has a 

 rotation round his axis.'' 4. What is said of the spots that have been 

 seen in the sun .'' 5. What does Dr. Herschel consider the sun to be ? 

 — The spots ? 6. Describe the zodiacal light. 7. In what proportion 

 do the planets receive light and heat from the sun ? {see Appendix.) 

 8. What rule is given ? 9. What is said of the attraction of bodies ? 

 10. What is the rule for finding the distances of the planets from the 

 sun? II. What was ascertained by Kepler? 12. What is the rule 

 for finding how many times one planet is greater than another? 

 [Note. When any body, revolving round the sun, is nearest to him, it is 

 said to be in its perihe'lion ; and when it is most distant, in its aphe'- 

 lion (pron. af-e'le-un.) The common centre aboui vh'ch the sun re- 

 volves in its periodical motion is always found to be exceedingly near 

 the sun, and most commonly within it : it may, therefore, without any 

 material error, be regarded as the centre of the planetary system.} 



LESSON 43. 



Mercury and Venus. 



Elonga'tion, a planet's elongation, or its angular distance from the 

 sun, IS an angle formed at the earth by two lines, one drawn 

 from the earth to the sun, and one from the earth to the planet. 



Disk, the face of the sun and 'moon, as it appears to us on the 

 earth. 



Mercury is seldom visible to the inhabitants of the earth, 

 for its greatest apparent distance from the sun, or its great- 

 est elongation, is not more than twenty-eight degrees, and 

 its reflected light is absorbed in the more powerful rays of 

 the sun. He always appears on the same side of the heavens 

 with the sun ; of course, he can be seen in the east, only in 

 the morning a little before sunrise, and in the west in the 

 evening a little after sunset. When viewed with a telescope 

 of high magnifying power, he exhibits nearly the same pha- 

 ses as the moon, and they are to be accounted for in the 

 same manner. Mercury revolves round the sun at nearly 



