the earth's orbit reached by the earth a month later; the least distance from the earth of Lexell's Comet (1770) was little more than seven times that of the moon; and it is probable that the earth passed through the tail of the Great Comet of 1861. Yet these near approaches produced no perceptible effect unless a certain yellow phosphorescent glare, noticed by some observers (June 30), was due to the presence of the comet of 1861.

10. Q. Why are the Periods of many Comets so uncertain?

A. Though often accomplished, the accurate prediction of a comet's return is a work of much difficulty; for it is frequently uncertain whether the curve described is a parabola or an ellipse, because a comet is only visible in that part of its orbit in which its path would be nearly the same for both curves. Three complete observations generally enable astronomers to determine the elements of an elliptic or a parabolic orbit.

11. Q. What are the Elements of a Comet's Orbit? A. The Elements of an Orbit are certain quantities from which the form and situation of the orbit, and the position of the body in the orbit, may be determined. They are the Time of the Perihelion Passage; the Longitude of the Perihelion; the Longitude of the Ascending Node of the Orbit; the Perihelion Distance; the Inclination of the Orbit to the Plane of the Ecliptic; and, in an elliptic orbit, the Eccentricity. Whether the motion is Direct or Retrograde must also be known (Lessons 2, 6, 31).

LESSON 25. PERIODIC COMETS: HALLEY'S, ENCKE'S, BIELA'S, LEXELL'S.

1. Q. Who first predicted a Comet's Return?

A. The Return of a Comet-that of 1682-was first predicted by Dr. E. Halley, who, having calculated the orbits of 24, found the paths of two (1531 and 1607), and suspected those of others, to agree with the orbit of the comet which now bears his name. This comet had previously taken 76 and 75 years to return, but Halley allowed 77 years for its next return, because on coming back it would be delayed by getting too near to Jupiter (Lesson 21).

2. Q. What was the date fixed for the Return of Halley's Comet?

A. About the year 1759 was the time named by Halley, and as this time approached the comet's period was carefully calculated by Clairaut and Lalande, assisted by Madame Lepaute. In Nov. 1758 Clairaut announced that the comet was being delayed 618 days-100 by Saturn and 518 by Jupiter and that it would pass its perihelion about the middle of April, 1759-a month either way being allowed for certain small quantities neglected in the calculation.

3. Q. How far were the predictions verified by this and the subsequent Return of Halley's Comet?

A. Astronomers eagerly watched for the returning wanderer, but it was first seen (December 25, 1758), without even a telescope, by a Saxon farmer; and it passed its perihelion (March 12, 1759) within the time predicted. On its next return, in 1835, the comet was seen on the 5th

of August within 1° of the place predicted for that day, and its perihelion passage, variously fixed by calculation for November 4th, 7th, 11th, and 26th, actually occurred on the 16th of Nov.

4. Q. What Other Comets have returned as predicted?

A. Among these are Encke's comet, discovered in 1795 with a period of about 3 years; Biela's, with a period of over 6 years; Faye's, discovered in 1843 with a period of nearly 7 years; and D'Arrest's, discovered in 1851, with a period of 6 years. In 1857 D'Arrest's comet returned within half a day of the time predicted, and again in 1870; the return of Encke's and Biela's (1845-6) comets, and the failure to return of Lexell's "Lost Comet," have excited much curiosity.

5. Q. Why has the return of Encke's Comet excited curiosity?

A. Because, at each successive return, its period becomes 2 hours shorter, so that the comet gets nearer to the sun, and, if the shortening continues,

it

may in time fall on his surface. This comet's period is supposed to be shortened by the resistance of a very rare ethereal medium pervading the regions in which it moves-a resistance which, by diminishing the velocity, would lessen the centrifugal force, and increase the sun's power to draw the comet nearer (Lesson 27).

6. Q. How would other Members of the Solar System be affected by such a resisting medium?

A. There are philosophers who maintain that the friction of the ether is actually shortening the periods and lessening the mean distances of all the planetary bodies, and that it is destined

ultimately to precipitate comets and planets alike on the body of the sun. But the periods of the planets are not known to have changed, and other explanations of the shortening of a comet's period do not necessarily point to such a catastrophe.

7. Q. What other Explanation of the Phenomenon is possible?

A. Part of the matter, which escapes in streams and jets from a comet when near its perihelion, may be driven off to distances from which it can never return to the comet; and since the lost matter seems to be of a nature to be repelled from, not attracted by, the sun, what remains may be more energetically attracted to the sun than was the whole comet before it lost the repulsive matter. Such a lessening of its substance, at each return, might shorten the comet's period.

8. Q. How would the Comet's Ultimate Destruction be averted?

A. If that part of the matter, driven from the nucleus into the tail in opposition to solar attraction, is of a nature to be repelled from the sun, the waste of the comet may only continue till all the matter so repelled is lost, after which the shortening of the comet's period would cease. Sir J. Herschel concludes that a comet's period may thus be shortened "until at length the whole of the repulsive matter is got rid of."

9. Q. By what Force is the Matter from the Nucleus driven into the Tail of a Comet?

A. It is described with reference to the sun as "a repulsive force enormously more powerful than the attractive force we call gravity." The

I

tail of the comet of 1843, which was whirled round unbroken through an angle of 180°, in little more than two hours, could hardly be the same tail; and if not the same, this force must have continually driven from the comet, with an amazing velocity, fresh matter for new tails, nearly 100 millions of miles long (Lesson 23).

10. Q. What was there to excite Special Curiosity in the Return of Biela's Comet in 1845?

A. This small comet, after returning regularly from 1826, astonished astronomers in December, 1845, by splitting into two complete comets which continued their journey together; but each apparently strove to outshine the other, for first the old, afterwards the new, and then again the old comet excelled in brilliancy, and this superiority the old comet retained till both disappeared. The two returned in 1852-11 millions of miles apart-and were again due in 1866 but failed to appear.

11. Q. What has become of Lexell's Lost Comet?

A. This comet, discovered in 1770, should have returned in 1776, but was not then seen, possibly through its passing its perihelion in the daytime. In 1779 it got so near to Jupiter that his attraction, being at the comet's distance 200 times that of the sun, threw the hapless wanderer into a new orbit, and it has, consequently, never since been seen. Jupiter is accused of having once before dragged Lexell's Comet from its path into a new orbit.

12. Q. Which are the most Remarkable Comets of the Present Century?

A. Among these are the Great Comet of 1811