in 



N0\'EMBER 1, 1898.] 



KNOWLEDGE. 



251 



estimated to be of the eleventh magnitude in August, from 

 which, and its distance from the sun and earth at the 

 time, we deduce that its diameter is some seventeen to 

 twenty miles. It is not likely in any case to exceed 

 twenty-five miles, so that when nearest to us its disc will 

 only subtend to us an angle of about ),", a quantity too 

 small to be measurable even in the largest telescopes. It 

 will, however, at such times, shine as a star of between 

 the sixth and seventh magnitudes, and may thus be visible 

 to keen eyes. It will at its nearest approach be situate in 

 Cancer, which is a very convenient position for northern 

 observers. If its density be assumed the same as 

 that of the moon, its mass is only about j^.ijoi.ouo o^ 

 hers, which is an altogether inappreciable quantity in 

 astronomy. 



We now naturally inquire when a favourable opposition 

 wiU next occur. For this purpose we must have the 

 planet at the nearest point to the sun, i.e., in perihelion, 



wards. The following list of perihelion passage was thus 

 deduced : — 



1894, January 21st. 



1895, October 28th. 

 1897, August 3rd. 

 1899, May 9th. 



1901, February 12th. 



1902, November 19th. 

 1904, August 25th. 

 190(>, May 31st. 

 1908, March 7th. 



1909, December 11th. 

 1911, September Kith. 

 1913, June 2l9t. 

 1915, March 2Hth. 



1917, January 3rd. 



1918, October 9 th. 

 1920, July 14th. 

 1922, April I9th. 

 1924, .January 2uth. 



AscffutiHii .Yoih 



Relative Disposition of the Orbits of Mars, DQ, and the Earth 



and the earth in the longitude of the planet's perihelion, 

 which is 122° 17'. Now, on reference to the " Nautical 

 Almanac," we find that the earth passes this longitude on 

 or about January 22nd in each year (the longitude of the 

 sun as seen from the earth is ISC' greater, or 302°). 



We therefore seek a year in which the planet passes 

 through its perihelion on or about January 22nd. We 

 find one perihelion passage by reckoning backwards half 

 the period, or 322-4 days from the aphelion passage on 

 1898, June 20th, and then we can find others by taking 

 successive intervals of 644-7 days backwards and for- 



We thus see that four and a half years ago an exception- 

 ally favourable opposition occurred. It is much to be 

 regretted that the planet was not detected on that occasion, 

 and it would be worth while for any who possess photo- 

 graphs of Cancer or its neighbourhood taken in January, 

 1891, to examine them carefully for traces of the planet. 

 It was moving south about 1 'i degrees per day, crossing the 

 ecliptic about January 21st, 

 * near longitude 122 degrees. 



An equally favourable op- 

 position will not occur till 

 1924 ; it may be noted here 

 that the planet's period is 

 almost exactly ^'.'ths of the 

 earth's ; hence it performs 

 seventeen revolutions while 

 the earth performs thirty, 

 and after this period its 

 motions nearly repeat them- 

 selves. We may find the 

 synodic period, or average 

 interval between two op- 

 positions, as follows : — In 

 thirty years the earth gains 

 thirteen revolutions on the 

 planet ; hence it gains one 

 revolution in ,' " years, which 

 is equal to two years and 

 one hundred and twelve 

 days. The next time that 

 the earth overtakes the 

 planet will bo in November, 

 1 900, when we shall approach 

 it more nearly than on any 

 other occasion till the year 

 1917 ; its least distance 

 from the earth will be some 

 thirty-one millions of miles, 

 which, although more tlian 

 double what it was in 1894, 

 is yet considerably less than 

 that of Mars at its nearest. 



The great value to as- 

 tronomers of such a near 

 approach lies in the means 

 it gives for improving our 

 knowledge of the sun's 

 distance, the fundamental unit of the solar system. It 

 has already been recognized that the minor planets 

 which approach us most closely afford a better means of 

 determining this than does Mars, in spite of its smaller 

 distance. The method adopted consists of measuring with 

 a heliometer the distances of the planets from a number 

 of neighbouring stars, the measures being made alternately 

 with the planet east and west of the meridian, so that the 

 observer has been carried in the interval by the earth's 

 rotation through a distance of several thousands of miles, 

 and the planet thus appears alternately on one side and on 



