THE EXPECTED RETURN OF HALLEY'S COMET 555 



bined approaches that had such a notable effect on the period. 

 The orbit of Saturn was crossed in February 1839, that of 

 Uranus just five years later; the orbit of Neptune, the most 

 distant known planet, 30 units from the sun, was reached in 

 December 1856. For sixteen more years the comet receded, 

 reaching its greatest distance, 35 units, early in 1873. It then 

 commenced its return journey, again crossing Neptune's orbit in 

 April 1889. Thus nearly half the periodic time is expended 

 in covering the small arc lying outside Neptune's orbit. The 

 similar arc in the sun's neighbourhood is covered in two years, 

 which gives a vivid idea of the great change in the speed. 

 The orbit of Uranus was crossed in March 1902, that of Saturn 

 in February 1907. Jupiter's orbit will be crossed in April of 

 this year, but the comet has already made its nearest approach 

 to that planet, which on this occasion is 5 units. 



We come now to the question, When is it likely to be visible 

 (i) with the telescope, (ii) with the naked eye? In 1759 it was 

 seen from yy days before perihelion to 102 days after; in 1835 

 from 102 days before to 185 days after. It must be remembered 

 that not only are large telescopes more numerous than formerly, 

 but photography has given us the power of recording intensely 

 faint objects, that would be wholly invisible even in the largest 

 telescopes. Consequently, it was thought not unlikely that the 

 comet might be photographed even some months ago ; however, 

 no certain trace of it has yet been found on the plates, though 

 there are some suspicious objects awaiting verification. If not 

 found by mid-March we must wait till the autumn, as the region 

 of Orion, where it now is, gets too near the sun to permit of 

 long-exposure photographs. It is almost a certainty that it will 

 then be found. Dr. Smart has prepared an ephemeris for 

 its motion in 1910, a copy of which is given. It is based on 

 the supposition that the perihelion passage will be on April 16, 

 1910, a date which I consider the most probable, though it is 

 right to add that some authorities give dates as much as two 

 months later. It will be nearly stationary in Pisces in January 

 and February, and probably visible with small telescopes, 

 possibly even with the naked eye. It will then pass behind the 

 sun, being a fairly bright morning-star in April, and the begin- 

 ning of May. It will again pass the sun on May 17, and for a 

 few days after this may be expected to be at its greatest 

 splendour, being only 12,000,000 miles from the earth. In 



