ASTRONOMICAL PROGRESS IN 1899. 



order to deal most effectively with stellar phe- 

 nomena a field of much greater width was neces- 

 sary than from 10 to 15 degrees, as formerly prac- 

 ticed. But not until the publication of Mr. Dall- 

 meyer's stigmatic lens, in 1897, was one able to 

 chart wide sky areas without great distortion 

 near the margin of the field. A photograph taken 

 by Mrs. Maunder, of England, with a stigmatic 

 lens only U inch in diameter and 9 inches in 

 focal length, has been published. The field shown 

 is 37 degrees on a side and 50 degrees diagonally. 

 A set of 40 plates of the same angular aperture 

 as the foregoing, but of larger scale, would pro- 

 vide a more complete chart of the heavens than 

 any that we now possess in a small compass and 

 at very little expense. The work lies very easily 

 within the powers of many small private ob- 

 servatories. 



The Planets. No remarkable advance in our 

 knowledge of the planets was made during the 

 year. Prof. Schiaparelli has published his fifth 

 memoir on the planet Mars, without increasing 

 our knowledge of the cause of its surface mark- 

 ings, which has excited the world during the past 

 two decades. The true explanation of the 

 changes visible on his surface at different sea- 

 sons, the duplication of the so-called canals, 

 which some astronomers, with indifferent optical 

 appliances, profess to see easily, while others 

 with far superior means can not see them, seems 

 as far off as ever. The gemination of the canals 

 is believed by the majority of astronomers to be 

 an optical illusion, inherent in some eyes, espe- 

 cially if wearied by overwork, the defect being 

 in the eye, and not in the telescope. Some ob- 

 servers say they see markings on Mercury and 

 Venus, and thence deduce conclusions as to their 

 rotation periods. Herr Leo Brenner says he has 

 conclusive evidence that Mercury rotates on its 

 axis once in thirty-three days and forty-five min- 

 utes. The general opinion that this period is 

 but about twenty-four hours has long held sway. 

 A day on Venus has been supposed to be about 

 the same as ours, yet some observers now .assert 

 that her period of rotation is 224.7 days. In the 

 presence of such discrepant conclusions it must 

 be held that nothing is positively known as to 

 the period of either planet, and the same remark 

 applies to the periods of Uranus and Neptune, 

 and all satellites except our own. 



Satellites of Mars. No. 7, Vol. IX, of the 

 Transactions of the British Astronomical Asso- 

 ciation contains some new and interesting facts 

 regarding the satellites of Mars, and, as they 

 are totally unlike anything known elsewhere in 

 the solar system, a few are briefly noticed: 



Phobos. This little speck of a world is only 

 10 or 15 miles in diameter (some estimate it at 

 7 miles), revolves around Mars in 7 h 39 m , and 

 therefore must move across the sky at the rate 

 of 47 an hour. But, as it rises in the west and 

 sets in the east, its apparent motion across the 

 sky will be the difference between its own and 

 the rotational velocity of Mars, which is 14 30' 

 an hour, or 32 30'. Owing to the satellite's 

 rapid motion, combined with its large parallax, 

 it is above the horizon at the equator for 4 h 15 m , 

 and below it for 6^ 45^. Owing to the combined 

 motion of both satellite and planet, it requires 

 llh gm f or Phobos to return to the same meridian, 

 during which time it goes through all the phases, 

 from new Moon to full, and from full to new, 

 that our Moon does in twenty-nine and a half 

 days. As the length of a night on Mars is 12^ 18 m , 

 Phobos can be seen by the same observer twice 

 full and once new^ or once full and twice new. 

 The eclipses of both the Sun and the satellite 



occur with great frequency. So rapid is the phase 

 change of Phobos that at a zenithal central 

 eclipse, which lasts 54 m , the satellite may be seen 

 to enter the shadow in a phase a little short of 

 full and emerge a little past full, each phase 

 being 0.937. When Phobos is in the zenith it 

 appears more than twice as large as on the 

 horizon. If Phobos and Deimos are exactly in 

 the plane of the planet's equator, Deimos will be 

 occulted by Phobos about every ten hours, the 

 moons approaching each other from opposite di- 

 rections. The average distance of Phobos from 

 the surface of Mars is only 3,730 miles. Assum- 

 ing that the satellite turns to Mars the same 

 phase as does our Moon to the Earth, the planet 

 will appear immovably fixed in the sky, with a 

 disk one thousand times larger than is shown 

 by our Moon. A Martian astronomer (if there 

 is one) could with a powerful telescope bring the 

 satellite within one mile. 



Deimos. This satellite completes a revolution 

 round the planet in 30 h 15 m , rising, as does our 

 Moon, in the east and setting in the west, travel- 

 ing across the sky at the slow rate of 2 45', 

 equal to the difference between its own hourly 

 velocity of 11 53' among the stars and the hour- 

 ly rotational velocity of the planet of 14 37'. 

 If the satellite is supposed to rise when full, it 

 can be seen twice full and twice new before it 

 sets. In the zenith Deimos from the surface of 

 Mars will appear about one third larger than 

 when in the horizon. Its diameter is uncertain, 

 but is considered to be from 10 to 15 miles, too 

 small ever to eclipse the Sun totally, though it 

 will cross the solar disk about 130 times during 

 a Martian year. Of course, it must occasionally 

 happen that both satellites will transit the Sun 

 at the same time, both passing across the sky 

 from west to east, Deimos slowly and Phobos 

 rapidly. If it be assumed that Deimos turns 

 the same face to Mars, which is probable, the 

 planet would appear fixed in the sky, the stars 

 sailing by in regular order. As Deimos is more 

 distant from the surface of Mars than is Phobos, 

 it will appear somewhat smaller, but would still 

 appear vastly larger than our Moon appears to us. 



Of Jupiter little is known that was not pre- 

 viously recorded. Mr. Denning, of England, has 

 examined his rotation period as given by dark 

 and bright spots in the region of the equator, 

 which furnishes a mean period of 9 h 50 m 23 s , 

 against 9*i 50 30s as previously adopted. The 

 period of the famous red spot, which is still 

 faintly visile, from 17,414 revolutions, is 9 h 55 m 

 39.4 s , but is not uniform. The variation is not 

 great perhaps not greater than might be ex- 

 pected from the nature of the observations. 



Nothing but ordinary, routine observations 

 have been made on the planets Uranus and N Q T>- 

 tune to determine their periods of rotation, but 

 Dr. E. E. Barnard, of the Yerkes Observatory, Ms 

 made a series of elaborate observations with the 

 mammoth telescope on Neptune's satellite, the 

 measures of which have enabled Prof. Hall to" 

 improve the elements of its orbit. He finds the 

 mass of Neptune to be TTTS'^T' that of the Sun 

 being 1. 



Asteroids. Since the last report the follow- 

 ing asteroids have received numbers to replace 

 the provisional letters given in the last volume: 



PL. 

 DM. 



DO. 



429 

 430 



432 



DP 437 



DQ 433 



DR 434 



DS 435 



Since then the following have been discovered 

 and received provisional letters. Some .of these 



