Work for the Telescope. 373 



WORK FOR THE TELESCOPE.— PLANETS OF THE 

 MONTH.— DOUBLE STARS. 



BY THE EEV. T. WEBB, F.R.A.S. 

 PLANETS OF THE MONTH. 



Theee may be a fair chance of getting a sight of Mercury in 

 the evening twilight at the beginning of the month, as he 

 attains his greatest elongation from the Sun, and his dicho- 

 tomy, or half-moon pkasis, on the 6th. A position, however, 

 near the horizon is seldom favourable for the employment of those 

 high powers which are requisite to make out details in an object 

 at that time scarcely 8" in diameter. Those who possess good 

 instruments equatorially mounted may, of course, always find 

 him (except when too near the Sun) at a more suitable altitude, 

 as his brightness renders him visible throughout the day ; and 

 it is in their power to supply one of the desiderata of planetary 

 astronomy, the confirmation of the phenomena discovered by 

 Schroter and Harding at the beginning of this century. These 

 consisted chiefly in a difference in the shape of the two cusps, 

 and in the occasional presence of dusky spots and belts, whence 

 Bessel deduced a rotation in 24h. Om. 53s. Schroter also per- 

 ceived slight irregularities of form not only in the terminator 

 or boundary of day and night, but in the circular limb, and a 

 difference between the breadth of the observed and calculated 

 phasis, which last has been confirmed by Beer and Madler. 



Venus is still conspicuous in the mornings, but is becoming 

 gibbous and comparatively uninteresting. 



Jupiter is passing away towards the west. The following 

 transits may be looked for, though the planet is getting more 

 distant and smaller, his diameter at the end of the month being 

 reduced to 32" -6. 



2nd. Shadow of I. goes off at 9h. 41m. 9th, I. and its shadow, 

 and III., are all on the disc together. I. emerges at lOh. 21m., 

 III. at 11m. 20m., the shadow at 11m. 36m. 12th, Shadow of 

 II. departs at lOh. 25m. 15th, Shadow of IV. enters 9h. 44m. 

 16th, I. is on the disc from 9h. 59m. to 12h. 16m. III. will be in 

 transit at the same time, emerging at lOh. 19m. ; the shadow 

 of I. enters at llh. 15m. 19th, Shadow of II. enters at lOh. 16m., 

 the satellite 15m. later. 25th, Shadow of I. goes off, 9h. 55m. 

 26th, II. enters, lOh. 20m. Some of these, it will be seen, are 

 interesting configurations. 



Saturn continues to turn to us the dark side of his ring. 



DOUBLE STAES. 



The rapid advance of summer twilight has somewhat over- 

 taken our work, and we are compelled to defer some remark - 

 vol. i. — no. v. c c 



