Page images
PDF
EPUB
[graphic][subsumed][merged small]

With the 40-inch refractor of the Yerkes Observatory, 1898 July 7, 9h. 20 m.-E. E. Barnard.

Cassini division and producing thus a kind of "black drop" effect by the junction of the black shadow with the black space.

1898 February 26. The shadow of the ball on the rings was not a uniform curve, but of an angular outline, as shown in the

[merged small][graphic]

Form of the shadow of the ball on the Ring of Saturn, 1898 February 26.

sketch. The north pole was dark-not black. The ball was clearly seen through the crape ring.

1898 March 6. The shadow of the ball on the rings preceding seemed "squarey." The north pole was dark.

1898 April 20. There was no definite dark polar cap such as was seen last year, but the polar regions were dark. The planet was beautifully seen through the crape ring. The shadow of the ball on the rings looked "squarey."

1898 July 7. The trace of the crape ring was hazy at both edges, or rather the inner edge of the bright ring was ill defined where it crossed the ball, but it was well defined on the sky between the ring and the ball. The crape ring was easily seen and well defined on the sky, but faint where it approached the ball. The two northern belts were more in contrast, or darker near the limbs of the planet. The inner bright ring was brightest toward the Cassini division, where the brightness was rather narrow. I could not see with certainty any division in the outer ring. There seemed to be a dusky shading where the Encke division is usually shown. The polar cap was darker than the darkest part of the ball.

The definition was superb. I have never seen the planet better, nor have I seen so much detail upon it before. The belts, as shown on the drawing (Plate 11), were seen with certainty. The ball was easily visible through the crape ring.

1899 April 7. 161. A heavy diffused belt at the equator. The north pole seemed dark. Seeing poor.

1899 April 25. The north pole was not very dark. The black, well-defined cap was not present. It was a little dusky at the pole.

1901 October 21. 17h 50m. There was no polar cap. There seemed to be a luminous appearance near the north pole, at a point inside the following limb near the shadow; seeing fair, but planet very low.

1904 June 17. north of the equator. 1904 July 2.

1904 July 4. 1904 July 11. colour.

There was a heavy, diffused, broad dark belt

The north belt was very heavy and diffused.
The north belt was heavy and broad.

The north polar region was of a light yellow

1904 August 27. The north polar region was light-coloured. 1904 December 5. Light at the equator, with broad dusky region toward the north, but all the north region was lighter.

I have purposely withheld (for another paper) the observations of Saturn in 1903, at the time of the appearance of the white spot on the ball of the planet.

Unless otherwise stated, all the observations in these papers were made with the 40-inch telescope.

Yerkes Observatory, Williams Bay, Wis. : 1908 January.

Photometric Measurements of Saturn, August to December 1907. By J. M. Baldwin, M.A., 1851 Exhibition Scholar (Melbourne).

(Communicated by Sir David Gill.)

During the recent opposition of Saturn, at the suggestion of Professor Müller, I undertook a series of photometric measures of that planet. This opposition has been of particular interest, for, owing to the small elevation of the Earth above the plane of the ring, the reduction to "ring invisible" is subject to very slight uncertainty, and so the variation in the brightness of the spheroid itself with phase angle can be obtained.

Seeliger bas theoretically arrived at an expression for the magnitude of Saturn, in which the magnitude depends on two quantities, the first of which, expressing the light received from the ring, shows a marked variation with phase, while the second, expressing the light from the spheroid, is almost independent of phase. Müller,† on the other hand, has arrived at empirical formulæ for the magnitude, in which the variation in magnitude is proportional to the change in phase. In discussing his observations and those of Zöllner, he remarks (l.c., p. 343), "es scheint also, als ob auch bei gänzlich verschwundenem Ring ein Einfluss der Phase sich geltend mache"; and Pannekoek has further emphasised this point, showing that the observed change of brightness with phase when the elevation of the Earth above

*H. Seeliger, Abhandl. der Bayer. Akad. der Wissensch., Bd. 16, 403, 1888. + G. Müller, Publ. des Astrophys. Observ. zu Potsdam 8, 339 u. 341, 1893. Ant. Pannekoek, Ast. Nach. 4006, Bd. 167, 363, 1905.

the plane of the ring was small are greater than accounted for
by Seeliger's theory, and so rendering it probable that the spheroid
itself shows changes in brightness with the phase. The object of
this series of measurements was therefore to determine which of
the two formulæ agreed best with observation.

In order to avoid bias, I was careful not to find out the
magnitudes of the variation in phase given by the formulæ ; and
further, the observations were not reduced at all until after the
whole series was completed.

The instrument used throughout was the Zöllner photometer referred to in the Potsdam publications as C III, the objective of which is of 215 mm. aperture and 137 mm. focal length. (For description see Publ. Astr. Obs. zu Potsdam, Bd. viii., p 17, 1891.) In this instrument the image of the planet differs but little from that of a star. Unfortunately there was no suitable comparison star near Saturn; a Aquilæ, distant some 60°, was chosen as the most convenient and was used throughout. Its magnitude has been taken as o'96 (l.c., p. 235), so that Saturn should be referred to the same system as former planetary observations here have been. For the extinction correction the mean values for Potsdam have been used.

A typical set of observations and the method of reduction is shown in Table I.

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][ocr errors]

Saturn

[ocr errors]

36 32 2 26'1 29'0 29'0 29.07 9'3732 -372 572 +372

In a few cases, however, only eight readings for each were taken, and occasionally a Aquila was observed first.

The results of the observations are given in Table II.

« PreviousContinue »