Brightness and stellar magnitude of the third Saturnian satellite, Tethys

Abstract

At about 8" 30m as I had just completed the measure of the distance of Enceladu8, the sky became covered with fl'ying clouds, which continually passed ovel' Saturn, partially obscuring the planet and dimming the light of the Ratellites. The clouds were probably formiIlg and disappearing conswntly, as the air was full of moisture, the difference of the wet and drv bulb thermometers being less than 10 F. ." I had noticed before beginning to observe that large portions of the sky were alternately obscured and clear, evideutly owing to thin clouds formed on the spot. The disappearance or dimming was gradual and it seemed suited to a photometric experiment similar to that tried by Bond on the nebula of Urion during twilight, when the order of the appearance of the details, with the times, were noted.* Accordingly, Mr. Anderson, assistant, watched Saturn with the naked eye, looking along the telescope as a guide at the planet, while I watched the appearance of the satellites throuO'h the eq uatorial. The interior of the dome was perfectly dark.'" As a cloud grad nally (and as nearly as could be j udO'ed uniformly) darkened Saturn as seen in the telescope, tht appearances were carefull'y noted and when the planet disappeared to the naked e'ye Anderson called" now." At this instant I endeavored to note the visibility of the satellites. Enceladus, which was faint at the best, always disappeared before this instant. In foul' (tolerabl'y satisfactory) experiments it was found that Tethys disappeared in the telescope at the same instant that Saturn did to the naked eye. In several reappearances I found that Dione reappeared in the telescope * Annals Harv. 0011. Obs., vol. v, p. 164, etc.