THE SATELLITES OF JUPITEK^ 



By Seth B. Nicholson 

 Mount Wilson Ohservatory, Carnegie Institution of Washington 



[With 1 plate] 



The four large moons of Jupiter were, as you all know, discovered 

 by Galileo in 1610 with a telescope which he himself had made. This 

 telescope, with an aperture of about 2 inches,* magnified some 30 

 times and was probably the largest and most powerful telescope in 

 existence at that time. The story of how this famous discovery 

 was made is best told in Galileo's own words which are quoted from 

 his account in The Sidereal Messenger as translated by E, S. Carlos.' 



Discovery of Jupiter's Satellites. — I have now finished my brief account of 

 the observations which I have thus far made with regard to the Moon, the 

 Fixed Stars, and the Galaxy. Tliere remains the matter, which seems to me 

 to deserve to be considered the most important in this worlc, namely, that I 

 should disclose and publish to the world the occasion of discovering and observ- 

 ing four Planets, never seen from the very beginning of the world up to our 

 own times, their positions, and the observations made during the last two 

 months about their movements and their changes of magnitude ; and I summon 

 all astronomers to apply themselves to examine and determine their peiiodic 

 times, which it has not been permitted me to achieve up to this day, owing to 

 the restriction of my time. I give them warning, however, again, so that they 

 may not approach such an Inquiry to no purpose, that they will want a very 

 accurate telescope, and such as I have described in the beginning of this account. 



On the 7th day of January in the present year, 1610, in the first hour of the 

 following night, when I was viewing the constellations of the heavens through a 

 telescope, the planet Jupiter presented itself to my view, and as I had prepared 

 for myself a very excellent instrument, I noticed a circumstance Miiich I had 

 never been able to notice before, owing to want of power in my other telescope, 

 namely, that three little stars, small but very bright, were near the planet ; and 

 although I believed them to belong to the number of the fixed stars, yet they 

 made me somewhat wonder, because they seemed to be arranged exactly in a 



* Public lecture delivered under the auspices of the Astronomical Society of the Pacific 

 in San Francisco on the evening of Monday, January 9, 1939. Reprinted by permission, 

 with slight revision, from Publications of the Astronomical Society of the Pacific, vol. Bl, 

 No. 300, April 1939. 



* It was Galileo's custom to malte a much larger lens than the apertni-e to be used and 

 then to cover with a diaphragm the outer portion of the lens where the figure was poor. 

 The actual aperture used in the discovery of Jupiter's satellites was probably between 1 

 and ly^ Inches. 



* Shapley and Howarth, A source book In astronomy, p. 49, 1929. 



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