'I'lll': niAMI'.'ITI-l ol' A MKIOMS \\\ MICIllvl. SOX'S 



INTI'.Rl'I'.ROMI'. I I'R MI'.TIIODS 



By V. G. Pease 



(Courtesy Popular Astronomy) 



hi 1890 Michclson pointed out the possihililx- df incasuriiij: by interference 

 inelliods. the diameter of planetoids and satellites and the distance between dou- 

 ble stars, and also showed how the method mii^'ht be used in determining the 

 diameter of a star. 



lie measured the diameters of Jupiter's satellites with the 12-inch I.ick re- 

 fractor in 1891. and in August. 1919, obtained interference fringes with the 40- 

 ineh refractor of the Yerkes Observatory, and with the 100-inch reflector at 

 Mount Wilson in September of the same year. In December, 1919, and the 

 months following, Anderson obtained measures of the distance and position 

 angles of the components of Capella with great accuracy. At Professor Michel- 

 sons suggestion, an interferometer beam 20 feet long, provided with movable 

 auxiliary mirrors, was then constructed to test conditions of interference at dis- 

 tances greater than the diameter of the one hundred-inch mirror itself. In Au- 

 gust, 1920, fringes were obtained with separations of the mirrors as great as 

 18 feet, the visibility of the fringes for Vega at this distance being as great as 

 that at 6 feet. 



Meanwhile. Eddington. Russell and Shapley had obtained values for the 

 diameter of a number of stars based on estimates of apparent surface brightness, 

 and their results indicated that several of these lay with the range of the 20- 

 foot beam. Orionis in particular was so large that Merrill investigated it with 

 the apparatus used in the measurement of Capella and found a definite decrease 

 in visilsility for the maximum separation of the slits, (100-inch aperature), this 

 holding true for all position angles. 



On December 13. 1920, with the outer mirrors of the 20-foot beam at 121 

 inches separation, no fringes were visible on aOrionis,, while observations on 

 rOrionis before, and on aCanis Minoris afterwards yielded strong fringes with 

 practically no change of setting. All measures were checked by Dr. i\nderson. 

 The seeing was very good on this night, but poor on the following nights, and 

 it appears that better conditions are required than when working with fringes 

 produced by apertures placed directly before the telescope. Nevertheless ob- 

 servations made December 14-17 indicate that rtCeti, flTauri and (!?Geminorum 

 will come within the range of the 20-foot beam. 



Assuming that the effective wave-length for aOrionis is rS7S0 its angular 

 diameter from the formula a^\.22 r/d proves to be 0" .047, and with Schles- 

 inger's parallax of 0" .016 its linear diameter turns out to be 271,000.000 miles 

 or slightly less than that of the orbit Mars. 



The uncertainty of the present measurement is about 10 per cent. The 

 effect of a possible darkening at the limb, which has been disregarded, would 

 tend to make the measured results too small. 



INTERESTING FACTS ABOUT OUR OWN PLANET 

 By Charles Nevers Holmes 



An equatorial journey "around our World" equals a distance approximat- 

 ing 24.900 miles — a distance crossed by light in less than 2 /10th of a second, 

 which would be traveled by a snail in about 34 years. Now, the Earth rotates 

 once upon its axis in 24 hours, so that at a velocity approximating 24.900 miles 

 per day a point on the equator would travel during the 20th Century about 

 909.000.000 miles. That is, it has tra^'eled since the beginning of the Christian 

 Era about 17,500,000,000 miles, or a distance equal to 188 times as far as it is 

 to the Sun. 



A journey to the centre of our Earth would approximate 3959 miles, and 

 if we should ever sink a shaft there, excavating at the rate of 10 miles a year, 

 it would take 4 centuries to reach the terrestrial centre. Now, 3959 miles do 

 not seem very far. the extreme length of the United States. Atlantic to Pacific 

 being about 2800 miles, and the extreme length of North America being about 

 4500 miles. However, if we attempted to tunnel downward to the terrestrial 



