Willson, Robert Wheeler, 1853-1922 · Boston, New York Ginn & company · leaf 173
-pointed, hard pencil,against the edge of a ruler passing through the sun and the point ofthe graduated circle whose reading equals the planets true helio-centric longitude. It is quite an advantage to have the ruler of atransparent substance in order that its edge may be correctly placedon the… … Fig. 84. Plotting with a Loop held between the thumb and forefinger, with the threads slightlyseparated (about j^ of an inch). The loop is then drawn taut, andthe middle of the white space between the threads may be bisectedby the proper point on the graduation; the place of the planet isthen…
Astronomy
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