A primary astronomy .. . the Sun, and A the Earth in herorbit. When she is at A, a spotis seen upon the disk of the Sunat B. The Sun revolves in thedirection of the arrows, and in25 days, 10 hours, the spot comesround to B again, or opposite thestar E. This is a sidereal revo-lution. 2. During these 25 days, 10hours, the Earth has passed onin her orbit some 25°, or nearly ^fto C, which will require nearlytwo days for the spot at B to get directly toward the Earth, as shown at D. This last is a synodic revolution. It con-sists of one complete revolution of the Sun upon his axis, and about 27° o
A primary astronomy .. . the Sun, and A the Earth in herorbit. When she is at A, a spotis seen upon the disk of the Sunat B. The Sun revolves in thedirection of the arrows, and in25 days, 10 hours, the spot comesround to B again, or opposite thestar E. This is a sidereal revo-lution. 2. During these 25 days, 10hours, the Earth has passed onin her orbit some 25°, or nearly ^fto C, which will require nearlytwo days for the spot at B to get directly toward the Earth, as shown at D. This last is a synodic revolution. It con-sists of one complete revolution of the Sun upon his axis, and about 27° over.] 70§. Where are these Spots situated ? They are usually on each side of the Suns equator, and within a zone of 60°. 709. How is the Suns Axis situated with respect to the Ecliptic 1It is inclined toward it 7° 20. 710. How was this inclination ascertained ? By observing changes in the direction of the solarspots, at different seasons of the year. VARIOUS DIRECTIONS OF THE SOLAR SPOTS. C. March. June. September. [1. Let E F represent the plane of the ecliptic. In March the spots describe a curve,which is convex to the south, as shown at A. In June they cross the Suns disk innearly straight lines, but incline upward. In September they curve again, though inthe opposite direction; and in December pass over in straight lines, inclining down-ward. 2. The figures B and D show the inclination of the Suns axis.] 711. How does this prove that the Suns Axis is inclined?If the Suns axis were perpendicular to the ecliptic,the spots would revolve in circles parallel to the eclip- PEIMARY ASTRONOMY. 149 tic, and apparently in straight lines; whereas the in-clination of his axis would give the spots precisely thesemotions during the year. [This subject will be fully understood by consulting the following figure, in con-nection with the preceding: SOLAR SPOTS OBSERVED FROM DIFFERENT POINTS. DEC.
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Keywords: ., bookcentury1800, bookdecade1850, booksubjectastronomy, bookyear18
