Half-hours with the Telescope eBook

Richard Anthony Proctor
This eBook from the Gutenberg Project consists of approximately 116 pages of information about Half-hours with the Telescope.
------------------- Date. |Dec. 22|Jan. 5|Jan. 20|Feb. 4|Feb. 19|Mar. 5 |Mar. 21 (Circiter.) | |June 6|May 21 |May 5 |Apr. 20|Apr. 5 | -------------------+-------+------+-------+------+-------+--
-----+-------- Inclination of |Left |Left |Left |Left |Left |Left |Left Ecliptical Diameter| | | | | | | of Sun to the |0 deg. 0’ |6 deg.24’ |12 deg.14’ |17 deg.3’ |20 deg.36’ |22 deg.44’ |23 deg.27’ Horizon.[17] |Right |Right |Right |Right |Right |Right |Right -------------------+-------+------+-------+------+-------+--
-----+-------- Date. | |Dec. 7|Nov. 22|Nov. 7|Oct. 23|Oct. 8 | (Circiter.) |Jan. 21|July 7|July 23|Aug. 6|Aug. 23|Sept. 7|Sept. 23 ------------------------------------------------------------

Now if our observer describe a circle, and draw a diameter inclined according to above table, this diameter would represent the sun’s equator if the axis of the sun were square to the ecliptic-plane.  But this axis is slightly inclined, the effect of which is, that on or about June 10 the sun is situated as shown in fig. 14 with respect to the ecliptic ab; on or about September 11 he is situated as shown in fig. 13; on or about December 11 as shown in fig. 12; and on or about March 10 as shown in fig. 15.  The inclination of his equator to the ecliptic being so small, the student can find little difficulty in determining with sufficient approximation the relation of the sun’s polar axis to the ecliptic on intermediate days, since the equator is never more inclined than in figs. 12 and 14, never more opened out than in figs. 13 and 15.  Having then drawn a line to represent the sun’s ecliptical diameter inclined to the horizontal diameter as above described, and having (with this line to correspond to ab in figs. 12-15) drawn in the sun’s equator suitably inclined and opened out, he has the sun’s actual presentation (at noon) as seen with an erecting eye-piece.  Holding his picture upside down, he has the sun’s presentation as seen with an astronomical eye-piece—­and, finally, looking at his picture from behind (without inverting it), he has the presentation seen when the sun is projected on the screen.  Hence, if he make a copy of this last view of his diagram upon the centre of his screen, and using a low power, bring the whole of the sun’s image to coincide with the circle thus drawn (to a suitable scale) on the screen, he will at once see what is the true position of the different sun-spots.  After a little practice the construction of a suitably sized and marked circle on the screen will not occupy more than a minute or two.

[Illustration:  Fig. 12.]

[Illustration:  Fig. 13.]

[Illustration:  Fig. 14.]

[Illustration:  Fig. 15.]

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Half-hours with the Telescope from Project Gutenberg. Public domain.
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