Since the sun’s diameter is about 850,000 miles, each graduation (in the case above specified) corresponds to one-64th part of 850,000 miles—­that is, to a length of 13,256 miles on the sun’s surface.  Any other case can be treated in precisely the same manner.

It will be found easy so to place the screen that the distance between successive graduations (as seen projected upon the screen) may correspond to any desired unit of linear measurement—­say an inch.  Then if the observer use transparent tracing-paper ruled with faint lines forming squares half-an-inch in size, he can comfortably copy directly from the screen any solar phenomena he may be struck with.  A variety of methods of drawing will suggest themselves.  Mr. Howlett, in the paper I have quoted from above, describes a very satisfactory method, which those who are anxious to devote themselves seriously to solar observation will do well to study.

It is necessary that the observer should be able to determine approximately where the sun’s equator is situated at the time of any observation, in order that he may assign to any spot or set of spots its true position in relation to solar longitude and latitude.  Mr. Howlett shows how this may be done by three observations of the sun made at any fixed hour on successive days.  Perhaps the following method will serve the purpose of the general observer sufficiently well:—­

The hour at which the sun crosses the meridian must be taken for the special observation now to be described.  This hour can always be learnt from ‘Dietrichsen’s Almanac’; but noon, civil time, is near enough for practical purposes.  Now it is necessary first to know the position of the ecliptic with reference to the celestial equator.  Of course, at noon a horizontal line across the sun’s disc is parallel to the equator, but the position of that diameter of the sun which coincides with the ecliptic is not constant:  at the summer and winter solstices this diameter coincides with the other, or is horizontal at noon; at the spring equinox the sun (which travels on the ecliptic) is passing towards the north of the equator, crossing that curve at an angle of 23-1/2 deg., so that the ecliptic coincides with that diameter of the sun which cuts the horizontal one at an angle of 23-1/2 deg. and has its left end above the horizontal diameter; and at the autumn equinox the sun is descending and the same description applies, only that the diameter (inclined 23-1/2 deg. to the horizon) which has its right end uppermost, now represents the ecliptic.  For intermediate dates, use the following little table:—­

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