There will now be seen upon the screen a picture of the sun very brilliant and pleasing, but perhaps a little out of focus. The focusing should therefore next be attended to, the increase of clearness in the image being the test of approach to the true focus. And again, it will be well to try the effect of slight changes of distance between the screen and the telescope's eye-piece. Mr. Howlett considers one yard as a convenient distance for producing an excellent effect with almost any eye-piece that the state of the atmosphere will admit of. Of course, the image becomes more sharply defined if we bring the screen nearer to the telescope, while all the details are enlarged when we move the screen away. The enlargement has no limits save those depending on the amount of light in the image. But, of course, the observer must not expect enlargement to bring with it a view of new details, after a certain magnitude of image has been attained. Still there is something instructive, I think, in occasionally getting a very magnified view of some remarkable spot. I have often looked with enhanced feelings of awe and wonder on the gigantic image of a solar spot thrown by means of the diagonal eye-piece upon the ceiling of the observing-room. Blurred and indistinct through over-magnifying, yet with a new meaning to me, there the vast abysm lies pictured; vague imaginings of the vast and incomprehensible agencies at work in the great centre of our system crowd unbidden into my mind; and I seem to feel—not merely think about—the stupendous grandeur of that life-emitting orb.

To return, however, to observation:—By slightly shifting the tube, different parts of the solar disc can be brought successively upon the screen and scrutinized as readily as if they were drawn upon a chart. "With a power of—say about 60 or 80 linear—the most minute solar spot, properly so called, that is capable of formation" (Mr. Howlett believes "they are never less than three seconds in length or breadth) will be more readily detected than by any other method," see Plate 7; "as also will any faculae, mottling, or in short, any other phenomena that may then be existing on the disc." "Drifting clouds frequently sweep by, to vary the scene, and occasionally an aerial hail- or snow-storm." Mr. Howlett has more than once seen a distant flight of rooks pass slowly across the disc with wonderful distinctness, when the sun has been at a low altitude, and likewise, much more frequently, the rapid dash of starlings, which, very much closer at hand, frequent his church-tower."

An eclipse of the sun, or a transit of an inferior planet, is also much better seen in this way than by any other method of observing the solar disc. In Plate 7 are presented several solar spots as they have appeared to Mr. Howlett, with an instrument of moderate power. The grotesque forms of some of these are remarkable; and the variations the spots undergo from day to day are particularly interesting to the thoughtful observer.

A method of measuring the spots may now be described. It is not likely indeed that the ordinary observer will care to enter upon any systematic series of measurements. But even in his case, the means of forming a general comparison between the spots he sees at different times cannot fail to be valuable. Also the knowledge—which a simple method of measurement supplies—of the actual dimensions of a spot in miles (roughly) is calculated to enhance our estimate of the importance of these features of the solar disc. I give Mr. Howlett's method in his own words:—

"Cause your optician to rule for you on a circular piece of glass a number of fine graduations, the 200th part of an inch apart, each fifth and tenth line being of a different length in order to assist the eye in their enumeration. Insert this between the anterior and posterior lenses of a Huygenian eye-piece of moderate power, say 80 linear. Direct your telescope upon the sun, and having so arranged it that the whole disc of the sun may be projected on the screen, count carefully the number of graduations that are seen to exactly occupy the solar diameter.... It matters not in which direction you measure your diameter, provided only the sun has risen some 18 deg. or 20 deg. above the horizon, and so escaped the distortion occasioned by refraction.[16]

"Next let us suppose that our observer has been observing the sun on any day of the year, say, if you choose, at the time of its mean apparent diameter, namely about the first of April or first of October, and has ascertained that" (as is the case with Mr. Howlett's instrument) "sixty-four graduations occupy the diameter of the projected image. Now the semi-diameter of the sun, at the epochs above mentioned, according to the tables given for every day of the year in the 'Nautical Almanac' (the same as in Dietrichsen and Hannay's very useful compilation) is 16' 2", and consequently his mean total diameter is 32' 4" or 1924". If now we divide 1924" by 64" this will, of course, award as nearly as possible 30" as the value in celestial arc of each graduation, either as seen on the screen, or as applied directly to the sun or any heavenly body large enough to be measured by it."

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:—

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.



It must be noticed that the sun's apparent diameter is not always the same. He is nearer to us in winter than in summer, and, of course, his apparent diameter is greater at the former than at the latter season. The variation of the apparent diameter corresponds (inversely) to the variation of distance. As the sun's greatest distance from the earth is 93,000,000 miles (pretty nearly) and his least 90,000,000, his greatest, mean, and least apparent diameters are as 93, 91-1/2, and 90 respectively; that is, as 62, 61, and 60 respectively.

Mr. Howlett considers that with a good 3-inch telescope, applied in the manner we have described, all the solar features may be seen, except the separate granules disclosed by first-class instruments in the hands of such observers as Dawes, Huggins, or Secchi. Faculae may, of course, be well seen. They are to be looked for near spots which lie close to the sun's limb.

When the sun's general surface is carefully scrutinised, it is found to present a mottled appearance. This is a somewhat delicate feature. It results, undoubtedly, from the combined effect of the granules separately seen in powerful instruments. Sir John Herschel has stated that he cannot recognise the marbled appearance of the sun with an achromatic. Mr. Webb, however, has seen this appearance with such a telescope, of moderate power, used with direct vision; and certainly I can corroborate Mr. Howlett in the statement that this appearance may be most distinctly seen when the image of the sun is received within a well-darkened room.

My space will not permit me to enter here upon the discussion of any of those interesting speculations which have been broached concerning solar phenomena. We may hope that the great eclipse of August, 1868, which promises to be the most favourable (for effective observation) that has ever taken place, will afford astronomers the opportunity of resolving some important questions. It seems as if we were on the verge of great discoveries,—and certainly, if persevering and well-directed labour would seem in any case to render such discoveries due as man's just reward, we may well say that he deserves shortly to reap a harvest of exact knowledge respecting solar phenomena.



THE END.



FOOTNOTES:

[Footnote 1: Such a telescope is most powerful with the shortest sight. It may be remarked that the use of a telescope often reveals a difference in the sight of the two eyes. In my own case, for instance, I have found that the left eye is very short-sighted, the sight of the right eye being of about the average range. Accordingly with my left eye a 5-1/2-foot object-glass, alone, forms an effective telescope, with which I can see Jupiter's moons quite distinctly, and under favourable circumstances even Saturn's rings. I find that the full moon is too bright to be observed in this way without pain, except at low altitudes.]

[Footnote 2: Betelgeuse—commonly interpreted the Giant's Shoulder—ibt-al-jauza. The words, however, really signify, "the armpit of the central one," Orion being so named because he is divided centrally by the equator.]

[Footnote 3: I have never been able to see more than four with a 3-3/4-inch aperture. I give a view of the trapezium as seen with an 8-inch equatorial.]

[Footnote 4: Sir W. Herschel several times saw [epsilon] Lyrae as a double. Bessel also relates that when he was a lad of thirteen he could see this star double. I think persons having average eye-sight could see it double if they selected a suitable hour for observation. My own eye-sight is not good enough for this, but I can distinctly see this star wedged whenever the line joining the components is inclined about 45 deg. to the horizon, and also when Lyra is near the zenith.]

[Footnote 5: They were so described by Admiral Smyth in 1839. Mr. Main, in 1862, describes them as straw-coloured and reddish, while Mr. Webb, in 1865, saw them pale-yellow and lilac!]

[Footnote 6: Or the observer may sweep from [omicron] towards [nu], looking for R about two-fifths of the way from [omicron] to [nu].]

[Footnote 7: Here a single period only is taken, to get back to a convenient hour of the evening.]

[Footnote 8: Here a single period only is taken, to get back to a convenient hour of the evening.]

[Footnote 9: I have constructed a zodiac-chart, which will enable the student to mark in the path of a planet, at any season of the year, from the recorded places in the almanacs.]

[Footnote 10: It is convenient to remember that through precession a star near the ecliptic shifts as respects the R.A. and Dec. lines, through an arc of one degree—or nearly twice the moon's diameter—in about 72 years, all other stars through a less arc.]

[Footnote 11: Mercury is best seen when in quadrature to the sun, but not (as I have seen stated) at those quadratures in which he attains his maximum elongation from the sun. This will appear singular, because the maximum elongation is about 27 deg., the minimum only about 18 deg.. But it happens that in our northern latitudes Mercury is always south of the sun when he attains his maximum elongation, and this fact exercises a more important effect than the mere amount of elongation.]

[Footnote 12: It does not seem to me that the difficulty of detecting Mercury is due to the difficulty "of identifying it amongst the surrounding stars, during the short time that it can be seen" (Hind's 'Introduction to Astronomy'). There are few stars which are comparable with Mercury in brilliancy, when seen under the same light.]

[Footnote 13: I may notice another error sometimes made. It is said that the shadow of a satellite appears elliptical when near the edge of the disc. The shadow is in reality elliptical when thus situated, but appears circular. A moment's consideration will show that this should be so. The part of the disc concealed by a satellite near the limb is also elliptical, but of course appears round.]

[Footnote 14: From a paper by Mr. Breen, in the 'Popular Science Review,' October, 1864.]

[Footnote 15: 'Intellectual Observer' for July, 1867, to which magazine the reader is referred for full details of Mr. Howlett's method of observation, and for illustrations of the appliances he made use of, and of some of his results.]

[Footnote 16: As the sun does not attain such an altitude as 18 deg. during two months in the year, it is well to notice that the true length of the sun's apparent solar diameter is determinable even immediately after sun-rise, if the line of graduation is made to coincide with the horizontal diameter of the picture on the screen—for refraction does not affect the length of this diameter.]

[Footnote 17: The words "Left" and "Right" indicate which end of the sun's ecliptical diameter is uppermost at the dates in upper or lower row respectively.]



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