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well to be 'well acquainted with the general appearance of the unenlightened hemisphere. As is well known to lunar observers, the dark and comparatively level plains, termed seas, are always visible with more or less distinctness, as are also the most conspicuous ring mountain systems, as Aristarchus, Copernicus, and others; but the light specks, supposed to be active volcanoes, differ in many important ways from the general light appearance of the objects just named. On January 1, 1865, 1 had the good fortune to pick up one of these curious objects. At 6 P.M. I directed my telescope to the moon, and saw the dark side with its usual markings very distinctly, and my eye immediately caught sight of a little speck of light, very distinctly seen like a considerable star a little out of focus. I most carefully studied its position, and found it to be just at the east foot of the Alps, and very near the wedge-shaped valley. It appeared tolerably well defmed, and very distinct and well seen with power 50 on my 3-foot telescope of 2-inch aperture; and I consider it the more interesting, as Schroter picked up a similar object very near the same place, September 26, 1788, though Mr. Birt, whose researches on lunar matters give his opinion great weight, does not consider them identical.

As to the markings on the floor of Plato, I have watched it for some years, and believe them to be variable, and could never see them as drawn on Beer and Midler's map; and a careful inspection of Plato with the fine Hartwell equatorial, under various powers, on the evening of June 5, confirmed me in my former opinions. This object is well worthy the attention of those who have - time and means to devote to the enquiry, which I have not. Pardon my intruding so largely on your space, and believe me,

Yours obediently,

CHARLES GROVER.

Red Lion Street, Chesham, Bucks:
June 13, 1865.

THE MARE CR1SIUM.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,—Some time ago I wrote to the Register respecting the bright spot a little west of the crater Picard, which was answered, at some length by Mr. Birt. Since that time I have watched the spot very attentively, in all illuminations, and with a much better instrument than I formerly had. A few more remarks may, therefore, not be out of place.

Of late, the bright misty appearance surrounding this spot has been growing much fainter, and I will here quote a passage from my Journal:—

"April 10. Mare Crisium, 4h. before full moon. Not a trace of the spot W. of Picard, but in its place a most minute point of light, glittering like a star, with about 100 of brilliancy; in size could not have exceeded 1"."

On May 12, after full, this place was within only 6" of the terminator; and I then perceived a small hollow or deep pit, the sides having about 50 of light, and about 2" diam. (est.)

It is remarkable that Beer and Madler do not give the least indication of it in their great maps—for, though small, it is easily seen near the terminator and in full, is very apparent at times, but is very variable in brilliancy. May not this be a recent eruption, or crater in process of formation? I must confess that this is my own idea. I should very much like to hear more observations of tbis very interesting region.

I may here state, that on the occasions April 10-11, the whole of the Mare was intersected with bright veins, mixed with bright spots of light, giving a grand appearance to the "sea."

These observations were made with my dialyte of 4-5 inches aperture, and the powers used were from 150 to 1,000.

I am, Sir, yours very trulv,

HERBERT INGALL.

Camberwell, S.: June 13, 1865.

A REMARK FOR OBSERVERS OF THE LUNAR MOUNTAINS {Ant. Nach. 1517).

TO TIIE EDITOR OF THE ASTRONOMICAL REGISTER.

Sir,—A communication recently made to the Astronomische Nachrichten, by the illustrious Director of the Observatory at Athens, on the subject of New Formations -on the Surface of the Moon, seems to me so interesting at the present time, that I am induced to ask you to publish the following translation of it in the Astronomical Rcyister.

I am, Sir, yours faithfully,

W. T. LYNN, B.A., F.R.A.S.

Royal Observatory, Greenwich:
June 6, 1865.

Of late years, several observers in England have occupied themselves very perseveringly with the special study of the moon's surface, and have occasionally arrived at conclusions leading to the belief that newly-formed mountain ranges are now being discovered upon the moon. Although not in a position myself, after 25 years' close observations of the kmd, to bring forward even one certain example of new formation, when the question is confined to crater-forms, yet I am far from disputing the fact of the existence of new formations upon the moon. I do not, however, seek them so much amongst the craters, of which thousands of smaller ones wanting in Lohrmann and Madler are now from time to time being detected; but have for about 15 years past directed my attention especially to the rills, of which I have newly discovered a very great number, and amongst these several of remarkable form, easy to be recognised, which have not been seen since the time of Schroter. With regard to the craters noticed by Messrs. Webb and Birt, in Marius, in Mersenius, near Delisle b, &c., I have known these since the year 1846, and am in possession of various drawings of them. As importance is being attached to these individual isolated craters (for which there are, indeed, good grounds), it may be here mentioned that I detected, on the 2nd and 4th of January in the present year, in the inner surface of Picard A. (Mare Orisium), a small crater, which, together with two others in the northern wall, ban

escaped all the observers up to this time, although in a region frequently and particularly scanned both by Schroter and myself.

As objects repaying careful investigation, I recommend the extremely remarkable rill-systems, near Kamsden, and a little westerly of Aristarchus, discovered by me on Jan. 4, 1849, a* Bonn, and on May 10, 1862, at Athens. Anyone experienced in this department of observation, -who will consider, under favourable circumstances, the twelve or thirteen extraordinary rills and crater-furrows west of the known Aristarchus rill, will perceive for himself how small is the probability that phenomena so remarkable should for so many years have escaped the notice of practised observers, merely "on account of external circumstances."

Athens: Jan.s, 1865. (Signed) J. F. JULIUS SCHMIDT.

CONVERSION OF SIDEREAL INTO MEAN TIME.

TO THE EDITOR OF THE ASTRONOMICAL REGISTER.

Sir,—The transit instrument affords a. most interesting study, and many of your readers will doubtless enter into the spirit of the letters which have recently appeared on this subject in the Astronomical Register. The erection of the instrument, and of a meridian mark; the calculation of the distance of the latter, and of the equatorial intervals of the wires; the determination and elimination, as far as practicable, of the inevitable errors of collimation, level, and azimuth; the permanence or otherwise of the instrumental adjustments, the rate of the clock, the verification of the observer's geographical position;—all are sources of delight to the amateur.

But perhaps the most useful purpose to which a transit instrument may be applied is to keep accurate Greenwich mean time, to be derived from local sidereal time, obtained by previous observation. Greenwich mean time is, par excellence, the time as now used in Great Britain for all the affairs of life, local mean time having been pretty well annihilated in these days of rapid communication by telegraph and railway. And as there appear no complete directions on this head in any work hitherto published—except, indeed, in Mr. Darby's handbook, where, in common with much other information, it is given erroneously—I am tempted to transcribe the method practised by myself, with the view of eliciting opinions as to its correctness; and for convenience of illustration, we may adopt the example just referred to—viz., "to convert 14h. 45m. sidereal time at Manchester, to Greenwich mean time on 13th July 1864."

This conversion is effected in the following manner by Mr. Darby (p. 23 of his Introduction) :— h, m. S.

Mean time at preceding sidereal noon . .16 34 44-6
Add given sidereal time, 14h. 45m,, reduced to

meantime 14 42 34*9*

Add 8 m. 52s. (the difference of time between

Greenwich and Manchester) . . . o 8 52

31 26 n*5

Less . . . . 24 o o

Gives G.M.T.. . . 7 26 n-5 * This is wrong: the seconds should be 3501 not 34-9

The exact difference of time between Greenwich and Manchester will, of course, depend on the position which the observer is supposed to occupy. Neglecting its suburbs, Manchester may be said to be about 3 J miles across, and in this latitude the difference in longitude between two points separated (due East and West) to this extent is about 5' of arc, or 20 seconds of time. The difference, 8m. 52s., assumed in the above example, represents a longitude west of Greenwich of 20 13' o", and the corresponding meridian may be seen, on the Ordnance map, to fall within the limits of this city. But these 8m. 52s. denote an interval of sidereal time, not of mean time, and should have been reduced, for the purpose in view, to their corresponding value in the latter—viz., 8m. 50-55s. of mean time. It might have been inferred that the longitude had been brought into mean time, and that the 8 m. 52s. were mtended to be thus expressed, if, in Mr. Darby's following example of the opposite kind of conversion, the same figures did not appear as forming part of an addition of intervals of sidereal time. The probabittty is, therefore, that they were simply deduced from the longitude in arc, and are intended for sidereal, not for mean.

Also, the "mean time at preceding sidereal noon" requires a correction, which has been here neglected, and which is prescribed thus in the "Explanation" of the Nautical Almanac (p. 512) for 1865 :— "If the place of observation be not on the meridian of Greenwich, the sidereal time must be corrected by the addition of 9-85653. for each hour of longitude, if to the west of Greenwich; and by its subtraction if to the east." And the correction proportionate to the assumed longitude of 8m. 52s. is 1-46s., by which amount the "first point of Aries" is later in coming to the assumed meridian than to that of Greenwich.

These two points being attended to, there are two ways in which the calculation can be made—viz., by comparing the sidereal time found by observation (i.) with the known instant of Greenwich mean time at which the transit of the first point of Aries occurred at Greenwich; or (ii.) with the known instant of Greenwich sidereal time at which the transit of the mean sun occurred at Greenwich; these data being respectively given at pp. xx. and ii. for each month in the Nautical Almanac.

And the operation would stand as follows:—

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M. M. T. corresponding given to M. S. T. . . 71718-15
Add for longitude . o 8 50-55

G. M. T 7 26 87

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May 1865.

P.S.—" Wherever there ia a church and a clock, there also should there be a transit instrument and a clock-rate, or no salary."—Admiral Smyth, Celestial Cycle, vol. i. p. 388.

THE "ASTRONOMICAL OBSERVER:'

TO THE EDITOR OF THE ASTRONOMICAL REGISTER.

Sir,—In his letter pointing out mistakes in this work, Mr. Dawes says the method for converting time is wrong; but, as stated on p. xxii. of the Introduction, the formulse are given according to the Nautical Almanac; so that if the " method" is incorrect, Mr. Darby should not be blamed for that. I notice that there is a small error; but it is so evidently one made in the transcription, that it could never mislead.

Extreme accuracy is desirable, no doubt, in such a work as Mr. Darby's; but I doubt the possibility of such accuracy in the first edition of any work of a similar nature. In the absence of "Smyth's Cycle" I consider Mr. Darby's book stands first on the list of works for the use of amateurs. To the majority of such observers the R.A.s and Declinations of objects are of no value; but the other directions, with the assistance of the maps of the S.D.U.K., are of the greatest value. Again, for the possessors of equatorials, the seconds of declination might have been altogether omitted (as they are in some works), without impairing the usefulness of the book: why, then, complain of an error of a few seconds of declination, or even of time? The amateur wants to Jind his objects; and if his instrument is tolerably well adjusted, my experience of Mr. Darby's book is that very few of the objects fail to come pretty close to the centre of the field.

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