THE TRAPEZIUM OF ORION. TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-Since I last wrote to you, I have gauged the 5th and 6th stars in the Trapezium of Orion; and I find the 5th star is about 10'6 magnitude, the 6th star about 111 mag. The greater difficulty of seeing the 6th star results from its proximity to the large star, and not from want of lucidity. March 24, 1865. Yours truly, FREDERICK BRODIE. P.S.-The magnitudes given are estimated by the method that Mr. Dawes adopts. VISIBILITY OF THE UNILLUMINED DISC OF VENUS. TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-Upon turning my Cooke refractor of 7.8 in. aperture, power employed being 85, upon Venus at 7h. 55m. P.M. of the 16th April, I was much surprised to find the unillumined portion of the planet's disc so plainly visible. It is the first time that I have observed this phenomenon during the present year, and it would be desirable to multiply observations of it about the time of approaching inferior conjunction. With the power above named, the whole disc was instantly seen on looking into the telescope, but was not so bright after a few minutes' examination. I am, Sir, yours, &c., Spondon Observatory: April 17, 1865. JOHN T. BARBER. AMATEUR OBSERVATIONS. TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir, Mr. Hunt's letter, in the last number of the Register, is of a kind one would wish to see more of in its pages-that is, the account of difficulties met with by amateurs only moderately furnished with instrumental appliances, and how they have surmounted them, and what degree of accuracy in observations they have been able to reach. I have long noticed, as Mr. Hunt has done, the inconvenience of too strong light in making transit observations. I have always found the wires better defined with too little rather than too much light. In an instrument in which the wires are not moveable, it is also of consequence to keep the eye as nearly as possible opposite to each row in succession, in order to avoid the error of parallax. When, as in my own case, the instrument has no better foundation than a pier of brick and cement on a boarded floor, it is desirable in levelling to stand so that the weight of the person may affect the floor as much as possible in the same manner and degree as when the observations are being taken. By attending to such particulars, I have for years obtained results as accurate as those of your correspondents, although I have no other means of ascertaining the azimuthal deviation than that of high and low stars, with a difference, at most, of 50° Declination. This must leave the results open to a small measure of uncer-tainty, as observations may agree beautifully among themselves, and yet be affected by some constant fractional error. I imagine that, to get the time, anything within one second is very good work for most observers, either with a small transit or the sextant. When great precision is not wanted, I find Dent's Dipleidoscope a very handy little instrument. When carefully fixed, it will often not vary more than a second or two for days together; and the greatest variations in the weather do not affect it, as far as I have noticed, more than a quarter of a minute, and such an error is very rare. Just for curiosity, I tried, a night or two ago, how near I could get my longitude by taking the transit of the moon's limb and Saturn with this little instrument. The result came within 8 seconds of the truth; of course an exceptional and happy hit, though the observations were good. I wonder if the numerous private owners of telescopes and transit instruments in the country have in many cases been at the pains of determining the exact positions of their places of observation. It might be useful to print them in the Register. Of course I do not allude to the highclass private observations. Observations with reflecting instruments, the observing of eclipses, occultations, and lunar transits, readily come within the province of the amateur, who, if anything of a good workman, will not be so apt to quarrel with his tools as to endeavour to compensate for their defects and make the most of them. And when the sky is unpropitious for observing, it is a pleasing work to reduce such observations as above mentioned; an incentive, too, for those who have the leisure and inclination to employ, or to acquire, or recover the moderate amount of mathematical knowledge required. Yours faithfully, April 1865. GEORGE J. WALKER. LUNAR CRATER PLATO: THE SOLAR SURFACE. TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir, I should very much like to know if any of your correspondents have noticed the ground markings of the lunar plain Plato. Beer and Mädler give in their map four bright streaks running across the plain. On March 8, 1865, I examined it attentively, and it then appeared to be bright at both ends only, but with an irregular narrow streak of light across the centre. The sun still being rather low on the plain, I could not see these markings so well as no doubt I should if there had been a higher light on the plain, and the weather has been so bad that I have not been able to obtain any view since. With great attention I could see the central speck (perhaps crater?). My telescope is a "Dialyte" of 4'5 inches aperture, and the power used in these observations did not exceed 200-indeed, I found 140, would show the central speck, but a higher power brought out the markings better. On the 5th inst. at about 9h. 30m. I viewed the sun with a power of 60, and a transparent diagonal eye-piece, and was greatly astonished at the appearance of the disc, which was covered with luminous objects (analogous I should imagine to the rice grains), crossing each other in all directions, and covering the entire surface of the sun. Very shortly afterwards hazy clouds put a stop to all observations. I am, sir, yours very faithfully, HERBERT INGALL. March 14. LUNAR ATMOSPHERE. TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-In my letter of the 7th October I stated and explained that the sudden occultations of the stars by the moon was no proof that the moon had no atmosphere. No one, I presume, contends that Jupiter has no atmosphere, yet the satellites continue bright until the moment of contact with the disc of their primary. At the meeting of the Astronomical Society, January 13, Mr. Huggins stated that he thought the question as to lunar atmosphere admitted a test by the "disappearance of the spectrum of a star when the moon approached.' "He considered that a lunar atmosphere might introduce fresh absorption bands at the time of immersion; that it might produce a gradual fading of the colours," &c. "It was found that the obscuration took place, not by a gradual fading, but by the cutting off the light as with an opaque shutter." Mr. Huggins, therefore, concluded that "the experiment was against the existence of a lunar atmosphere." Were this conclusion correct, Jupiter could not have any atmosphere, inasmuch as the immersions of the satellites are observed as occurring as instantatively as the occultations of stars by the moon. Such tests, for reasons explained in my former letter, cannot decide the question whether the moon has an atmosphere or not. I quite agree with the President's remark, viz. that "he still thought one would be found, as it was difficult to conceive any chemical formation of matter in the absence of an atmosphere." It is gratifying to find the chair of a scientific body occupied by one who declines to accept mere assumptions, and demands more satisfactory demonstrations, even in questions connected with the established astronomical theories. 15 Clarendon Gardens: February 9. I am, sir, yours faithfully, E. HOPKINS, F.G.S. P.S.-It is demonstrated and generally admitted that the entire half of the spherical surface of our globe could not be illuminated had it not been for the atmosphere refracting the rays of the sun towards the external margin. The moon, when in quadrature, is seen half illuminated. How could half the spherical surface of the moon be illuminated with such bright light, even to the terminator, by means of the direct rays of the sun in the absence of a lunar atmosphere ?— E. H. SURFACE OF THE MOON. TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-As there appears to be some doubt as to there being at the present time changes going on by volcanic action on the surface of the moon, allow me to ask information, as an amateur, of some of your more learned readers on the following points : - As the moon is generally supposed to have no atmosphere, can volcanic action exist in its absence? But supposing that such could be the case, from a sufficient supply of oxygen being provided by materials composing the lunar crust, what has become of the gases generated by the combustion of so great a mass of matter, and must they not now form a stratum on the lunar surface of considerable height, but of which we have no appearance in the telescope? 2. Supposing the moon to have an atmosphere, and water in bulk or vapour, would not the exhaustion of its internal heat by volcanic eruption cause all the water to be frozen, and the vapour to be precipitated from the atmosphere as snow, leaving the air utterly dry and devoid of aqueous vapour; and does not terrestrial analogy favour such a conclusion, as the higher we ascend, and the more rarified the atmosphere, the less the heat (as proved by the recent balloon ascent), so that, could we ascend into space, or beyond the atmosphere, the cold must be most intense? Of course this is taking the view that, although the sun is by some subtle and mysterious influence the source of heat to all the planets, yet the heat is contained in the planets and their atmospheres, and that no heat can exist without an atmosphere. If this is not so, we must suppose our atmosphere to exert one of two actions, either as absorbing heat from space in ratio to its density, the less density the less capacity of absorption or a resistive action, the less density the greater resistance, which I think are both illogical and anti-analogical, and that we must conclude that the sun's rays produce the heat in our atmosphere, the greater the density, the greater the action and heat. 3. Does not the telescopic appearance of the moon indicate snowcovered peaks and mountain ridges, the bright streaks and points radiating from the higher craters ? I should not trouble you with these enquiries, but that your Register is the only means of gaining this information. Feb. 14, 1865. Remaining, dear sir, yours truly, IGNORAMUS IN SEARCH OF TRUTH. THE MOON CONTROVERSY. TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir,-The fallacy of W. M.'s reasoning in your last issue is transparent even to the most ordinary reader. He attaches a globe to a straight iron bar in such a manner that, when it is made to slide along from one end to the other, it cannot rotate "on its own axis." He then bends the bar into a large circle, slides the globe along, and presto! that which cannot rotate on its own axis immediately begins to do so! He does not see that, since the centre round which the globe turns lies outside, there is revolution but no rotation. Finally, he reduces the size of his circle till the globe just fits into it, and then he gets rotation alone. In the last case it so happens that the centre round which the globe turns is the common axis of the globe and ring, and of course the globe now has "rotation on its own axis." It is really marvellous how men gifted with common sense can deceive themselves in the way they do about the rotation of bodies "on their own axes," which their own diagrams show to be merely revolving round some distant centre. Feb. 8, 1865. Yours faithfully, TO THE EDITOR OF THE ASTRONOMICAL REGISTER. Sir, I do not think that the absolute non-rotation theory of the moon can gain many advocates; but the rotation being granted, it seems, according to Academicus, a matter to be considered as to the kind of rotation. There are two motions where a body is considered to rotate on its own axis. 1. Where a body rotates on its own axis, which axis is considered fixed in space (and therefore the motion is rotation alone). 2. Where a body rotates on its own axis, but at the same time progresses in space. The moon cannot be required to fulfil the conditions of No. 1, for even the sun while rotating is considered to move its axis through space. We must, therefore, fall back on the second interpretation of rotation of a body on its own axis: viz., where a body rotates on its own axis and at the same time progresses in space. Now the question is, does there exist anything in the earth's attraction to compel rotation as well as revolution in the moon? As far as is known there is not, and the moon in her one revolution round the earth might have rotated fifty times, or not once; therefore, if it be granted that rotation exists in the moon, it must be due to some motion within herself; in fact, that she turns, and is not turned (a radial rotation is where a body is rigidly connected with the centre of its orbit, and is turned). Again, if the moon's rotation be considered radial, how can her libration be accounted for? Again, if her rotation were really radial, the earth should only make one rotation in 28 days. It is certainly odd that satellites should always present the same face, or nearly so, to the body round which they revolve, and the chances seem greatly against a body (unrestrained as to its rotation) doing so; but until the above objection can be got over, how can it be considered radial? It is one condition of axial rotation which, to a certain extent, puts on a false appearance of being radial. A man walking round a pole with one side turned to it rotates on his own axis. A man strapped tightly to the pole and the pole made to turn, rotates on the axis of the pole. In Mr. Little's top, if the peas are packed tight, each pea rotates, but only those in the exact central vertical line of the top rotate on their own axis (for in this case the axis of each pea in this line coincides with the axis of the top); the exterior peas each rotate as well as revolve round the central line of peas, but, forming a part of a solid body, I believe them to rotate on the axis of that body. |