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TELESCOPE, THE
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Telegraph Messages, Astronomical Venus

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VARIABLE STAR, THE NEW, IN CORONA

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ZETA HERCULIS

Names of new Subscribers, Instruments for Sale &c.
Notices to Correspondents &c. &c.

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The Astronomical Register.

No. 37.

JANUARY.

1866.

OURSELVES.

WITH the present number we commence our 4th annual volume. In accordance with precedent, we shall venture on a few observations. The Astronomical Register has been a decided success; that is to say, our endeavours to fill a blank in periodical literature have been well received, and we take this opportunity of tendering our best thanks to that very numerous circle of penmen who have enriched our pages with a vast variety of interesting and useful information. We have done our best to deal impartially with all; if we have occasionally pruned a letter of matter which its writer considered ought not to have been omitted, we have done so, for the most part, in order to avoid the necessity of excluding other communications having at least an equal claim, or perchance to avoid unnecessarily wounding the feelings of those subjected to hostile criticism.

As regards the future we do not wish to make any sweeping promises which it will not be in our power to redeem. We prefer to be judged by our performances, but we must remind our readers that in real truth a great deal depends on them. The current expenses of a technical publication like the present are by no means inconsiderable, and a punctual and regular payment of the subscriptions we do not think it too much to ask for. If, likewise, each of our readers would make known the Register to his friends, a large increase in our circulation would be promptly and easily obtained, and in the various facilities for improvement which an increased sale places within the reach of proprietors and editors, all parties will share. Meanwhile, we wish our readers one and all

A HAPPY NEW YEAR.

A DESCRIPTIVE ACCOUNT OF THE SUN AND PLANETS.

[In the series of papers, of which the first is here presented, it will be sought to give, in popular language, a representation of the present state of our knowledge of the subjects treated of. Especially will be taken into account the changes rendered necessary by the alteration in the accepted value of the solar parallax. It is believed that this is the first attempt on any considerable scale to take cognizance of recent discoveries.-G.F.C.]

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If there is one material object more than other which may be regarded as occupying the foremost place in the mind of the astronomer, it is the Sun, Speaking generally, there is scarcely any branch of astronomical enquiry with which, directly or indirectly, the Sun is not associated in some way or other. Even in sidereal astronomy, with which a connexion would at first sight scarcely seem possible, it comes before us (as we shall in due course see) in reference to the supposed proper motion of the solar system through space. Under these circumstances, and bearing in mind the position it holds in the universe, it will not appear unreasonable if we devote to it the very foremost place.

By common consent, the mean distance of the Earth from the Sun has been accepted as the usual unit of astronomical measurement. The ordinary and most approved method of determining the value of this has been by the aid of observations on transits of the planet Venus across the Sun (to be dealt with generally hereafter). The problem is an intricate one for various reasons, and an examination of it does not fairly come within the scope of this work; suffice it then to say, that the observations above alluded to, place us in possession of the amount of the Sun's equatorial horizontal parallax; in other words, give us the angular measure of the Earth's equatorial semi-diameter as seen from the Sun, the Earth being at its mean distance from that luminary. With this element given, it is not then difficult to determine, by simple trigonometry, the Sun's distance expressed in radii of the Earth, reducible thereafter to common miles.

Professor Encke, of Berlin, executed an able discussion of the observations of the transit of Venus in 1769, and deduced 8.5776" as the amount of the angle in question.* From this we find the mean distance of the Earth from the Sun to be 24046-9 times the equatorial radius of the former, equal to 95,283,115 British statute miles. We shall hereafter see this has ceased to be a definitive value.

At a meeting of the Royal Astronomical Society, held on May 8, 1857, the Astronomer Royal proposed a method for

* Der Venusdurchgang von 1769, p. 108. Gotha, 1824.

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determining the absolute dimensions of the solar system, founded upon observations of the displacement of Mars in right ascension, when it is far east of the meridian and far west of the meridian, as seen at a single observatory; such observations to be made a fortnight before and a fortnight after the opposition of the planet. In consequence of the great eccentricity of the orbit of Mars, the method is only applicable to those oppositions during which the planet is nearly at its least possible distance from the Earth. Mr. Airy pointed out the advantages of this method in the various respects that Mars may then be compared with stars throughout the night; that it has two observable limbs, both admitting of good observation; that it remains long in proximity to the Earth; and the nearer it is, the more extended are the hours of observation, in all of which matters, Mars is a better object upon which to found deductions than Venus likewise observed for right ascensional displacement. He also entered into some considerations relative to some of the forthcoming oppositions, and named those of 1860, 1862, and 1877, as favourable ones for determining parallax in the manner he suggested.* The general advantage of having observations made on this plan was explained to be, that transits of Venus would not occur till 1874 and 1882, and that, apart from the inconvenience of holding so long in suspense our accepted determination of the Sun's distance, there were terrestrial difficulties which might interfere with observations when the time actually did arrive.

Another astronomer now appears on the scene. M. Le Verrier announced in 1861 † that he could only reconcile discrepancies in the theories of Venus, the Earth, and Mars, by assuming the value of the solar parallax to be much greater than the usually received value of 8'5776". He fixed 8.95" as its probable value.

The importance of a re-determination was thus rendered more and more obvious, and Ellery, of Williamstown, Victoria, New South Wales, succeeded in obtaining a fine series of meridian observations of Mars, at its opposition in the autumn of 1862. These have been reduced by Stone, of Greenwich, and the mean result is a value of 8.932" for the solar parallax, with a probable error of only o'032", supposing the probable error of a single observation to be 0.25". This result is singularly of accord with Le Verrier's theoretical deduction. Winnecke's comparison of the Pulkova and Cape observations of May, yield 8.964".

Though confirmation is requisite, there appears to be good grounds for believing that the Sun is nearer than has hitherto been considered to be the case. The distance amended to accord with

* Month. Not. R.A.S. vol. xvii. pp. 208–21.

† Annales de l'Observatoire Impériale. Paris, 1861.
Month. Not. R.A.S. vol. xxiii. p. 185.

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