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Equation of Time. Apparent time, or that which is shown by a good sun-dial, requires the following numbers to be added to it to give that which ought to be indicated by a well-regulated clock at the same instant :
14 35 Sunday, 16th,
14 27 Friday, ............ 21st,
14 0 Wednesday, 26th,
Phases of the Moon.
5 ........... 3 in the morn.
1 Full Moon, 25th
5 Moon's Passage over the Meridian. The Moon will make her transit over the first me. ridian of this country at the following times during this month, which will afford good opportunities for observation, should the weather be favourable: viz. February 2d, at 1 m. past 5 in the morning
16 7th, 6 16th,
4 in the afternoon 17th, 57
4 18th, 55
6 in the evening
10 PHENOMENA PLANETARUM.
Phases of Venus. The phases of this beautiful planet will bear the following proportions to each other at the commencement of this month : viz.
Illuminated part = 11.84896 digits
= 0·15104 Eclipses of Jupiter's Satellites. The following are such of the eclipses of the first and second satellites of this planet as will be visible at the Royal Observatory this month: viz.
8 in the evening
10 at night
6 in the evening
6 in the evening.
O in the morning.
13th 19th 25th
h. Mercury 1 10 aft. 1 4
0 36 11 34 mor.11 3 Venus 0 40 aft. 0 45
0 49 0 54 0 59 Mars 6 46 aft.
6 35 6 29 6 24 Jupiter 6 40 aft.
5 55 5 34 5 13 Saturn 5 12 aft. 4 49
4 27 4 5 3 44 G. Sidus 9 44 even. 9 21
8 58 8 37 8 15
MERIDIONAL ALTITUDES, Mercury 26055
310 40 30024 27054 Venus 22 44
27 57 30 50 30 50 Mars 23 41
27.1 30 47 30 36 Jupiter 57 13
57 25 57 33 57 43 Saturn 49 7
49 24 49 34 49 43 G. Sidus 15 3
15 5 15 6 15 7
Other Phenomena. Venus and Mars may be seen together on the 4th of this month, when Venus will only be 24 south of Mars. The Moon will also be in conjunction with a in Scorpio at 29 m. after 10 the same evening. Mercury will be stationary on the 10th, and in con
junction with the Moon at 27 minutes past 5 in the morning of the 12th : this planet will likewise attain his greatest elongation on the 4th, and his inferior conjunction at half past 7 in the morning of the 19th. Jupiter will also be in quadrature at 45 m. after 2 in the morning of the 16th.
While the youthful astronomer is contemplating the celestial phenomena of the month, let him reflect that, in reference to the Power who called these mighty orbs into existence, and gave them laws by which these very phenomena are produced, it may truly be said, “ these are but part of his ways;" and thus reflecting, may he realize the conception of the poet:
Night opes the noblest scenes, and sheds an awe
DESCRIPTION AND USE OF SOME OF THE MOST
INDISPENSABLE ASTRONOMICAL INSTRUMENTS To assist our astronomical students in their application of the principles we have already explained, and in observing the phenomena already described, we shall now explain the nature and use of a few of the most indispensable astronomical instruments. In doing this, however, we shall not enter into the scientific principles of their construction, but merely present such observations as may elucidate their practical purposes. The first that deserves attention is
THE ASTRONOMICAL TELESCOPE. An astronomical telescope consists of two convex lenses, placed in a tube at a distance from each other equal to the sum of their focal distances. That lens which is nearest the object is called the objectglass, and that nearest the eye, the eye-glass. Remote objects appear, through a telescope of this kind, distinct and inverted; and the apparent diameter of an object seen through this telescope is to
its diameter, when viewed by the naked eye, as the focal length of the object glass is to that of the eye glass. The following figure will illustrate these properties. If CD be one convex lens, whose focal distance is cf, and EF another, whose focal distance is gf, and if these be so placed that the distance between them is equal to cf +gf, they form an astronomical telescope.
Let AB be the object emitting the several pencils of rays, ACD, BCĎ, &c. which is supposed to be at so great a distance from the object-glass CD, that the rays of the same pencil may be considered as parallel to each other, the breadth of that glass being nothing in comparison to the distance of the object. These rays are therefore supposed to be collected into their respective foci in the points a and b, situated at the focal distance of the object-glass, CD. Here they form the image ab of the primary object, and, crossing each other, proceed diverging to the eye-glass EF; which being placed at its own focal distance from the points a and b, the rays of each pencil, after passing through that lens, will be nearly parallel to each other. The pencils, however, will have a great convergency, and will intersect each other at e, very little further from the lens EF than its focal distance gf; for, in a long telescope, the diameter of the eye-glass bears but a small proportion to the distance between the lenses. So that the place of the eye, to obtain distinct vision, will be nearly in the focus of the eye-glass, and the rays of each pencil being nearly parallel among themselves, and their axes crossing each other in a larger angle than they would have done if the object had been seen with the naked eye, the vision will be distinct, and the object magnified. As, in comparison with the distance of any celestial object, the length of the telescope may safely be neglected, the increase in the apparent magnitude of the object will obviously be in proportion of the angle A'B' to the angle AcB, or to its equal acb; or, which amounts to the same thing, the magnifying power of the telescope is equal to the focal distance of the object-glass, divided by the focal distance of the eye-glass; that is,
cf Mag. power =
gf By this, it is simply to be understood that the angle under which the object is seen by means of the telescope is equal to that under which it would appear if brought so many times nearer, as is indicated by the above fraction.
As the axes of the pencils of rays that flow from the object AB cross each other in the axis of the object-glass c, the image, as seen at A'B', will necessarily be inverted. The apparent motion of the object will also be reversed; so that, if it be from left to right in the heavens, it will be from right to left in the telescope. But as this inversion of appearance and motion pertains to all the bodies that fall within the field of the telescope, it is not productive of any practical inconvenience. It will, therefore, be sufficient to have pointed it out, that the young observer may not lose time in searching for the cause of a phenomenon apparently so contradictory to his previous conceptions. It may be