Abbildungen der Seite
PDF
EPUB

THE MOON. Of all the secondary planets the earth's satellite is by far the most interesting and important. The inoon completes her circuit around the earth in a period whose mean or average length is 27 days 7 hours 43. 2 minutes; but in consequence of her motion in common with the earth around the sun, the meau duration of the lunar month, that is, the time from new moon to new moon, is 29 days 12 hours 44.05 minutes, which is called the moon's synodical period. If the earth were mohence her dlstance from the earth varies during the course of 'a lunar month. Her mean distance Lionless in space the moon's

orbit would be nearly an ellipse, having the earth in one of the foci; from the earth is 238,850 miles. Her maximum distance, however, may reach 252,830 miles, and the least distance to which she can approach the earth 'is 221.520 mites. Her diameter is 2,162 miles, and if we deduct from her distance from the earth the sum of the two radli of the earth and moon, viz., 3,962 and 1,081 miles, respectively, we shall have for the nearest approach of the surfaces of the two bodies 216,477 miles. Her orbit is a very intricate one, hecause the earth in moving around the sun carries the moon along with it: hence the latter is sometimes within and sometimes without the earth's orbit. Its form is that of a serpentine curve, always concave toward the sun, and inclined to the plane of the earth's orbit at an angle of 50 91, in consequence of which our satellite appears sometimes above and sometimes below the plane of the earth's orbit, through which she passes twice in a revolution. These points or positions are called nodes, and iro two consecutive podes occupy positious diametrically opposite on the lunar orbit. The nodes have a retrograde motion, which causes them to make an entire revolution in 18 years, 218 days, 21 hours, 22 minu'es aud 46 seconds. This motion was well known to the ancients, who called it the Saros, aud was made use of by them in roughly predicting eclipses.

The moon always presents the same face to us, as is evident from the permanency of the varions markings on its surface. This circumstance proves that with respect to the earth she revolves on an axis, and the time of rotation is exactly equal to the time of revolution around the earth. viz., 27.32166 days. The moon's axis is not perpendicular to the plane of her orbit, but deviates therefrom by an angle of about 60 411. In consequence of this fact, and of the inclination of the innar orbit to that of the ecliptic, the poles of the moon lean alternately to and from the earth. When the north pole leans toward the earth we see somewhat more of the region surrounding it, and somewhat less when it leans the contrary way. This displacement is known by the name of libra.. tion in latitude.

The moon's motion on her axis is uniform, but her angular velocity in her orbit is subject to slight variations hy reason of the form of her orbit; hence it happens that we sometimes see a little more of the eastern or western edge at one time than at another. This phenomenon is known as libration in longitude.

The moon's surface contains about 14,685,000 square miles, or nearly four times the area of Farope. Her volume is 1-49 and her mass 1-81 that of the earth, and hence her density is about 3-5 that of the earth, or about

3 2-5 that of water. At the lunar surface gravity is only 3-20 of what it is at the earth, and therefore a body which weighs 20 pounds here would weigh only 3 pounds there. The centre of gravity of the earth and moon,

or the point about which they both actually revolve in their course around the sun, lies within the earth; it is 1,063 miles below the surface.

The attractive force of the moon acting on the water of our oceans is mainly instrumental in raising them into protuberances or tides in such a manner as to give the total mass a spheroidal figure whose principal axis would continually coiucide with the line joining the centres of the earth and moon, but in consequence of the resistance which this movement of the water encounters from continents and islands, as well as from the liquid molecules themselves, the tidal wave can never arrive at any place until about one hour after the moon has crossed the meridian of the place.

The moon has no atmosphere and no water. The suddenness with which stars are occulted hy the moon is regarded as a conclusive proof that a Junar atmosphere does not exist, and the spectroscope furnishes negative evidence of the same character,

In remote ages the lunar surface was the theatre of violent volcanic action, being elevated Into cones and ridges exceeding 20,000 feet high, and at other places rent into furrows or depressions of corresponding depth. The lunar volcanoes are now extinct. A profound silence reigus over the desolate and rugged surface. It is a dead world, utterly unfit to support auimal or vegetable life.

THE EARTH'S ATMOSPHERE. The earth's sensible atmosphere is generally supposed to extend some forty mles in height, probably further, but becoming at only a few miles from the surface of too great a tenuity to support life. The condition and motions of this aerial ocean play a most important part in the determination of climate, modifying, by absorbing, the otherwise intense heat of the sun, and, when laden with clouds, hindering the earth from radiating its acquired heat into space. – Whitaker.

[blocks in formation]
[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][subsumed][ocr errors][merged small][merged small][ocr errors][merged small][ocr errors][merged small][ocr errors][ocr errors][ocr errors][ocr errors][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][ocr errors][merged small][ocr errors][ocr errors][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][ocr errors][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][ocr errors][merged small][merged small]
[merged small][merged small][graphic][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][subsumed][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

EXPLANATION. -The white spaces show the amount of moonlight each night. January 1, March 1, etc., full moon, when moonlight lasts the whole night; January 7, February 6, etc., when the moon rises at or near midnight, when the latter half of the night has moonlight; January 15, February 14, etc., new moon, no moonlight during the whole night; January 23, February 22, etc., the moon sets at or near midnight, when the first half of the night has moonlight.

POLE STAR.
MEAN TIME OF TRANSIT (AT WASHINGTON) AND POLAR DISTANCE OF POLARIS.

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

1 11 21

P. M.
A. M,
A.M.
*M,

P. M.

P. M.
H. a.s.
H.M.S. 0111 H.M.S. OIII

H.M.S.

011
H.M.S
o 11

H.M.S.OI
6 53 11 18 57 58 35 18 55 ? 52 96 1 84 19 54 48 1 8 87 10 494 1895 8 50 56 1 S 15
6 14 4 18 57 4 14 40 1 8 53 ? 13 20 1844 12 15 39 1 8 33 10 944 1 8 29 11 30 1
5 84 56 18 56 3 %5 32 1 8 51 1 34

11 32 19 PM 18 30 9 30 21 18 19 + 7 84 31 8 10 From June 16 to August 1 both the upper and lower transits take place during daylight. The azimuth at the time of greatest Eastern or Western elongation can be easily computed írom the formula:

sin A=

sin where A denotes the azimuth, p the polar distance, and I the latitude of the place.

DATE OF GREATEST ELONGATION. To find the time of greatest Eastern or Westerii elongation, let H denote the hour angle, and I and p as before, then we shall have

cos IT = tan p tan l. And the hour angle in mean time is

Im = Ho X 0.0664846. This quantity, Hin, added to or subtracted from the time of transit given above, according to the elongation required, will give the mean time of the greatest elongation at any place whose north latitude is L.

STAR TABLE.
FOR IDENTIFYING THE PRINCIPAL FIXED STARS.
NAME OF STAR.
Declination On Meridian.

NAME OF STAR.

Declination On Meridian
Upper. Lower

Upper, Lower.
H. M. H.M.

H. M. H.M.
a Andromed (Alpher’z) + 28 36 - 1 18.0 +10 40.0 a Leonis (Regulus).. + 12 24 + 8 40.1 +20 38.1
v Pegasi (Algenib). + 14 42 - 1 13.2 +10 44,8 a Virginis (Spica). -10 43 +11 56, 5+:3 54.5
aCassiopeize (schedir).... + 56 8 - 0 42.2 +11 15.8 a Boouis (Arcturus). + 19 38 +13 47.5 + 0 45.5
a Arietis
+ 23 3 + 0 40.0 +12 33.0 Ursa Minoris

+ 74 31 +13 37.5 + 125.5 BPersei (Algol).. + 40 37 + 1 39.9 +13 37.9 aCoronæ Borealis. +27 113 49.7 + 1 47.7 a Tauri (Aldebaran). + 16 20 + 3 8.2 +15 6.2 aScorpii (Antares). -26 14 1+14 59.3 + 2 57.3 Aurigre (Capella).... + 45 64 + 3 47.6 +13 45.1 (Lyra (Vega)

+ 38 42 +17 9.3 + 5 7 3 a Orionis (Betelguese).. 23 + 4 97.6 +16 26.6 a Aquilæ (Altair). + 8 38 +18 21.4 + 619 4 aCanis Majoris (Sirius). -16 36 + 5 18.4 +17 16. aCygni (Deneb)

+ 44 78 +19 13. 5 ? 11.5 aGeminorum (Castor) + 32 5 + 6 5.7 +18 3.7 aCephei

+ 62 13 +19 51, 5 + 7 49.5 BGeminorum Pollux).. + 28 14 + 6 16.6 +18 14. 6 a Aquarii

0 45 +20 38, 8 + 8 33.8 aCanis Minor (Procyou) + 5 27 + 6 11.6 +18 9.6 a Piscis Australis..

30 5+21 27.1 + 9 25,1

la Pegasi (Markab).. + 14 44 +1 31.7 + 9 32.7 To tud the time of the star's transit add or subtract, according to the sign, the numbers in the second columu of figures to the date of the transit of the pole star given above. Thus, for a Andromeda February 1. Lower Transit of Pole Star is 4 h, 45 m. 4 s. AM. to which add 10 h. 40 m. and we have 3 h. 25 m. 45. P. M.; for December 1, we find 7 h. 31 m. 22 s. P. M., etc. APPROXIMATE PARALLAX AND DISTANCE IN LIGHT-YEARS OF SOME OF THE

PRINCIPAL FIXED STARS.
By light-years is to be understood the number of years light requires to travel from the star to us.
Light-

Light-
Parallax
Yers.

Parallax

[ocr errors]

Vears.

[ocr errors]

Polaris (Pole Star)........

0.073
45 a Lyra (Vega)...

0.140

23 a Auriga (Capella)........ 0.046

61 Cygnl

0.348 -0.564 6-8 a Canis Majoris (Sirius).

0233
15 B Cassiopeia

0.187

17 a Canis Minoris (Procyon).. 0 123

27 y Draconis

0.127 a Bootis (Arcturus)...

0.127
28 8. Pegasi

0.054

60 a Centauri ..

0.216

3.6 The determination of stellar parallax is one of the most difficult and refined problems in practical or observational astronomy. It is to find the angle which the semi-diameter of the earth's orbit subtends at the star-an angle always very small, as seen from the above table, and which camuot be measured directly, but by various processes too complicated to be explained here.

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors]

Mercury

115 4 3.26 76 5 10.9 + 5.7 47 17 17.4 7.6 10.58 4.59 Venus.. 165 4 20.94 130 19 58.0 +0.4 75 53 15.5

-17.9 1.94 1.91 Earth.. 99 47 20.22101 25 37.7 +11.6

1.03 0.97 Mars.

70 45 5.47334 26 21.8 +15.9 48 52 426 -22.2 0.52 0.36 Jupiter.

242 24 21,96 12 54 18.0 + 7.6 99 33 33.3 -13.9 0.041 0.034 Saturn.

53 23 10.90 91 19 26.1 +20.2 112 53 17.7 -18.9 0.012 0.010 Uranus.

294 57 2.33 169 14 25.8 + 7.4 73 33 2.1 -32.0 0.003 0.0025 Neptune

111 24 32.14) 43 51 38.2 -18.9 130 48 38.9 10.7 0.001 0.001 •Epoch 1912 January 0d Greenwich mean time.

SEMI-DIAMETER.
SUN

Gravity at
At
At Mean

In
Volume. Mass.
Density. Axial

Surface.
AND
Unit
Least Mlles

Rotation.

1 PLANETS. Distance. Distance. (Mean).

D. H. M. S. Bun. 15 59.6

432183.68 1303371.8 329390 0.2527 25 7 48 0 27.6057 Mercury 3.34

1504.24 0.054955 0.054898 0.99895 24 5 ? .37979 Venus.

8.55

30.90 3850.67 0.921875 0.807328 0.87574 23 21 ? .85236 Earth.

1.000000 1.000000 1.00000 23 56 4.09 1.00000 Mars.

*5.05 *9.64 2274.37 0.189953 0.106478 0.56055 24 37 23 .32222 Jupiter 1 37.16 23.12 43758.03 1352.809 314.4985 0.23247 9 55 20 2.57115 Saturn.

1 21.17 9.55 36558.86 788.934 94.0684 0.11923 10 14 24 1.10175 Uranus

33.5 1.84 16096.43 55.550 14.4033 0.25928 Unknown .98932 Neptune.. 38.7

1.33 17411.34 85.224) 16.71991 0.196191 Unknown .86338

[ocr errors]

5:45

Astronomical Phenomena for the ¥ear 1915.

ASTRONOMICAL SIGNS AND SYMBOLS.

[ocr errors]
[ocr errors]

The Sun,
Mars.

Conjunction.
The Moon.
Jupiter.

Quadrature.
Mercury.
Saturn.

Opposition.
Venus.
Uranus,

Ascending Node.
The Earth.
Neptune.

Descending Node. Two heavenly bodies are in conjunction" (0) when they have the same Right Ascension, or are on the same meridian, i. e.. When one is due north or south of the other; if the bodies are near each other as seen from the earth, they will rise and set at the same time; they are in ..opposition when in opposite quarters of the heavens, or when one rises just as the other is setting. "Quadrature (D) is half way between conjunction and opposition. By "greatest elongation" is meant the greatest apparent anguuar distance from the sun; the planet is then generally most favorably situated for observation. Mercury can only be seen with the uaked eye at this time. When a planet is in its “ascending': (7) or “descending' (0) node it is crossing the plane of the earth's orbit. The term "* Perihelion" means nearest, and “Aphelion' furthest, from the sun. An “occultation" of a planet or star is an eclipse of it by some other body, usually the moon.

(8)

« ZurückWeiter »