Imágenes de páginas

server. This will only happen when the planet, at the time of this conjunction, is in or near its node.

Trine, an aspect where the bodies are at the distance of of the ecliptic or 1200. Twilight. See Crepusculum. Venus, the second primary planet from the sun, at the

distance of about 68 millions of miles. Year, a period of time generally considered as compre

hending a complete revolution of the seasons. The year is variously distinguished, viz.

1. Tropical Solar year, the time in which the sun appears to perform a complete revolution through all the signs of the zodiac 365d. 5h. 48m. 48s.

2. Sidereal year, the time in which the sun appears to revolve from any fixed star to the same again 365d. 6h. 9m. 17s. The difference between the tropical and sidereal year (20m. 29 s.) is the time of the sun's apparent motion through 50''1, the arch of annual precession.

3. Lunar astronomical year, consists of 12 lunar synodical months=354d. 8h. 48m. 38s. and therefore 10d. 21h.Om. 10s. less than the solar year--a difference which is the foundation of the epact.

4. The common lunar civil year, consists of 12 lunar civil months,

= 324 days. 5. The embolismic or intercalary lunar year, consists of 13 lunar civil months = 384 days.

6. The common civil year, contains 365 days, divided into 12 calendar months.

7. Bissextile or leap-year, containing 366 days. See cal. endar. Zenith, the upper pole of the horizon. Zenith-distance of a celestial body, its distance from the ze. nith, measured on the azimuth-circle passing through the body, and is equal to the complement of the altitude to 90%. Zodiac, a zone or broad circle in the heavens including all

the planets, and extending about 10°. on each side of the

ecliptic. Zodiacal light. a pyramidal lucid appearance, sometimes ob

served in the zodiac, resembling the galaxy, or milky way. It is most plainly observable after the evening twi. light about the latter end of February; and before the morning twilight about the beginning of October. For at these times it appears nearly perpendicular to the horizon. This appearance is generally supposed to be occasioned by the sun's atmosphere.

[ocr errors]

Zone, in astronomical geography, is applied to a division of the earth's surface by certain parallels of latitude.

The Zones are 5 in number, viz.

1. The torrid zone, lying between the two tropics. It comprehends the West India Islands, the greater parts of South America and of Africa, the southern parts of Asia, and the East India Islands.

2. The north frigid zone, lying round the north pole, and bounded by the north polar circle. It comprehends part of Greenland, of the northern regions of North America, and a little of the northern parts of Europe and Asia.

3. The south frigid zone, lying round the south pole, and bounded by the south polar circle. It contains no dry land, so far as yet discovered.

4. The north temperate zone, lying between the torrid and north frigid. It comprehends almost the whole of North America, Europe, and Asia, with the northern part of Africa.

5. The south temperate zone, lying between the torrid and south frigid. It comprehends the southern part of South America, of Africa, and of the great island of New. Holland.

In the torrid zone, the sun is vertical twice a year to every part of it, and there is very little diversity in the length of the day throughout the year, the longest day varying only from 12 to about 13 hours. In the temperate zones the sun is never vertical, and the length of the longest day varies from about 134 to 24 hours. In the frigid zones, the length of the longest day (or time between the sun's rising and setting) varies from 24 hours to 6 months.


Table of the Planets' motions, distances, &c.



Longitude of perihelion. 730.5661
Proportional mean distance

from 0.
Proportion of excentricity

to mean distance.
Proportion of diameters. .4012
Proportion of densities, wa-

ter being 1.
Proportion of polar to equa-
torial diameters.



[ocr errors]


Satel- ! Period. revol. Period. revol.

Period. revol. Period. revol.


Dist. lites. in days.

in days.



9.0659 1.37024



1.88780 4.893 10.966 19.

25.436 16.68902 2.73948 6.268



4.51749 18.754



15.9453 20.295 107.694 88. 6


REMARKS. 1. There is a small variation in the inclination of planets' orbits, the longitude of
nodes, the longitude of perihelion, and excentricity of orbits, the amount of which in 100 years is
usually inserted in astronomical tables, and termed secular variatione.

2. The diameter of the earth being 1. that of the sun will be 111.45 and that of the moon .2731.
The density of the sun, (water being 1.) will be 1.1468, and that of moon, 3.339.

3. The inclination of the sun's axis is about 8°, and the time of his rotation about 25 days 6h.
4. The inclination of the moon's orbit to the ecliptic is about 5° 8', that of her axis about 2o.

and the time of her rotation — that of her revolution round the earth.

5. The periodical revolutions in the tables are those termed sidereal, and the distances of the
sateilitcs from their primaries are reckoned in semidiameters of their respective primaries.

6. The orbits of the 1st, 2d, and Sd, satellites of Jupiter are very nearly circular, and coincident with the orbit of Jupiter: but that of thc 4th is very sensibly elliptical, and inclined to the orbit of Jupiter in an angle of about 1o. 448.

7. The elements of the above Tables are taken chiefly from Laplace and Lalande, the places being calculated for the beginning of the year 1750.


The Earth's. Jupiter's.


in clays.

in days.




[ocr errors]
« AnteriorContinuar »