« AnteriorContinuar »
preceding values; but gradually diminish a little every century, because the motion of the Moon is accelerated; and these changes become sensible after considerable intervals of time.
In general the movement of the Moon, like that of the Sun, is composed of a small number of elements, the mean value of which is very nearly the same, with several secular inequalities, which operate gradual changes in these values at long periods of time; and a great number of smaller inequalities which are successively destroyed and re-established in short periods. These last sometimes accelerate and at others retard the lunar motion; while the secular inequalities preserve a constant progress of either increase or decrease for a great number of centuries, before their periods are accomplished, and their effects recommence. Our limits, however, preclude us from attempting any explanation of these perturbating causes; and we must therefore refer such of our readers as are desirous of becoming acquainted with them, to Vince's complete System of Astronomy; or, Biots
Traité élémentaire d’Astronomie Physique; from which we have derived many of the statements com tained in this article.
We have already explained (see T. T. for 1814, p. 180), that by the word node is to be understood the points where the orbit of the Moon and the ecliptic cross each other. These points are also de. termined by observing the instant when the latitude of the Moon is nothing. The nodes are not fixed points in the heavens, but have a slow retrograde motion. At the commencement of the present century, the distance of the ascending node from the vernal equinoctial point was 150.92397; and which therefore determines the position of the lunar orbit for that epoch.
The preceding observations relate to the proper motion of the Moon; that of its nodes is ascertained in the same manner, by the differences of their longitude
at different epochs. The time which they employ in returning to the same longitude, gives the tropical revolution of the node. The mean secular and tropical movement, in 1800, was 1936o.94073, which gives 6788.5492 days for their tropical, and 6793.7212 days for their sidereal revolution. This motion of the.nodes, however, is not uniform, but, like that of the Moon, subject to several small irregularities.
Having given some account of the phases of the Moon in a former volume (see T.T. for 1814, p. 180), we shall now offer a brief explanation of the princi. ples upon which they depend; and these principles are very simple when we only take into the account the mean movements. If the motions of the Sun and Moon were exactly the same, we should always see the same phase of the latter. The distance, on a circle of latitude at which the Moon is obsery. ed, at any time, from the Sun, is equal to the excess of her movement in longitude. Now the mean secular movement of the Sun, as stated in the preceding part of this article, is 36000°.7625 ; that of the Moon is now equal to 4812670.8793; their difference, 4452670.116799, is the quantity which the Moon removes from the Sun in 100 Julian years, in consequence of its mean motion. Whence by a simple proportion, in the same manner as before explained, we obtain 29.530588 days for the synodic revolution, or lunar month,
In the same manner, also, the time elapsed between two consecutive returns of the Sun to the node of the lunar orbit may be found. The secular movement of the node is 1934°.18748, that of the Sun is as stated above. Now as their motions are in opposite directions, the relative movement is equal to their sum, or 37934°.94998. This is the quantity which the Sun and the node are removed from each other in 100 years; from which it is concluded that they separate 360° in 346.61963 days; and this period is denominated the synodic revolution of the node. This
period is less than the tropical year, because the motion of the node is retrograde, and the Sun meets it before he has completed one entire circle of the heavens. As the calculations are founded upon the mean quantities, these results will be in some measure affected by the lunar inequalities, but they will never be far from the true values.
Several other points relative to the lunar orbit deserve attention ; such as those where the radius vector coincides with the radius vector of the Sun; then the two bodies, together with that of the Earth, are in the same plane perpendicular to that of the ecliptic, and the longitude of the Moon is either the same as that of the Sun, or 180° different from it. These points are called the syzigies. When the Moon is in these, it is either situated between the Sun and the Earth, or the Earth is between it and the Sun; in the former case the Moon is said to be in conjunction, in the latter, in opposition. · İt is in the conjunctions which new Moons and eclipses of the Sun take place. In order that these latter phenomena may happen, it is not absolutely necessary that the Moon should be in the plane of the ecliptic; it is sufficient that her distance from it is less than the apparent semidiameter of the Sun. On the contrary, the moment of opposition is that of full Moon, and it is near that epoch that eclipses of the Moon happen. In order that these may occur, the distance of the Moon from the plane of the ecliptic must not exceed the semidiameter of the Earth's shadow at that distance.
In the next article of our observations we shall explain the method of calculating the extent of this shadow, and of determining the circumstances under which the Moon will be immersed in it, with other particulars relative to the eclipses of the two great luminaries.
The Naturalist's Diary.
Now comes July, and with his fervid noon
HURDIS. SUMMER may be said to commence with this month. In May and June, the Spring glowed with all the mixtures of colorific radiance, but now opening beauty and increasing variety are succeeded by the more uniform scenes of maturity and perfection. Nature, in our temperate regions, appears now to have nearly finished her annual work. Something of her variety she begins to lose in this season. Nothing, indeed, can be more beautiful than the verdure of the orchards and woods, but the shades of hue which they exhibit are no longer so agreeable. The meadows begin to whiten, and the flowers that adorn them are mowed down. The corn gradually assumes a yellow hue, and the colours that decorate the rural scene are no longer so numerous.
Groves, fields, and meadows, which charm us so much in the SPRING, when they are all new and fresh, with their first gloss upon them, in the Summer, and in proportion as we advance towards Autumn, lose insensibly their pleasing effects; the song of the nightingale is no longer heard ; and that favourite enjoyment of the country, a walk through fields of verdure, becomes inconvenient and unpleasing, on account of the great heat' which sometimes prevails
Yet Summer has still inexpressible charms, and exhibits proofs every day of the unbounded goodness of the Great Creator. It is that season of felicity in which he dispenses his blessings more abundantly to every living creature. Nature, after having reanimated and enlivened us by all the pleasures of the Spring, is incessantly employed, during the Summer, to provide those enjoyments which are most agreeable to the sensei, to facilitate the means of subsistence, and to excite in our breasts the correspondent sentiments of gratitude and love.
Hay-making, which usually commences about this time, or rather earlier, in fine seasons, is thus pleasingly described by Dodsley, in his “ AGRICULTURE,' a poem:
The ripening mead
From winter's want the flocks and lowing herds, July is generally accounted the hottest month in the year. In consequence of the excessive heat, an evaporation takes place from the surface of the earth and waters, and large clouds are formed, which pour down their watery stores, and deluge the country with floods, frequently laying the full-grown corn.
The storm subsided, and the day begun,