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6.

The best time to see Mercury in the evening is in the spring, at the time the planet is east of the Sun, and at the greatest distance from that body. It will then be visible for several minutes, and will set about one hour and fifty minutes after the Sun. But if the planet is west of the Sun, and at its greatest distance, it will rise about one hour and fifty minutes before that body, and will be most advantageously seen in the morning, at the latter end of summer or beginning of

autumn.

7. When Mercury is viewed at different times in a good telescope, it presents to us phases similar to those of the Moon, and directed in the same manner towards the Sun.

This planet never appears quite full, because its enlightened hemisphere is never turned directly towards the Earth, except when it is so near the Sun, in or near its superior conjunction, as to be either hidden by the Sun's body, or by his beams, and therefore to us invisible. The enlightened hemisphere of Mercury being thus always turned towards the Sun, proves that it shines not by any light of its own; for if it did, it would always appear round, and fully enlightened.

8. Mercury is sometimes in inferior conjunction near one of its nodes: it then appears as a dark and well defined spot on the disc of the Sun; and a transit of Mercury takes place, which can only be seen by the assistance of a telescope.

The apparent diameter of Mercury, viewed in a good telescope, at the time of its transit, is about 11". The transits of Mercury are very frequent, arising from the proportion of the periodic time of Mercury to that of the Earth, being nearly expressed by several pairs of small whole numbers. If an inferior planet be observed in conjunction its node, (or in a certain place in the zodiac,) it will be in conjunction at the same node, (or place,) after the planet and the Earth have each completed a certain number of revolutions. Now it is easily computed from the periodic times of Mercury and the Earth, that nearly

near

7 periodic revolutions of the Earth are = to 29 of Mercury.

13 per.

of the Earth

Therefore transits of Mercury, at the same node, may happen at intervals of 7, 13, 33, &c. years.

At present the ascending node of Mercury, is in 1s. 16° nearly; and the descending node in 7s. 160. The Earth, as seen from the Sun, is in the former longitude, in the beginning of November, and the latter in the beginning of May. Hence the transits of Mercury will happen for many ages to come in November and May. The first transit of Mercury was first observed by Cassendi, in November, 1631; since which time seven transits of this planet have been observed. The last appearance of this kind was in November 5th, 1822: the next four will take place May 5th, 1832; November 7th, 1835; in 1845, and 1848, all of which will be visible in the United States.

TABLE.

Showing the mean distance of Mercury from the 'Sun, and its eccentricity, in miles; longitudes of the ascending node and the perihelion, &c.

Mean distance in miles

Eccentricity

Longitude of ascending node at the be

ginning of 1801

Longitude of the peribelion at the same time

Greatest equation of the centre Heliocentric longitude on the 1st of January, 1825

36,668,873 7,434,424

1s. 150 57 81"

2 14 21 47

0

32 40 0

0 0 0 10

Geocentric longitude at the same time 0 10 0 15 Mean hourly motion The line of the apsides has a sidereal motion, according to the order of the signs, equal to 9' 44" in a century; or 10 83' 44" when referred to the ecliptic.

The sidereal secular motion of the node is retrograde about 13′ 2.3"; but if referred to the ecliptic, the place of the nodes will, on account of the recession of the equinoctial points, be direct about 42" in a year, or 10 10 27" in a century.

On account of the proximity of Mercury to the Sun, astronomers have not yet ascertained with any degree of certainty, whether the axis of this planet has any inclination to its orbit; and therefore, whether it has any difference of sea sons, is also quite uncertain.

QUESTIONS.

What is the length of Mercury's year? what is the length of its day?

What is the diameter of Mercury in miles ?

What is its relative magnitude with respect to the

Earth, &c.?

What is the inclination of Mercury's orbit to the ecliptic?

What is said of its eccentricity, &c.?

What are the limits of the greatest elongation of Mercury?

What is the mean arc of Mercury's retrogradation, and what is the duration?

What is the mean apparent diameter of Mercury? When Mercury is viewed in a telescope, does he present similar phases to those of the Moon ?

When does a transit of Mercury take place?

CHAPTER VI.

Of Venus. Q

1. Venus is the next planet in order after Mercury, and surpasses in brightness all the other stars and planets, being sometimes so brilliant as to be seen in full day and by the naked eye. This planet revolves round the Sun in 224 days, 16 hours, 49 minutes and 11 seconds, which is the length of its year; and the mean hourly motion in the orbit is about 80,062 miles.

Venus is denoted by the character, which is supposed to be a rude representation of a female figure, with a trailing