NOMY.
At particular times, sometimes once and sometimes twice in a year, the light of the sun is interrupted by an opaque body, which covers more or less of his disc. This is called an eclipse of the sun, but as it is occasion ed by the interposition of the moon, we shall refer the consideration of eclipses to another Section.
By observing the apparent motions of Mercury and Venus, it will appear that they always keep near the sun. On this account Mercury is very seldom seen. He appears a little before the rising, and a little after the setting of the sun. His distance from that luminary never exceeds 27 degrees, and the difference between the rising or setting of Mercury and that of the Sun is never greater than 1" 50'. When Mercury first begins to appear after sunset in the west, he resembles a bril liant twinkling star, nearly obscured by the glare of twilight. On the succeeding evening, his distance from the sun increases till it becomes about 22°, when he is said to be at his greatest elongation from the sun. The planet then appears stationary, and afterwards moves backwards in a retrograde direction to the sun. After having then plunged into the sun's rays, Mercury may be seen in the morning before sunrise. his distance from the sun again increases till it is about 22°, when being a second time stationary, he again returns towards the sun. The greatest elongation of Mercury is not always 22°, but varies from 17° 30' to 28° 20'.
The cause of these phenomena will be understood from Plate XXXVIII, Fig. 3, where GNT is a portion of the heavens lying in the plane of the ecliptic, a, b, e, d, e,f, g, h, &c. the orbit of the Earth, and 1, 2, 3, 4, 5, &c. the orbit of Mercury, the two planets revolving round the Sun S in the order of the figures and letters. The orbit of Mercury is divided into 12 equal parts, I, ; 2, 3, &c. each of which he describes in 74- days, and a portion of the Earth's orbit described by that body in the time that Mercury performs two complete revo lutions, is divided into 24 equal parts. Let us now sup pose that Mercury is at the point 1 in his orbit when the earth is at the point a, Mercury will then appear in the heavens at A, in the direction of the line aA. In the space of 74- days, Mercury will have got to 2, while the Earth has reached b, and therefore Mercury will appear at B. By taking the successive simultaneous positions of Mercury and the Earth, and producing the line which joins them to the heavens GNT, we shall find that Mercury will have successively the positions ABODE FGHIKLMNOPQRSTUVWXYZ. From A to C, his
apparent motion will appear direct, or in the direction of his real motion. From C to G, his motion will be retrograde, or contrary to his real motion. At G he will appear to stand still, and his motion will then be direct through the whole arch GOT. At 'I' he will again appear stationary, and will afterwards move in a retro grade direction from U along the arch TUV\VX. At X he will again appear stationary, and will then move directly from X to Z. The apparent longitude of Mer cury, or of any other planet as seen from the Earth, is called his geocentric longitude, and when seen from the Sun, his heliocentric longitude. When the Earth is at n, and Mercury at 7, so as to be between the Earth and the Sun, he is said to be in inferior conjunction with the sun ; and when the earth is at n, and Mercury at 12, so as to be behind the sun, he is said to be in eriperior con junction with that luminary. At 7 Mercury is also said to be in his perigee, or nearest the earth, and at 1 in his apogee, or farthest from the earth. The anomaly of com mutation of Mercury, or any other planet, is the angle formed at the sun by two lines, drawn to the sun from the earth and the planet.
If we suppose that Mercury is in his perihelion at the point of his first station, after being in superior conjunc tion with the sun, which is supposed to be in 8° of lon gitude, while the earth is in aphelion ; and that Mercury is in his aphelion while the earth is in perihelion, tie shall have the following results : The apparent motions of Venus are exactly the same as those of Mercury, and may be explained in the very same manner, with this difference only, that Venus goes to a greater distance from the sun than Mercury, vary ing between 44° 57' and 46', and that from the slow ness of her motion her stations and retrogradations re turn less frequently. When Venus appears in the even ing after sunset, she is called the evening star ; and when she rises in the morning before sunrise she is called the morning star. She sometimes shines with such a dazzling splendour, that she has been seen in the day time. This singular phenomenon was observed at Lon don in 1716 on the 21st of July, and at Paris in 1750. The light of Venus is most brilliant when her elonga tion from the sun is 39° 2' about 69 days before and after her inferior conjunction with the sun.