Triangulum Australis, the Southern Triangle, is a small constellation north of Apus, and south of Norma. A fine triangle, nearly isosceles, is formed by its three bright stars, α, β, γ, the brightest α being at the vertex. These three stars form with α Centauri an elongated cross. The stars β and γ are about 3rd magnitude. β is reddish. ε (4·11, Harvard) is also reddish, and is nearly midway between β and γ, and near the centre of the cross above referred to. α is a fine star (1·88 Harvard) and is one of the brightest stars in the sky—No. 33 in a list of 1500 highest stars given by Pickering. About 1° 40′ west of ε is the short-period variable R Trianguli Australis (R.A. 15h 10m·8, S. 66° 8′) discovered at Cordoba in 1871. It varies from 6·7 to 7·4, and the period is about 3d 7h·2. Although not visible to ordinary eyesight it is given here, as it is an interesting object and all its light changes may be well seen with an opera-glass. A little south-east of β is another short-period variable, S Trianguli Australis (R.A. 15h 52m·2, S. 63° 30′), which varies from 6·4 to 7·4, with a period of 6·3 days; and all its fluctuations of light may also be observed with a good opera-glass.
Circinus, the Compass, is a very small constellation lying between Triangulum and Centaurus. Its brightest star, α, is about 3½ magnitude, about 4° south of α Centauri.
Pavo, the Peacock, lies north of Octans and Apus, and south of Telescopium. Its brightest star is α, which is a fine bright star (2·12 Harvard). κ is a short-period variable. It varies from 3·8 to 5·2, and the period is about 9 days. This is an interesting object, as all the fluctations of light can be observed by the naked eye or an opera-glass. ε Pavonis was measured 4·10 at Harvard, but the Cordoba estimates vary from 3·6 to 4·2. Gould says “it is of a remarkably blue colour.”
Indus.—This constellation lies north of Octans, and south of Sagittarius, Microscopium, and Grus. One of its stars, α, is probably referred to by Al-Sufi in his description of Sagittarius; it lies nearly midway between β Sagittarii and α Gruis, and is the brightest star of the constellation. The star ε Indi (4·74 Harvard) has a remarkably large proper motion of 4″·68 per annum. Its parallax is about 0″·28, and the proper motion indicates a velocity of about 49 miles a second at right angles to the line of sight.
Toucan.—This constellation lies north of Octans, and south of Phœnix and Grus, east of Indus, and west of Hydrus. Its brightest star is α, of about the 3rd magnitude.
There are seven “celestial rivers” alluded to by the ancient astronomers:—
1. The Fish River, which flows from the urn of Aquarius.
2. The “River of the Bird,” or the Milky Way in Cygnus.
3. The River of the Birds—2, including Aquila.