The Summer Triangle, Scorpius and Sagittarius

On a clear evening in August, seen from Ankara, Vega stands almost overhead, only about one degree from the zenith. It is the fifth brightest star of the night sky, and together with Deneb and Altair it makes the Summer Triangle, the frame of the northern summer sky. The Milky Way runs through the triangle and on down to the southern horizon, where it is brightest in Scorpius and Sagittarius. From Ankara those two constellations stay low, and Antares, the red heart of the Scorpion, is never more than 24° up. Seen from Sydney the same evening the picture turns over. The Summer Triangle is low in the north, while Scorpius and Sagittarius pass almost overhead, and the Teapot of Sagittarius passes within a degree of the zenith. In this section we learn the Summer Triangle first, then the hops that lead from it to Scorpius and Sagittarius, and then the fainter constellations between them.

O1.4.1The Summer Triangle

The three corners of the Summer Triangle belong to three constellations. Vega (α Lyr) is the brightest star of Lyra, the Lyre, a small figure of a triangle and a parallelogram just south-east of it. Deneb (α Cyg) marks the tail of Cygnus, the Swan, and Altair (α Aql) is the brightest star of Aquila, the Eagle. The triangle is not equilateral. Its sides are 23.8° from Vega to Deneb, 38.0° from Deneb to Altair and 34.2° from Altair to Vega, so Altair sits at its sharpest corner, with an angle of about 41°.

Cygnus is also called the Northern Cross. Its long beam runs from Deneb through Sadr (γ Cyg), the middle of the cross, to Albireo (β Cyg), 22° from Deneb at the foot of the cross, and the crossbar runs through Sadr at right angles to it. The Swan flies down the Milky Way, with Deneb at its tail and Albireo at its head. A small telescope splits Albireo into two stars, one golden and one blue.

Altair is easy to tell from other bright stars because it has a fainter star close to it on each side, Tarazed (γ Aql) 2.1° away and Alshain (β Aql) 2.7° away, the three in a short, nearly straight line. Close to Vega, the parallelogram of Lyra has Sheliak (β Lyr) and Sulafat (γ Lyr) at its southern end, 2° apart, and between them, a little nearer Sheliak, lies the Ring Nebula, M57, the glowing shell of gas round a dying star, which needs a telescope.

O1.4.2From the Summer Triangle to Scorpius and Sagittarius

Two hops lead from the Summer Triangle to the brightest part of the Milky Way. The first follows the long side of the triangle. A line from Deneb through Altair, continued about as far again beyond Altair, reaches Nunki (σ Sgr), magnitude 2.05, at the handle of the Teapot of Sagittarius. The second starts at Vega. A line from Vega through Rasalhague (α Oph), the brightest star of Ophiuchus, continued about one and a half times as far, reaches Antares (α Sco), the red supergiant at the heart of Scorpius, the Scorpion.

Antares has magnitude 1.06 and a deep orange-red colour, and its name means the rival of Mars, because the two look alike and Mars often passes close to it. Antares lies 4.6° south of the ecliptic, so the Moon and the planets pass near it as they cross this part of the sky. About 1.3° from Antares lies the globular cluster M4, a ball of old stars that binoculars show as a faint, round glow.

O1.4.3Scorpius and Sagittarius

Scorpius is one of the few constellations that looks like its name. From Antares a curve of stars runs down and round to the south-east, ending in the sting, a close pair of bright stars, Shaula (λ Sco) and Lesath (υ Sco), only 36′ apart and 17° from Antares. The head of the Scorpion is a short, nearly upright line of three stars west of Antares, with Dschubba (δ Sco) in the middle and Acrab (β Sco) at its northern end. Near the sting lie two open clusters, M6 and M7, about 5° from Shaula. M7, at magnitude 3.3, is easy to see with the naked eye from a dark site.

East of Scorpius, Sagittarius looks like a teapot. Its body is a quadrilateral with Kaus Australis (ε Sgr), magnitude 1.79 and the brightest star of the constellation, at its lower right corner, the lid is the star Kaus Borealis (λ Sgr), the handle is on the east side with Nunki, and the spout points west towards Scorpius, ending in Alnasl (γ Sgr). The Milky Way rises from the spout like steam, and it is brightest here because this is the direction of the centre of the Galaxy, which lies in Sagittarius only 4.6° from Alnasl. The centre itself is hidden behind dust, 26 000 light-years away. The steam holds the Lagoon Nebula, M8, about 6° from Alnasl, and the globular cluster M22, 2.4° from Kaus Borealis, both visible in binoculars.

O1.4.4Fainter neighbours

Three large, fainter constellations fill the sky between the Summer Triangle and Scorpius. Hercules lies between Vega and Arcturus, and its middle is the Keystone, a quadrilateral of stars of magnitude 2.8 to 3.9. On the western side of the Keystone, about a third of the way from η Her to ζ Her, lies the Great Globular Cluster in Hercules, M13, a ball of several hundred thousand stars that the naked eye can just see under a dark sky. South of Hercules lies Ophiuchus, the Serpent Bearer, a large figure shaped like a coffin with Rasalhague at its top. The Serpent, Serpens, is the only constellation split into two parts, the head west of Ophiuchus and the tail east of it, and the head holds its brightest star, Unukalhai (α Ser). The ecliptic passes through the southern part of Ophiuchus, which Section O1.7 takes up.

Draco, the Dragon, winds round the north celestial pole between the two Dippers, and its head is a small quadrilateral 14.5° north of Vega, with Eltanin (γ Dra), magnitude 2.24, as its brightest star. Two small constellations lie close to Altair. Delphinus, the Dolphin, about 14° north-east of it, is a neat diamond of four faint stars with a short tail, and Sagitta, the Arrow, about 10° north of it, is a short line of faint stars pointing east, between Altair and Albireo. Two stars of Delphinus have strange names, Sualocin (α Del) and Rotanev (β Del), which read backwards spell Nicolaus Venator. That is the Latin form of the name of Niccolò Cacciatore, the assistant of Giuseppe Piazzi at Palermo Observatory, and both cacciatore and venator mean hunter. The names first appeared in the Palermo star catalogue of 1814.

O1.4.5The summer sky from the southern hemisphere

Seen from Sydney, Santiago or Johannesburg, the constellations of this section are seen in the winter, from June to September. Scorpius is then almost overhead, with Antares 83° up when it crosses the meridian, and the Teapot is turned upside down, with its spout to the left and the lid below the body. Kaus Australis passes within one degree of the zenith of Sydney. The Summer Triangle is low in the north, where Vega rises to only 17° and Deneb to 11°, while Altair, 47° up, is the easiest of the three to see. Many observers in the south find Scorpius first, from its shape alone, and use it to find the rest of the Milky Way.