July sky at night — click on the image below to see a larger version

If you have a clear southern horizon, then one target in July’s night sky stands out; the giant planet Jupiter shining at magnitude -2.7 among the stars of Sagittarius. Rising at 10pm BST at the start of the month (and at 7.50pm at the end of it) the planet reaches opposition on the 9th. As a result, it is visible all night long, although the low altitude will prevent observers based in Britain from seeing the planet clearly.

Although none of the three rocky planets is well placed this month, they are all visible. Mercury is visible in the morning sky at the start of the month, reaching its farthest point from the Sun (greatest western elongation) on the 1st. Look in the east-northeast just before dawn, but even at its best it is only 8 degrees above the horizon when the Sun rises. Venus is similarly placed in the evening sky towards the end of the month, being just 5 degrees above the horizon at sunrise.

Mars keeps Venus company early in the evening, and while, at 200 million miles from Earth, it’s too distant and therefore appears too small to interest telescopic observers, it plays a role in a series of conjunctions. At the start of the month it lies next to the Regulus, the base of the sickle that is the most distinctive feature of the constellation of Leo. The Red Planet is joined on the 6th by the four-day old moon, providing a striking sight.

By then Mars will be roughly equidistant from Regulus and the brighter Saturn, the ringed planet making its last appearance before heading into the evening twilight and the three forming a striking trio. The two planets are closest on the 10th, separated by just over the diameter of the full Moon.

The stars of the July night sky are the three constellations that contribute to the so-called Summer Triangle. The triangle is made up of the stars Vega, in the constellation of Lyra, the harp; Deneb in Cygnus, the swan; and Altair in Aquila, the eagle. These three stand out from their neighbours, but the name given to their grouping is unofficial and seems to be recent.

The easiest constellation of the three to identify is Cygnus, with Deneb at the swan’s head forming the tip of a cross. Moving down the main stem of the cross we reach gamma, chi and beta Cygni. The latter, best known as Albireo, is one of the most glorious double stars in the summer sky. Binoculars are sufficient to split the pair, revealing what I see as a sky-blue star and a golden-yellow companion. Astronomical colour is often highly subjective as the eye strains to make the most of faint light, but there seems to be little argument about the colours — or the beauty — of this pair.

The densest part of the northern Milky Way runs along the swan’s back too. A dark sky will not only reveal the band of our galaxy in all its glory, but it contains intrigue too. Early astronomers were puzzled by a region within the main band that is apparently devoid of stars, known as the Cygnus rift. We now know that rather than an absence of stars, this is a region where dust blocks light from more distant stars from reaching us.

The neighbouring constellation of Lyra is much smaller. It’s brightest star, Vega, marks the harp’s handle, while its body is a parallelogram of stars. Forming a triangle with Vega and the nearest of the four parallelogram stars to it is epsilon Lyrae. Epsilon is not just a double, but a double double; two pairs of two stars visible in even the smallest telescopes. The pairs complete one orbit in 585 years and 1,200 years respectively, and the two pairs are also physically connected, but must take hundreds of thousands of years to complete even a single orbit.

The third constellation of the trio, Aquila, is often overlooked as it is missing the distinctive shape of the other two. Looking away from Altair, the constellation appears as a cross with bent arms, stretching down toward the southern horizon, just to the west of Jupiter.