The word planetes is Greek in origin, meaning 'wanderer'. As the planets orbit around the Sun, they move slowly across the starry sky. This month, Jupiter wanders into a very good position, being very bright and visible throughout the evening in the south. Saturn and Venus are also visible for the first half of the month - look towards the west just after sunset.
Let's look at these 3 planets in more detail.
The very bright planet Jupiter is rising in the east just before the sun sets throughout July, and is prominent until after midnight. Looking at Jupiter through binoculars, you can see up to four moons that orbit the planet. These are collectively called the Galilean Moons after the man who discovered them through the first astronomical telescope 400 years ago.
At the start of the month, Venus and Saturn are visible to the west after sunset. The pair are close to each other all month, with Saturn just to the right of Venus. They are visible until 22:30, after which they follow the Sun and set in the west. By the end of the month, both Venus and Saturn are hidden by the glare of the Sun. Because Venus orbits the Sun quicker than the Earth does, it will move in front of the Sun during July until, on the 18th of August, Venus lies between the Earth and Sun. Saturn takes 29.4 years to orbit the Sun, and so it is the relatively rapid motion of the Earth that causes Saturn to be hidden behind the glare of the Sun.
At this time of year, the Earth is on the side of the Sun which allows us to see constellations such as Hercules, Cygnus and Lyra.
In Cygnus, the star that makes up the head of the swan is called Alberio. Although it looks like a lone star to the unaided eye, looking at the star through a small telescope or binoculars reveals that this is actually a double star. It is the stark difference in colours that makes this double star such a delight. Although there has been no sign of any orbital motion of the last hundred years of observation, calculations suggest that they orbit around each other once every around 69,500 years!
Close to Alberio, in the adjoining constellation of Vulpecula the fox, is the Dumbbell nebula. It is visible with a small telescope, some 1,300 light-years away. In the centre of the Dumbbell is a faint white dwarf star, the left over core of the star that puffed out its outer layers to form the nebula. From observing the speed of expansion, the nebula is estimated to be about 11,000 years old.
In Lyra, another nebulae is visible through a small telescope - the ring nebula. Looking like a ring of smoke, it can be easily found between beta & gamma Lyra 1500 light years away. At the centre is faint but very hot blue dwarf star, which is emitting intense UV radiation which causes the ring to glow. The inner edge of the gas is green due to the illuminated Oxygen, and the outer edge is red due to illuminated Hydrogen and Nitrogen. The gas is in the shape of a rugby ball around the white dwarf, half a light year across.
Visible towards the North-West this month is the great globular cluster in the constellation Hercules. From a dark site the cluster is just visible to people with sharp eyesight, but through a telescope the sight is stunning. It contains about one million stars! The cluster is about 24,000 light-years away and some 160 light-years across, with an age of about 10 thousand million years.
The neighbouring constellation to Hercules is Bootes, containing the bright red-giant star Arcturus. Arcturus is 16 times wider than our own Sun, and over 100 times brighter. The handle of the Plough points towards Arcturus and on towards another giant star called Spica. This can be remembered by the ditty "Follow the arc to Arcturus, and speed on to Spica!".
Spica is in the constellation of Virgo, which also contains the Virgo group of over 1000 galaxies. Many of the brighter galaxies are visible in small telescopes, including M87, the 87th member of Charles Messier's catalogue of interesting astronomical objects.