An incantation of moons

THE Scottish night sky promises rather lean pickings for planet watchers in the month ahead. Of our Earth's five solar system companions visible to the naked eye only one, Jupiter, offers much to look at during July and even then only for early birds (or very late owls). Venus will soon move behind the sun. Mercury - always a challenge at the best of times - might be glimpsed in the post sunset afterglow. Mars and Saturn are low in the pre-dawn sky but faint enough to need binoculars, Mars getting slowly brighter as Saturn fades.

Part of the reason for the poor show ought to be obvious, there isn't much night sky at the moment! Another is the angle of the ecliptic. The solar system is a remarkably flat disc. Most planets orbit the sun in the same plane with not much more than a few degrees of variation. On summer nights the sun dips only shallowly below the northern horizon. Correspondingly, the planets are low too. Light from objects at low elevations reaches us through more atmosphere. The atmosphere with all its movement and pollutants disturbs the light rays, resulting in fainter, fuzzier images.

But Jupiter is fun to watch, often by far the brightest thing in the sky (our moon excepted of course). Even viewed through binoculars its disc grows larger, where stars remain the same size. I use a birdwatching telescope and on good nights I can clearly see Jupiter's famous coloured bands and the four Galilean moons, so called after their first observer Galileo Galilei four hundred and one years ago. The moons have an ecliptic of their own and can often be seen strung out in a line to one side of the planet.

The Galilean moons! As a child I used to chant their mysterious names over and over, as if through my incantation I might summon the mythological lovers of Jupiter (or Zeus) in the flesh. Io, Europa, Ganymede and Callisto... Io, Europa, Ganymede and Callisto. Conquests of Jupiter might be a better collective description as all, three girls and a boy, were pursued, deceived and taken by force. Io, Europa, Ganymede and Callisto...  In fact it wasn't Galileo who named them but a rival, Simon Marius, a German astronomer. His claim to their discovery has been largely forgotten but his apt names have persisted.

An incantation of Jovian moons

Attempts to get my camera and telescope to speak to each other have so far yielded disappointing results so this picture isn't mine, but it gives a good idea of what you should expect to see. Galileo was a generous, or perhaps just prudent, man. He dedicated his discovery to his patron and former maths pupil Cosimo de Medici, Grand Duke of Tuscany.  He was also genius, devising a means of telling the time and longitude from nightly observing the relative positions of  'Cosimo's stars'. And he was fortunate to live in a part of the world and an age where clearer skies were guaranteed!

As July goes on Jupiter will rise earlier, so by the end of the month it will be up by midnight. I'll be away in my tent again by then, free of Edinburgh's light pollution. Telescope at the ready I will start up my incantation. Io, Europa, Ganymede and Callisto... Io, Europa, Ganymede and Callisto. Join me?