The Night Sky in May
Venus is a brilliant beacon, visible low above the westnorth-west horizon after sunset. The planet presents an
88 per cent lit gibbous disc when viewed through a telescope at the start of May, reducing to 80 per cent lit by the end of the month.
However, it is Jupiter which steals the show as it reaches opposition on May 9. Opposition is the term used to describe when a planet is on the opposite side of the sky to the Sun. In this position, the distance between the planet and Earth is minimised for the current period of observation and the planet appears brighter and, through a telescope, larger than at other times.
Being a fast spinning gaseous planet, Jupiter’s equatorial regions bulge outwards. A small telescope will show two main belts encircling the planet’s globe, running parallel either side of Jupiter’s equator. Larger instruments reveal additional belts as well as irregularities in and around them. One of the most impressive atmospheric features is the Great Red Spot, a persistent storm in Jupiter’s atmosphere that has been observed for at least 188 years. A telescope with an aperture diameter of at least four inches is required to see this feature.
A telescope of any size will show up to four star-like dots that appear to move either side of Jupiter’s disc. These are the Galilean moons Io, Europa, Ganymede and Callisto. Discovered by Galilei Galileo in 1610, these are the four largest and brightest Jovian moons. In total, Jupiter has 69 moons, but the remaining 65 are too small and too faint to be seen easily.
When a Galilean moon passes between Jupiter and the Sun it casts a dark shadow on the planet’s atmosphere. Before opposition the shadow precedes the moon across the globe, while after opposition the shadow follows its moon. The transition point occurs at opposition when a moon will transit Jupiter’s disc lined up with its shadow. Typically the north-south alignment is imprecise from Earth allowing us to see both moon and shadow progressing across Jupiter’s disc together. Two such events can be seen this month. On May 8 from 04:00 BST, Io appears virtually on top of its shadow, with a similar event visible on May 9 from 22:25 BST.
The real shadow show-stopper occurs when Ganymede transits Jupiter. This is the largest planetary moon in the Solar System, even larger than the planet Mercury. As it crosses Jupiter’s disc on May 14 it will be followed by its giant shadow at 02:00 BST.
To the naked eye, Jupiter looks like a bright star currently located within the Zodiacal constellation of Libra the Scales. This means it is visible low in the southern part of the sky at around the middle of the night. From the UK, look below and left of Jupiter to locate the bright orange-coloured star Antares. This marks the heart of Scorpius the Scorpion, a magnificent southern constellation which sadly never full rises from the UK.
North-east (above and left) of the scorpion is the large, ill-defined shape of Ophiuchus the Serpent Bearer, the overall shape of which is often described as resembling an upturned flower pot. The serpent bearer’s head is marked by Rasalhague, the brightest star at the top of the pattern. His eastern shoulder contains a dim yet distinctive V-shaped pattern known as Poniatowski’s Bull, named in honour of Stanislaus Poniatowski, the King of Poland from 1764 to 1795.