Welcome to the May night sky. The big event of this month is the transit of Mercury across the face of the Sun. These transits of Mercury can only happen at the moment during the months of May or November due to the relative positions of the orbits of Earth and Mercury, and they are not that rare. In fact, in the past 100 years there has been 16 transits, although not all of them visible from western Europe.
This time, the whole duration is possible to see starting at around midday on Monday, May 9 and ending at about 7.30pm local time. These timings are somewhat variable due to the parallax effect and these differences were used during the 17th century to help measure the distance from the Earth to the Sun.
In the case of Mercury, the results were not that good due to the small size and relatively fast movement of the tiny black disc of our innermost planet against the brilliant glare of the Sun.
The transit this time will suffer from the same problems due to Mercury being not much larger than our own moon, so when it passes in front of the Sun it will be a tiny target that will not be visible without proper safe optical aid.
If you miss it this time, the next will be on November 11, 2019, although only the start of this transit will be seen from western Europe as the Sun will set before the end.
The Moon is near new for the first week of May, so the sky will be dark for the Eta Aquarid meteor shower. This shower is dust from the tail of comet Halley and the peak of activity should be on the night of the 6th. This meteor shower has been observed since the year 401AD and like most long-lasting showers it tends to be reliable but not spectacular.
On the night of the 22nd the red planet Mars comes to opposition; this means it will rise at Sunset and will be due south and at its highest at local midnight. Mars is in the constellation of Scorpius but this means that it will be low in the southern sky as seen by us in western Europe.
The brightest star in this constellation is the red giant Antares, and it will be close to Mars during May and also of a similar reddish hue. It should be easy to tell one from the other as Antares will twinkle and Mars should shine with a steady light.
The ringed planet Saturn is also close to Scorpius this month and comes to opposition early next month.
On the night of the 21st on to the morning of the 22nd the full Moon will be close to all three of them.
The moon is new on the 6th, first quarter on the 13th, full on the 21st and last quarter on May 29.
By Clive Jackson
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Clive Jackson is the Director of the Camera Obscura (next to the Castle in Tavira), specialising in education and public outreach.
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To see the May Sky Map click on the pdf link below