The Night Sky This Month: May 2018

Jupiter-Spica-Arcturus Triangle
Image Credit: Bob King

The main zodiac constellations that can be seen in the night sky this month includes Gemini and Cancer, which can be in the south-west during the early evening, after which Leo rises to prominence in the south-eastern sky. During the late evening, Virgo can then be seen in the east, with its brightest star, Spica, and the planet Jupiter in Libra forming the base of a huge isosceles triangle, and Arcturus in Boötes marking its most northerly point.


Favorable illumination on the 6th, and again on the 22nd of the month offers excellent opportunities to view one of the few craters on the Moon that did not originate during an impact event, namely Hyginus Rille.

This lunar caldera runs diagonally from northwest to southeast across the Moon’s surface, with a large, 11 km-wide rimless pit almost in the middle of the rille. The absence of raised rim and a central peak in the pit is taken as evidence that the lava tube that forms the rille had either collapsed upon itself, or exploded, which blew a hole in the Moons’ surface.


Last Quarter: 8th
New Moon: 15th
First Quarter: 22nd
Full Moon: 29th


Mercury reached its greatest eastern elongation from the Sun on the 29th of April, which means it might still be possible to spot the little planet low on the western horizon during the first few days of May. However, Mercury will be too close to the Sun to be visible after the first week of the month.

Venus will dominate the western evening sky, shining at magnitude -3.9 in the constellation Taurus until the 19th, after which date it will pass into Gemini. Nonetheless, Venus will grow slightly from 11.5 seconds of arc to about 13 seconds of arc, while gaining a few degrees in altitude as the month wears on. Setting about two hours after the Sun at the start of May, it will set progressively later until by month’s end it will disappear from view about two and a half hours after the Sun. Note that Venus will remain about 20 degrees above the horizon at sunset throughout the month.

Mars is now an early morning object, and it starts the month rising at about 1:30 BST in the constellation Sagittarius, before passing into the constellation Capricornus by the middle of May. During the month, Mars will grow brighter fairly rapidly from magnitude 0.4 to magnitude -1.2 as its angular diameter increases 11.1 seconds of arc to 15.1 seconds of arc. Small telescopes should therefore be able to resolve major surface details like Syrtis Major, but note that Mars will be close to the horizon at the beginning of the month, which might prevent clear views of the planet.

Jupiter will come into opposition on the 8th, and will therefore be visible throughout the month in Libra, although it will culminate at only about 20 degrees above the horizon when it crosses the meridian into the southern parts of the ecliptic. However, since the ‘King of the Planets’ will have an angular diameter of 44 seconds of arc, and a magnitude of -2.5, it might be possible to tweak a reasonable view of it though the murk of the atmosphere with the aid of an atmospheric dispersion corrector.

Saturn is now well into its latest apparition, and it will rise about astronomical midnight at the start of May, and progressively earlier as the month wears on. The planet starts the month with a magnitude of +0.4 and an angular diameter of 17.5 seconds of arc, both of which will increase to magnitude +0.2, and 18.1 seconds of arc as the month progresses.

Note though that even though the planet’s ring system is still tilted towards us by slightly more than 25 degrees, the planet will remain below 15 degrees above the horizon for much of 2018, which could make obtaining clear views of it difficult. Nonetheless, the ringed planet can now be spotted just before dawn close to the topmost start of the Teapot asterism in the constellation Sagittarius, although an atmospheric dispersion corrector might be required to see the ring system clearly.

Meteor Showers

The Eta Aquariids is the only meteor shower that occurs in May, with the peak (as seen from the northern hemisphere) expected to occur during the early morning just before dawn on the 6th, although the waning gibbous Moon might spoil the view somewhat.

While the Eta Aquariids is visible from most of the earth’s surface, it is best viewed from the southern hemisphere, and northern hemisphere observers can expect to see a maximum hourly rate of only about 20 or so meteors. Note that the Eta Aquariids shower is one of two showers that occur when the Earth passes through debris left behind by Halley’s Comet, the other shower being the Orionids, which peaks during the second half of October each year.

Deep-Sky Objects

Messier 87

M87The constellation Virgo may not known for its prominence at any time of the year, but it is among the biggest constellations and contains a large number of galaxies, almost all of which are visible with “rich-field” amateur telescopes. A good case in point is the Virgo Cluster of galaxies in the upper right hand corner of the constellation; this region contains 13 Messier objects, all of which are visible even in small amateur telescopes.

The image opposite shows the most luminous member of the Virgo Cluster, M87, an enormous elliptical galaxy from which a super massive black hole is blasting material right out of galaxy, in a process that can be seen as the bright jet that is extending from M87 towards the lower right.

Sombrero Galaxy (M104)

Also in Virgo, and just to the right of the constellation’s brightest star, Spica is this spectacular, eighth magnitude spiral galaxy, known popularly as the Sombrero Galaxy. Its bloated appearance is the result of an enormous central bulge, which gives it the appearance of a wide brimmed hat in long exposure photographs. The Sombrero is an easy target for small telescopes under clear skies.

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