The Sky in May: Cool Arcturus gatecrashes Sun's neighbourhood

As the Plough wheels overhead, the brightest star visible during our May nights is the orange giant Arcturus in the kite-shaped constellation of Bootes the Herdsman.

Shining at magnitude -0.04, it stands in the middle of the south-eastern sky at nightfall and moves into the south by our star map times.

Arcturus has a name that means "Guardian of the Bear" from its proximity to Ursa Major, the constellation to which the Plough belongs. At 37 light years away, it is an interloper to the Sun's galactic neighbourhood and is speeding at 122 km per second towards Virgo so that, against the more distant stars, it traverses the apparent diameter of the Moon in only 800 years. Indeed, within half a million years it will be so far away that we will need binoculars to see it. Larger, cooler and more luminous than our Sun, it may be twice as old and traces an inclined orbit around the centre of the Milky Way that carries it far above the disk where most of the other stars reside.

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Another notable star in Bootes is Izar ("Girdle"), a pair of unequal stars, the brighter one golden-orange and the fainter white, that lie about 300 light years away and only three arcseconds apart. We need a telescope to separate them, but their beauty has led Izar to be given the more modern name "Pulcherrima".

The star Alphecca or Gemma, 10 to the east of Izar and of similar brightness, is the jewel in the Northern Crown, Corona Borealis. Within the crown's arc lies the enigmatic variable star R that normally shines at the sixth magnitude, near the naked eye limit, but sometimes plunges temporarily in brightness when clouds of obscuring soot condense around it. It is now almost three years into a record breaking fade that has kept it near magnitude 14, making it difficult to spot even through large telescopes.

Venus continues as a brilliant evening star, its visibility hardly changing at all during the month although it brightens slightly from magnitude -3.9 to -4.0. Each evening, it stands 19 high in the west-north-west at sunset and sinks to set in the north-west almost three hours later.

They won't be obvious in the twilight, but binoculars should show the stars of the Pleiades cluster 8 to the right of Venus tonight, and a little lower in the sky, while the red giant Aldebaran in Taurus holds a similar position to Venus's left. The stars, though, sink a degree lower with respect to Venus every day, so the Pleiades and Aldebaran will soon be lost from view. On the 15th, Venus lies between the stars that mark the tips of Taurus's horns and is 7 above-left of the slender young Moon, while it stands 5 below-right of the Moon on the 16th. It enters Gemini four days later and even makes it onto our star chart before the month's end.

Viewed through a telescope, Venus's dazzling cloud-shrouded disk swells from 11 to 13 arcseconds across while the sunlit portion falls from 89 per cent to 81 per cent .

Mercury has now slipped back into the Sun's glare where it swept through inferior conjunction between the Sun and the Earth on Wednesday. By 26 May it stands 25 west of the Sun, but it remains too low in our bright eastern morning twilight to be seen during May. Jupiter, emerging from the Sun's far side, rises in the east one hour before sunrise at present and almost two hours before the Sun at the month's end. It shines at magnitude -2.1 and lies 7 below-left of the waning Moon on the 9th.

Mars is high, bright and reddish in the south-west as darkness falls tonight, but is sinking in the west by our star map times. During May, it moves almost 14 eastwards from Cancer into Leo to draw to within 4 of Regulus, Leo's leading star. It also dims further from magnitude 0.7 to 1.1 and, with its small disk contracting further from 7 to 6 arcseconds, there is little to gain from telescopic study. Look for the first quarter Moon 5 below Regulus and 10 below-left of Mars on the evening of the 20th.

Saturn, on the other side of Regulus to Mars, is slow moving this month close to the star Zavijava in Virgo. Shining brightly at magnitude 0.8, it is 37 high on Edinburgh's meridian as the sky darkens tonight and moves into the south-west by our map times. By 31 May, its cream coloured light has faded slightly to magnitude 1.0, making it equal to Virgo's main star, Spica. Saturn lies 8 above the gibbous Moon on the 22nd when a telescope shows its globe to be 18 arcseconds wide, set within rings that stretch across 42 arcseconds but have their north face tipped only 1.7 in our favour.

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With less than two months to go to the summer solstice, the Sun climbs another 7 northwards during May as sunrise/sunset times for Edinburgh change from 05:30/20:52 BST tomorrow to 04:37/21:45 on the 31st. The duration of nautical twilight at dusk and dawn increases from 105 minutes to more than 190 minutes during May. Indeed, the final night of May has only 29 minutes of official darkness, our last until the night of 11/12 July.

The Moon is at last quarter on the 6th, new on the 14th, at first quarter on the 21st and full on the 28th. The bright Moon stands 10 to the right of the red supergiant Antares in Scorpius later tonight as it glides low across our southern sky; lower even than the midwinter Sun.

There is another closer meeting between the Moon and Antares on the night of May 27.

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