Cassini begins Grand Finale at Saturn
This month brings the final truly dark night skies for Scotland until mid-July or later. Our dwindling nights are dominated by Jupiter, bright and unmistakable as it passes about 30° high in our southern evening sky and sinks to the western horizon before dawn. Venus is brighter still but easily overlooked as it hovers low in our brightening eastern dawn twilight. Saturn is also best as a morning planet, though it rises at our south-eastern horizon a few minutes before our May star map times.
Saturn creeps westwards from the constellation Sagittarius into Ophiuchus this month and brightens a little from magnitude 0.3 to 0.1, making it comparable with the brightest stars visible at our map times – Arcturus, Capella and Vega. The ringed planet, though, climbs to only 12° high in the south by the time morning twilight floods our sky, which is too low for crisp telescopic views of its stunning rings. On the morning of the 14th, as Saturn stands only 3° below-right of the Moon, its rotation-squashed globe measures 18 arcseconds in diameter while its rings stretch across 41 arcseconds and have their northern face tipped at 26° to our view.
Saturn’s main moon, Titan, takes 16 days to orbit the planet and is an easy telescopic target on the ninth magnitude. It stands furthest west of the disk (3 arcminutes) on the 3rd and 19th and furthest east on the 11th and 27th.
The Cassini probe is now into the final chapter, its so-called Grand Finale, of its epic exploration of the Saturn system. On 22 April, it made its 127th and last flyby of Titan, while on 26 April it dived for the first time through the gap between the planet and its visible rings, successfully returning data from a region it has never dared to explore before. Cassini’s new orbit sees it make another 21 weekly dives until, come 15 September, its almost-20 years mission ends with a fiery plunge into the Saturnian atmosphere.
The Sun’s northwards progress during May, to within only 1.4° of its most northerly point at the summer solstice, changes the sunrise/sunset times for Edinburgh from 05:29/20:52 BST on the 1st to 04:36/21:46 on the 31st. The Moon reaches first quarter on the 3rd, full on the 10th, last quarter on the 19th and new on the 25th.
This crescent Moon on the 1st lies in the west, between the stars Pollux in Gemini and Procyon in Canis Minor, lower to its left, while on the 2nd it is 4° below-left of the Praesepe star cluster in Cancer, best viewed through binoculars. It lies near Regulus in Leo on the 3rd and 4th, and appears only 1.2° above the conspicuous Jupiter on the 7th.
The giant planet lies 10° above-right of Virgo’s leading star Spica and edges 2° to the west-north-west this month, drawing closer to the celebrated double star Porrima whose two equal stars orbit each other every 169 years but appear so close together at present that we need a good telescope to divide them.
Following its opposition on 7 April, Jupiter recedes from 678 million to 724 million km during May, dimming slightly from magnitude -2.4 to -2.2 as its diameter shrinks from 43 to 41 arcseconds. Any telescope should show its changing cloud-banded surface while its four main moons may be glimpsed through binoculars, although sometimes one or more disappear as they transit in front of the disk or are hidden behind it or in its shadow.
Some 30° above and to the left of Jupiter is the orange-red giant star Arcturus in Bootes the Herdsman. At magnitude -0.05, this is (just) the brightest star in the northern celestial hemisphere ahead of Capella in Auriga, low in the north-north-west at our map times, and Vega in Lyra, climbing in the east. It is also one of the closer stars to the Sun, but it is only a temporary neighbour for it is speeding by the solar system at 122 km per second at a distance of 36.7 light years. Even so, it takes 800 years to move a Moon’s breadth across our sky. It is also a corner star of a rarely-heralded asterism dubbed the Spring Triangle – the other vertices being marked by Spica and Regulus.
A useful trick for finding Arcturus is to extend a curving line along the handle of the Plough which passes overhead during our spring evenings but is always visible somewhere in our northern sky. That line, still pending, leads to Arcturus and then onwards to Spica. The traditional mnemonic for this is “Arc to Arcturus, spike to Spica” but, given current circumstances, we might amend this to “Arc to Arcturus, jump to Jupiter”.
Venus rises 65 minutes before the Sun on the 1st and climbs to stand 9° high at sunrise. By the 31st, these figures change only a little to 75 minutes and 10°, so it is far from obvious as a morning star, even though it blazes at magnitude -4.5 to -4.3. Through a telescope, it shows a crescent whose sunlit portion increases from 27% to 48% while its diameter shrinks from 38 to 25 arcseconds. Early rises, or insomniacs, can see it left of the waning Moon on the 22nd.
Mercury stands below and left of Venus but remains swamped by our dawn twilight. It is furthest west of the Sun (26°) on the 18th. Still visible, but destined soon to disappear into our evening twilight, is Mars. Shining at a lowly magnitude 1.6, it lies 7° above-right of Aldebaran as the month begins and tracks between the Bull’s horns as Taurus sinks below our north-western horizon in the early evening.