Jupiter rules our April nights
The maps show the sky at midnight BST on the 1st, 23:00 on the 16th and 22:00 on 30th. An arrow shows the motion of Mars during the final week of the month. (Click on map to enlarge)
Venus dominated our evening sky for the first quarter of 2017, but it is now Jupiter’s turn in the spotlight. The conspicuous giant planet lies directly opposite the Sun in the sky on the 7th so that it rises in the east at sunset, reaches its highest point in the south in the middle of the night and sets in the west at sunrise.
Our charts show it in Virgo to the east of south as Taurus and Orion dip beneath the western horizon and the Plough looms overhead, stretched out of its familiar shape by our map projection. Regulus in Leo is in the south-west and almost level with Arcturus in Bootes in the south-east. Vega in Lyra and Deneb in Cygnus are beginning their climb in the north-east.
Sunrise/sunset times for Edinburgh change from 06:43/19:51 BST on the 1st to 05:31/20:50 on the 30th. The Moon is at first quarter on the 3rd, full on the 11th, at last quarter on the 19th and new on the 26th.
Venus rises only a little more than one hour before sunrise and, though brilliant at magnitude -4.2, may be difficult to spot low in the east before dawn. However, the other inner planet, Mercury, remains nicely placed in the evening and stands furthest east of the Sun (19°) on the 1st.
Thirty minutes after Edinburgh’s sunset on that day, Mercury is 12° high in the west and shines at magnitude 0.0. It should be possible to spy it through binoculars and eventually with the unaided eye as the twilight fades and the planet sinks to set another 96 minutes later. By the 8th, though, it is a couple of degrees lower and a quarter as bright at magnitude 1.6 as it is engulfed by the twilight. Inferior conjunction on the Sun’s near side occurs on the 20th.
Mars, magnitude 1.5 to 1.6 and above and to Mercury’s left at present, tracks east-north-eastwards this month to pass 5° below the Pleiades on the 15th and a similar distance left of the star cluster on the 26th. By then it sets late enough to be plotted near our north-western horizon at the star map times.
Its opposition means that Jupiter is at its brightest and closest, shining more brightly than any star at magnitude -2.5 and a distance of 666 million km. It lies 6° north-west (above-right) of Virgo’s leading star Spica as the month begins and tracks 3.7° westwards during April to pass 10 arcminutes or a third of a Moon’s-width south of the fourth magnitude star Theta Virginis on the 5th.
Jupiter lies close to the full Moon on the night of the 10th-11th when the Jovian disk appears 44 arcseconds wide if viewed telescopically, one fortieth as wide as the Moon.
Jupiter’s clouds are arrayed in bands that lie parallel to its equator, the dark ones called belts and the intervening lighter hued ones called zones. There are numerous whirls and spots, the most famous being the Great Red Spot in the southern hemisphere. The planet spins in under ten hours, so a resolute observer might view the entire span of its clouds in a single April night. The four main moons, visible through decent binoculars and easy through a telescope, lie on each side of the disk and change their configuration from night to night.
The beautiful planet Saturn rises in the south-east less than three hours after our map times and is the brightest object (magnitude 0.4 to 0.3) less than 15° above Edinburgh’s southern horizon before dawn. It is a shame that its low altitude means that we miss the sharpest and most impressive views of it rings which span 39 arcseconds in mid-April, and are tilted at 26° around its 17 arcseconds disk. After appearing stationary on the 6th, Saturn begins to creep westwards against the stars of Sagittarius – look for it below and left of the Moon on the 16th and right of the Moon on the 17th.
It is not often that I advertise the annual Lyrids meteor shower. As one of the year’s lesser displays, it yields only some 18 meteors per hour at best, all of them swift and some leaving glowing trains in their wake as they diverge from a radiant point to the right of Vega. The event lasts from the 18th to the 25th and peaks on the 22nd when moonlight should not interfere unduly this year. The Lyrid meteoroids were released by Comet Thatcher, last seen in 1861.
Bright comets have been rare of late, but fainter ones are observed frequently. One such has the jaunty name of comet 41P/Tuttle–Giacobini–Kresák and takes 5.4 years to orbit between the paths of Jupiter and the Earth. It passes within 21 million km of us on the 1st as it nears perihelion, its closest point to the Sun, on the 12th. I glimpsed it through binoculars from a superb dark-sky site at Kielder Forrest, Northumberland, last week when it was a diffuse seventh magnitude smudge close to Merak, the southern star of the Pointers in the Plough.
Although its path is not depicted on our chart, the comet is poised to sweep close to three of the stars identified in Draco, between the Plough and Polaris, the Pole Star. It passes 0.6° north of Thuban on the night of the 2nd-3rd, 1.5° south-west of Eta on the 11th (sadly, in full moonlight) and 0.6° west of Beta on the 18th-19th. During past perihelia, it has been seen to flare by several magnitudes for a few days at a time, so, if we are lucky, it may reach naked-eye visibility.
Alan Pickup