May Night Sky
The duration of fully dark skies diminishes ever further in May, and by the end of the month skies are light until after 10 pm. The first brightest ‘stars’ to emerge will probably be planets Jupiter, Mars and Saturn, with Mercury also putting on a good show low in the evening twilight.
Having been so prominent over the last five months or so, Jupiter is finally dropping down into the west, so it’s best observed earlier in the month. The moon lies nearby on the 4th and again on the 31st. Having reached opposition early last month, Mars remains a conspicuous object to the naked eye, residing midway up in the south, upper right of Spica, chief star in Spica. Through a scope Mars exhibits a small orange disk, with a hint of surface detail – notably the north polar cap. The moon lies nearby on the 10th/11th.
Saturn takes centre stage this month, reaching opposition on the 10th, and is therefore visible from sunset to sunrise. You can locate the ringed wonder over in the southeast during late evening, and due south by midnight, a conspicuous ‘pearly star’ a couple of hand’s spans left of Mars. To view the beautiful rings you will need a telescope – any size will suffice. The moon lies nearby on the 14th. Mercury bounds back up into the evening twilight sky at the start of May and remains on show until the end. Look to the WNW 45minutes after sunset around 5 degrees above the horizon (width of a hand at arm’s length). It is highest around the 19th but is brightest before this. Use binoculars if you cannot initially spot this elusive planet with the naked eye in the bright twilight. Finally, Venus lingers in the early dawn sky low down in the east before sunrise.
The Eta Aquarid meteor shower peaks on the 6th, but will be best seen on the night of the 5th around midnight. Keep a watch on May 24th when a new meteor shower, the 209P-id meteors may occur.
Fri 23 May: Talk at Whitby Museum by top astro-enthusiast Paul Money ‘Images of the Universe vol. 5’, the story behind twelve of the most profound, significant and spectacular images ever taken of our celestial surroundings. Doors open 7 pm for 7.30 start. Admission £2.50 adults and £1 under 14s. Delicious homemade refreshments. Paul’s presentations are infotainment of the highest order, so if you fancy ‘a night at the Museum’ please arrive in good time, you will not be disappointed. Lecture in Normanby room; please use Chubb Hill entrance.
The Celestial Ocean – Twilight and Shadow
Twilight, twilight, twilight . . . no, not the teen vampire saga, but the changes in light levels encountered daily after sunset or before sunrise. If you were to step outside in autumn, winter or early spring, skies are fully darkened by around 9 pm. This deepest level of darkness, known as Astronomical twilight, is just one of three levels experienced, the others being Civil and Nautical twilight respectively. However, as Earth orbits around the Sun, the duration and intensity of these twilight levels varies and this is most noticeable at higher latitudes, including the UK. Indeed as we head towards summer three levels become two.
So let me explain a little further using evening twilight as an example. Immediately after sunset Civil twilight exists until the Sun is approximately 6 degrees below the horizon. At this point normal daytime activities that require light cease to be possible and ‘lighting up’ time occurs. If skies are really clear look out for an interesting phenomena half an hour or so after sunset, opposite the position of sunset. In this direction a hazy, darkish purple band just above the ESE horizon gradually ‘heaps up’ before being engulfed by deepening twilight. This is not mist but the shadow of Earth itself, being cast back into space!
As the Earth spins a little further round and the Sun’s elevation below the horizon reaches approximately 12 degrees, the marine horizon is no longer apparent and Nautical twilight exists. In the heavens only the brighter navigational stars are visible.
Finally, when the Sun drops approximately 18 degrees below the horizon (a hand’s span), Astronomical twilight commences and the remaining fainter stars become visible. Obviously the faintness of these depends on your local circumstances – it stands to reason someone observing from a dark rural site will see more stars than someone observing from a light-polluted urban site. Then, with the approach of dawn the same lighting levels are experienced – only in reverse.
As we move towards summer the northern hemisphere is inclined towards the Sun and twilight durations alter significantly. At our latitude, for 82 nights from late May until mid-August, astronomical twilight is totally absent as the Sun’s journey beneath the horizon is so shallow it never actually reaches 18 degrees. Matters become worse (for the astronomer) around the summer solstice when even nautical twilight levels barely exist here, the Sun setting by a mere dozen degrees. Not surprising then that many astronomers ‘mothball’ their telescopes, waiting for darker skies to return.
All is not lost with regard to stars themselves. In the coming months stellar luminaries such as Altair, Antares, Arcturus, Capella, Deneb, Spica, Vega and others will become familiar friends, especially if you enjoy a late evening stroll. Other phenomena to look out for include noctilucent cloud and even aurora. I shall talk about all of these in due course, and there is always the moon.
At least from northern England summer astronomy is just about possible, but as you travel further north nocturnal conditions grow ever lighter and only 13 degrees north of Whitby at latitude 67 degrees N (the Arctic Circle) twilight on any level is completely absent for part of summer and the Sun is visible above the horizon 24 hours a day. Only solar observers are completely happy for in the land of ‘midnight sun’, star gazing is limited to just one, that is if it’s not cloudy!
by Mark Lawson, Whitby & District Astronomical Society
For more info visit www.whitby-astronomers.com