Brian Jones: Summer sky sights
The orbits of the planets Mercury and Venus lie inside that of the Earth, which means that they never appear far from the Sun when viewed from our planet. Indeed, Venus graced our evening skies during spring, being visible as a brilliant star-like point of light in the western sky and setting several hours after the Sun. However, the orbit of Venus is now swinging the planet closer to our parent star (as viewed from Earth) and it will shortly pass between the Earth and Sun. When at these points in its orbit Venus would normally pass either just above or just below the Sun as seen from Earth. As a result, it would generally not be observable, being lost from view in the bright solar glare. However, there are occasions when the lining up of the Earth, Sun and Venus (or Mercury) are more or less exact, and at these times the planet in question can be seen silhouetted against the disc of the Sun.
Such events are known as transits, and on the 5th and 6th of June astronomers and sky gazers will witness a transit of Venus as the orbit of the planet carries it between our planet and the Sun. When this happens, Venus will pass directly between the Sun and Earth and will be visible as a tiny black disc against the Sun. The upcoming transit will actually start shortly after 11.00pm on 5th June, at which time the Sun is below the horizon as seen from the British Isles. As a consequence, only the latter stages of the transit will be visible to British observers, Venus being located near the edge of the solar disc and finally moving away from the Sun a little over an hour after sunrise.
Great care must be taken when observing the Sun, even when the Sun is located low down in the sky either on or near the horizon. Never look directly at the Sun, either with the naked eye or with optical aid, as you may permanently damage your eyesight. The best way to observe the transit is to project an image of the Sun onto a white screen, such as a piece of card. This can be done either by focussing the Sun’s image through a pinhole in a piece of card onto a second piece of card, or by using a telescope or one half of a pair of binoculars to the same effect. If binoculars are used they can be held steady by clamping them to a tripod. If you use a telescope, put a cover over the end of the finder scope so that you don’t accidentally look at the solar disc through it. Never align the telescope or binoculars by looking through them. The best method is to line the binoculars or telescope up in the direction of the Sun (bearing in mind the main lens of the telescope or binoculars should be capped during this process) and then manoeuvre the instrument round until the shadow of the telescope tube or binocular barrel on the screen is perfectly round. Then, keeping your eye away from the eyepiece, uncap the telescope or binoculars and you should see a projected image of the Sun on the screen. If you don’t, then some fine adjustment should bring the image of the solar disc into view. Once you have this, Venus should be visible as a tiny black disc situated near the edge of the solar image. From here, the movement of Venus across the face of the Sun will become noticeable over the period of a few minutes or so.
Transits of Venus are rare and only occur in pairs with an eight-year gap between each, the next pair of transits taking place over a century later. The last transit of Venus was in June 2004. The previous ones were in December 1874 and December 1882 and the next pair will not be seen until December 2117 and December 2125. So, if you miss this one you won’t get a second chance – there won’t be another taking place in your lifetime!
The night sky at this time of year is dominated by the three constellations Cygnus (the Swan), Lyra (the Lyre) and Aquila (the Eagle) which lie close to the overhead point during summer evenings. Particularly prominent is the triangle formed from the bright stars Deneb in Cygnus, Vega in Lyra and Altair in Aquila. Known as the Summer Triangle, this trio of stars is unmistakable and from here many of the other constellations on the chart can be located. (For more on the Summer Triangle click here).
If you look in the region of sky between, and slightly to the east of, Aquila and Cygnus you will spot the three smaller groups: Vulpecula (the Fox), Sagitta (the Arrow) and Delphinus (the Dolphin) (click here for more on Delphinus), while to the immediate southwest of Aquila is the faint but distinctive shape of Scutum (the Shield). All four of these constellations should be visible to the naked eye if the sky is dark and clear, although a pair of binoculars will help you to pick them out…
To the west of Lyra we see the conspicuous quadrilateral of stars marking the constellation of Hercules. Known as the ‘Keystone’, from it the rest of Hercules can be seen spreading away. Look immediately to the west of Hercules and you’ll spot the distinctive circlet of stars forming Corona Borealis (the Northern Crown), just beyond which is the distinctive shape of Bootes (the Herdsman).
If the sky is really dark and clear, you should be able to trace the winding pattern of Serpens Caput (the ‘head’ of the Serpent) snaking down towards the south. If you follow the line of stars shown here you’ll arrive at Ophiuchus (the Serpent Holder) with, to the east of Ophiuchus, the smaller line of stars Serpens Cauda (the ‘tail’ of the Serpent). According to Greek mythology, Ophiuchus depicts Asclepius, the god of medicine and son of Apollo. He is holding the head of the serpent in his left hand and the tail in his right hand, thereby splitting the constellation Serpens into two parts.
To the southwest of Ophiuchus, and lying a little way above the southern horizon, we can see the brilliant red Antares, the leading star in Scorpius (the Scorpion). The name Antares means ‘rival of Mars’ from the fact that, when Mars (often referred to as the red planet) and Antares are in the same area of sky, the two objects rival each other for prominence.
So, if the evening is warm, and the skies are clear and moonless, why not take yourself outside and check out the summer star patterns? And if you’re an early riser, look out for the transit of Venus on 6th June. Good luck… and happy sky gazing!