IT WAS 100 years ago this June that the Earth was rocked by its most violent impact in recent history. And it was sheer luck that it occurred over a remote area of Siberia and led to no known loss of human life.
Even so, the explosion did destroy more than 2,000 square kilometres of forest near the Tunguska River and throw enough dust into the upper atmosphere to light up our summer nights for weeks. No-one reached the epicentre for another 13 years, but the
investigations and theories have continued ever since.
We now believe that the dazzling fireball that streaked north-westwards across the Siberian sky on the morning of 30 June, 1908, had at its heart a small asteroid, no more than a few tens of metres across. As this ploughed into the atmosphere at perhaps 30km per second, the heat and stresses became so great that it detonated with the force of five megatons of TNT while it was five to ten kilometres above the ground.
The detonation felled trees in a radial pattern, though those directly beneath had only their branches blown off, leaving their trunks standing like a vast field of telegraph poles. With no obvious crater on the ground, it seems that the asteroid was pulverised almost instantly into pebbles and dust.
It is also believed that the asteroid was a piece of Comet Encke, following the comet's orbit in the stream of meteoroids that produce the annual Beta Taurids meteor shower, one that occurs mainly in daylight and is usually only "seen" by radar. Few asteroids as small as the Tunguska impactor are ever spotted by the surveys being made for "near-Earth objects" but they must be plentiful and we might expect similar impact events every 100 to 1,000 years.
Our June nights are characterised by night-long twilight and, if we are lucky, by the silvery-blue glow of noctilucent clouds. Literally "night-shining", these form at altitudes near 82 km, where they catch the sunlight long after our more familiar clouds are in darkness. June and July are their peak months and Scotland sits at just the right latitude to view them – wait for about one hour after sunset and scan the lower reaches of the northern quarter of the sky. Often tenuous and lacy in appearance, they sometimes take on a rippled or herring-bone pattern.
Believed to consist of ice particles around motes of dust, they appear to have increased in frequency over recent decades, perhaps because of growing pollution.
The Sun is furthest north of the equator at 00:59 BST on 21 June, the moment of the summer solstice. In the middle of that night the Sun stands only 10.6° below the northern horizon for Edinburgh, not enough for the sky to be officially dark. The persistent twilight swamps the fainter stars, but from further north, where the dip of the Sun is even less below the horizon, the twilight may hide all but the brighter stars.
Sunrise/sunset times for Edinburgh change from 04:35/21:47 BST on the 1st to 04:27/22:03 on the 21st and 04:31/22:02 BST on the 30th. The Moon is new on the 3rd, at first quarter on the 10th, full on the 18th and at last quarter on the 26th.
Unless you are a keen solar observer, June can be a fallow month for astronomy from our latitudes. Most nebulae are overwhelmed by the twilight, though M13, the glorious globular cluster of stars in Hercules, is in prime position high on the meridian at our star map times. This ball of 100,000 stars some 25,100 light years away appears as a fuzzy blob through binoculars and will still be on show to be glimpsed by the naked eye after the twilight subsides in August.
Just above our southern horizon, and with its red colour accentuated in the twilight, is the supergiant Antares in Scorpius, which lies close the Moon on the night of the 16th. Three nights later the Moon stands below-right of Jupiter, now our brightest night-time object after the Moon as it edges westwards in the constellation Sagittarius. The planet rises above Edinburgh's south-eastern horizon at 00:23 BST on the 1st and by 22:20 on the 30th, but climbs no more than 12° high in the south three hours after the map times. It is too low in the sky to get a clear view but telescopes show its disk swell from 45 to 47 arcseconds in diameter as it brightens a little from magnitude -2.6 to -2.7.
The only other planet visible at our map times at present is Saturn, which shines at magnitude 0.7 only 2.7° above-left of Regulus in Leo and is now creeping away from the star as they sink the lower in the western evening twilight. We can still find them some 20° high as the sky darkens, but this is down below 10° by the month's end.
Mars lies almost 20° to the right of Saturn and Regulus tonight and is a few degrees lower in the sky to the left of the Praesepe star cluster in Cancer. A little fainter than Regulus, it fades slightly from magnitude 1.5 to 1.6 this month as its speeds eastwards into Leo to lie only 0.8° north of Regulus by the 30th. Look for the young Moon almost in line with Saturn and Regulus, and 11° to the left of Mars on the evening of the 8th.
Neither Mercury nor Venus will be seen this month. Mercury navigates between the Earth and the Sun as it passes through inferior conjunction on the 7th, while Venus reaches superior conjunction on the Sun's far side two days later.
