The mysterious noctilucent clouds of summer
If we are prepared to do battle with June’s night-long twilight, and provided the weather improves at last, there is plenty of interest in our June sky. Saturn is the pick of the planets while the bright star Vega in Lyra leads the onslaught as the constellations of summer invade from the east at our star map times. We also need to be alert for noctilucent clouds as they make their seasonal appearance low in our northern sky.
The Sun is furthest north at 11:51 BST on the 21st, the instant of our summer solstice. On that day, the Sun dips only 10.6° below Edinburgh’s northern horizon in the middle of the night, so that our sky remains bathed in twilight throughout the night while from further north in Scotland the sky is brighter still. This obviously impedes our ability to see the dimmer stars and “faint fuzzies” such as galaxies and nebulae. On the other hand, it means that satellites remain sunlit whenever they pass overhead. Indeed, the International Space Station is conspicuous two or three times each night until 10 June as it transits from west to east across Scotland’s southern sky – visit heavens-above.com for predictions customised for your location.
The Sun’s shallow sweep below our northern horizon overnight also allows us occasional views of noctilucent or “night-shining” clouds. Composed of tiny ice crystals in a thin layer at a height near 82 km, they catch the sunlight long after our usual low-level clouds are in darkness and can appear like chaotic banks of electric-blue cirrus, sometimes in a herringbone pattern. Their preferred direction follows the Sun around the horizon, so they are more commonly seen low in the north-west after nightfall and towards the north-east before dawn. They occur from mid-May to mid-August but why they are more frequent than they were a century ago remains a mystery. Could the rise be due to global warming, increased industrial pollution or even particles from rocket launches?
Sunrise/sunset times for Edinburgh change from 04:35/21:47 BST on the 1st to 04:26/22:03 on the 21st and 04:31/22:02 on the 30th. The Moon is at first quarter on the 5th, full on the 13th, at last quarter on the 19th and new on the 27th.
At magnitude -1.9, our brightest evening planet continues to be Jupiter, but we must look lower into the west to catch it below Pollux in Gemini as the twilight fades. Shining at magnitude -1.9, it stands 9° above-right of the Moon on the 1st. Jupiter sinks to set in the north-west almost three hours after the Sun as June begins but by the 30th it is only 6° high at sunset and may already be lost from view.
Mercury lies 18° below and to the right of Jupiter on the 1st but is one twentieth as bright at magnitude 1.4 and fading rapidly as it moves to pass through inferior conjunction between the Sun and Earth on the 19th.
The bright star Arcturus in Bootes stands high on the meridian at nightfall but has moved to the middle of our south-western sky by the map times. This leaves our high southern sky devoid of bright stars until we come to Vega in Lyra high in the east-south-east. Directly below Vega is Altair in Aquila while Deneb in Cygnus, almost due east, completes the Summer Triangle. The arc from Vega to Arcturus cuts through Hercules and Corona Borealis, the pretty semi-circular Northern Crown whose main star has the dual names of Alphecca or, perhaps more appropriately, Gemma.
Mars fades from magnitude -0.5 to 0.0 as it tracks eastwards in Virgo towards Spica. It also recedes from 119 million to 148 million km during the month as its small disk contracts from 12 to 9 arcseconds if viewed through a telescope. Look for its reddish light about 26° high in the south-west at nightfall and catch it above the Moon on the 7th. Our maps show it sinking towards the west where it sets two hours later.
Saturn, magnitude 0.2 to 0.4, stands almost 20° high in the south at nightfall at present and continues to creep westwards in Libra almost 4° above-left of the double star Zubenelgenubi. After standing close to Spica on the 8th, the Moon lies near Saturn on the 10th when the planet appears 18 arcseconds wide, its disk set within rings that span 41 arcseconds and have their north face inclined 21° towards us. Don’t miss an opportunity to observe it this month for it will soon be following Mars lower into the south-west at nightfall, and it stands even further south in our summer sky during every year until 2022.
Continuing as a brilliant morning star of magnitude -4.0 to -3.9, Venus rises above Edinburgh’s east-north-eastern horizon 61 minutes before the Sun tomorrow and in the north-east 102 minutes before sunrise on the 30th. Before dawn on the 24th, it lies 5° left of the slender waning Moon and 6° below the Pleiades in Taurus.
Last month, I reported the prediction that the Earth would slice through streams of particles from Comet 209P/LINEAR on the morning on 24 May and that the resulting meteor shower might be spectacular. In fact, it appears that the encounter occurred as forecast, but that the resulting display was disappointing with only a few bright meteors, even for observers in the Americas for whom the timing of the outburst was ideal. Radar studies suggest that the vast majority of meteoroids were unusually small and their meteors too dim to be seen by the unaided eye.