Both Jupiter and Mercury have now dropped out of sight in the evening sky, but together with Mars will reappear later next month in the early dawn sky.
Saturn and Venus form the planetary interest in the late evening twilight, which at this time of year is 10.30pm. Look for Venus very low in the WNW around 5 degrees above the horizon (four fingertips at arm’s length).
Even in twilight Venus is pretty conspicuous. Saturn resides up in the WSW as twilight falls, visible to the naked eye to the left of Spica, chief star in Virgo. Through the eyepiece Saturn is a glorious sight with the ring system favourably orientated. Saturn’s largest moon, Titan, may also be glimpsed as a speck of light close by. Our moon lies closest to Saturn on July 16.
‘Super mega moon’
Following May’s ‘mega’ moon, the full moon on June 23 will appear even more impressive being the largest and closest of 2013, making it a ‘super mega moon’!
Mega moons events occur when full moon appears at, or near perigee, when it is closest to Earth in its orbit.
On this occasion, the Moon will be just 356,991 kilometers (221,824 miles) away.
The moon will look approximately 14% larger and 30% brighter than normal. The image of an apparently huge moon looming large over the horizon is one etched onto our minds from a very early age.
It is however a deep rooted subconscious trick of the brain and only a few times a year is the moon actually fractionally larger than normal.
With bright twilight now persisting well into the night, observers of the sky should watch out for a particularly beautiful type of cloud formation seen only at this time of year.
Known as Noctilucent cloud, these delicate formations appear above the northern horizon, long after sunset, often around midnight. Shining quite brightly, Noctilucent cloud is filamentary in structure, having a characteristic silvery-blue colour.
It forms almost exclusively between latitudes 50 and 60 degrees north, high in the upper atmosphere: 50 miles up- five times higher than normal clouds.
The cloud forms when water vapour condenses at the low temperatures that prevail at such altitudes onto particles suspended in the air. More frequent sightings of Noctilucent cloud over the last 40 years may indicate that these particles could be a result of industrial pollution, perhaps from increased air traffic.
As far as observers of the sky are concerned, the right type of cloud, for the wrong reasons.
One of my observing highlights last summer was a display of Noctilucent cloud – simply stunning. There have already been sightings this year so do keep watch!
The Celestial Ocean - The Summer Solstice
The official start of summer in the northern hemisphere commences on June 21 – the date of the summer solstice this year, when the Sun reaches its greatest altitude on the ecliptic; the path it takes across the sky during a year. From our latitude this equates to almost 59 degrees above the southern horizon at midday.
The word ‘solstice’ is derived from the Greek for ‘Sun’ and ‘stoppage’. From our perspective the Sun stops moving north in the sky and starts to retreat southwards once again as the axial tilt of Earth begins to shift away from the direction of the Sun.
Many people wrongly assume that Earth is closer to the Sun at this time of year; in fact the opposite is true. Earth is actually furthest from the Sun on July 4; almost 95 million miles distant, three million miles further than when at its closest approach in early January!
The reason days feel warmer in summer is due to the higher concentration of sunlight per unit area as a result of the axial tilt toward the Sun. The intensity of solar radiation is then equivalent to approximately 1.2KW of heat for each square metre, although in recent years this seems hard to comprehend!
Although known as the longest day, earliest sunrise and latest sunset do not fall on the summer solstice date. From Whitby earliest sunrise actually occurs on June 16th at 04:25am, whilst latest sunset occurs on June 25th at 21:42pm.
It is the duration of useable daylight which does reach a maximum on the 21st, equating to around 17 hours from our latitude. Then the Sun only dips below the horizon a meagre 12 degrees, barely enough for nautical twilight to exist.
Thousands of years ago the position of the summer solstice stood before the stars of Cancer in the northern hemisphere, but has subsequently shifted due of the effects of precession, Earth’s slow axial wobble.
From our latitude the Sun never appears overhead, but may do so anywhere between latitude 23.5 degrees North and latitude 23.5 degrees South, casting no shadows, a phenomena astronomers in antiquity duly noted. Because the summer solstice then stood before Cancer, the latitude of 23.5 degrees north eventually became known as the tropic of Cancer. Similarly then, the Sun then stood before the stars of Capricorn when at its southern limit on the ecliptic (the winter solstice) 23.5 degrees south of the celestial equator and the origin of the tropic of Capricorn.
A glance at the night sky at this time of year offers up relatively few stars before midnight and for the casual observer not familiar with the changing aspect of the heavens, these scattered jewels may seem unfamiliar. So let me reveal the identity of these stellar friends ‘incognito’.
We’ll start high to the NE with the second brightest star visible during the summer months, Vega -chief star in Lyra - the Harp. Vega is a young adolescent star, a fact reflected by its lovely steely blue-white luster. During the 1980’s it was discovered that Vega is still surrounded by material left over after its formation, which is thought eventually will coalesce into a planetary system. Vega is situated just 26 light years away. Our next star, Altair, is located across in the ESE in the constellation of Aquila the Eagle.
Also considered a young star, Altair is a star in a spin, an unusually rapid one, rotating in less than six hours. As a result, Altair is not spherical like our Sun, but decidedly rugby ball shaped. Altair also has the distinction of being the nearest first magnitude star visible from Britain during the summer, a mere 16 light years away.
Both Altair and Vega play a prominent role in one of the few star legends to have come down to us from ancient China, the appealing story of the Weaving girl and the Herd–boy. According to this tale, Vega the weaving-girl and Altair the herd-boy, were deeply in love and lost in amorous dalliance neglected their duties. The two were suitably punished, eternally separated by the celestial river, the impassable Milky Way. Only once a year, on the seventh night of the seventh moon were the lovers allowed to meet, when a bridge of doves temporarily spanned the stellar river.
The two will embrace once again on July 15, no voyeurs please!
Our next port of call is more associated with the winter sky. Capella, chief star in Auriga the Charioteer, is actually visible all year round from our latitude (circumpolar) but during summer is located low above the N horizon, hence it is often mistaken for the North Star.
It is situated 46 light away.
The brightest star visible throughout summer and early autumn, Arcturus, is presently located high in the SSW. In the distant past Arcturus was probably comparable to our Sun but has evolved into an orange giant’ star 30 times its original diameter. Arcturus is 37 light years away.
Further down in the SW two conspicuous stars are apparent. The left hand one is Saturn, whilst the star ahead of it is Spica in Virgo, a blue giant some 260 light years distant. Our next luminary, Antares, meaning ‘rival of Mars’ because of deep orange hue, lies in the constellation of Scorpius and is one of the largest stars visible to the naked eye. Classed as a red super giant nearing the end of its life, Antares has a diameter estimated to be approaching 250 million miles.
Our final destination is the chief star of Cygnus, currently located mid-way up in the NE. Together with Vega and Altair, Deneb is the third member of the ‘summer triangle’ asterism.
Ranked only seventh brightest star in our summer sky, its immense distance (around 1700 light years) mask its true nature, a genuine heavy weight amongst luminaries, 25 times more massive and 140 thousand times more luminous than our Sun. In terms of true age Deneb is one of the youngest of the stars on our tour, however it consumes its fuel at such a prodigious rate that perhaps in little more than 15 million years it will fully evolve into a star almost twice the size of Antares, ending its days in cataclysmic fashion as a supernova. Then, for a few weeks Deneb will outshine all but the Sun and Moon and our descendants will marvel at the new light in the sky.
The Whitby and District Astronomical Society will be hosting public Star Parties on June 21, 22 and July 13 from the West Cliff – Captain Cook’s monument headland. 9.30pm.