June Night Sky
Jupiter continues to linger low in the evening twilight sky over in the NW, but by the end of June finally drops into bright solar glare and is lost to view. The moon lies nearby on June 1st. The ochre hue of Mars remains quite conspicuous in the late evening sky over in the SW and is now once again in prograde motion (moving west to east as seen against the background stars). Telescopically Mars appears quite small with just a hint of surface detail visible. The moon lies nearby on June 7th. Saturn is now the best placed in the evening/overnight. Look for it due south around 11pm, a bright pearly ‘star’ a couple of hand spans above the horizon. Through a scope the stunning rings should be quite apparent – one of nature’s finest sights. Look for Titan, Saturn’s largest moon, as a speck of light nearby; it circles round Saturn twice a month in an anti-clockwise direction. Saturn lies near our moon on the 10th. Finally, Mercury is once again lost as it drops below the horizon, whilst in the dawn sky Venus lingers just above the SE horizon.
Moon Phases: 1st Qtr 7th, Full 12th, Last Qtr 21st, New 28th
The Celestial Ocean – The Summer Solstice
The Summer Solstice occurs on June 21st at which point from the northern hemisphere the Sun reaches its greatest altitude on the ecliptic; the path the Sun follows 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 4th; 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, caused by the axial tilt of the Northern hemisphere towards the Sun. The intensity of solar radiation is then equivalent to approximately 1.2KW of heat for each square metre, although sometimes you would find this hard to believe!
Although known as the longest day, earliest sunrise and latest sunset do not occur 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 21:42 pm. It is the duration of useable daylight that does reach a maximum on the 21st, which, from our latitude, is just over 17 hours. The Sun only dips below the horizon a meagre 12 degrees, barely enough for nautical twilight to exist.
Thousands of years ago this position of the summer solstice stood before the stars of Cancer in the northern hemisphere, but has subsequently shifted due to 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, 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.
With bright twilight now persisting well into the night, observers of the sky may like to keep an eye open for a particularly beautiful type of cloud formation seen only at this time of year. Known as Noctilucent cloud, these delicate formations appear in the twilight sky above the northern horizon, long after sunset, even after 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.
by Mark Lawson, Whitby & District Astronomical Society
For more info visit www.whitby-astronomers.com