Juno’s all-seeing eye will cast a glance at turbulent Jupiter
AUGUST will soon be upon us and with it the return to a respectable allocation of darkness. The advent of astronomical twilight is on the night of the 18th when, with the centre of the sun sinking more than 12 degrees below the horizon for the first time in almost four months, all astronomical observation will be possible again – assuming the skies will be clear enough for earnest watchers to make their observations; should the weather continue its present trend that may well not be the case.
In some respects there won’t be all that much to see with Mercury, for example, at inferior conjunction on the 17th. This is the astronomical term for being between us and the sun and, as might be expected, the smallest of the planets is entirely swallowed in our star’s almighty glare for much of the month.
However, this swift-moving object does not stay long anywhere and will make its best appearance of the year for us in the last week of August onwards into mid-September. Mercury’s orbit is one of the most eccentric in the solar system and, on an average of 13 times per century, it passes exactly between us and the sun in a process called a transit which is readily observable with a minimum of equipment.
The last transit took place in November 2006 and the next is due in May 2016 which gives us all something to look forward to within a reasonable timescale.
For just now, look out for Mercury about half an hour before dawn from August 24 to 31. It will be on a bearing of around 72 degrees and a mere two degrees above the horizon on the 24th but will climb to around nine degrees and move to a bearing of around 80 degrees by the 31st.

Use binoculars to pluck it from the twilight but put them away before you catch the slightest glimpse of the sun’s incandescent face.
Venus passes through superior conjunction – opposite us on the other side of the sun – on August 17 and will not be visible at all this month.
It is set to reappear as the evening star in late September. Like Mercury, Venus makes transits of the sun but much less frequently with its passages across the sun’s face taking place in pairs every 109 years. We are very lucky as we were able to witness the first transit of the current pair on June 8, 2004 and, weather permitting, will be able to catch the second part on June 5 to 6, 2012 – another astronomical date to look forward to.
If you miss it you will have to wait till 2117, which will be a bit long for most of us!
MARS is gradually brightening and is now visible from shortly after midnight until dawn but remains almost indistinguishable from the brighter stars. Jupiter is the king of the early morning, very bright in the dawn sky and on view from shortly after midnight by the end of the month.
Saturn is extremely low in the south-western sky and, as it is setting by 11 at the beginning of the month and by nine towards the end, is almost unobservable from our latitude.
August is, of course, the month of the Perseids meteor shower, one of the most prolific showers in the annual calendar. Its peak usually coincides with Glorious Twelfth, a bad day for grouse, but this year is predicted for between 2am and 4am on the 13th.
Last year’s shower produced a zenithal hour rate (the number of meteors theoretically visible to an observer under dark skies if the radiant was at the zenith) of 173 and this year’s is equally promising.
Unfortunately, the peak coincides with the full moon so, even if the meteors do make their appearance, we will not necessarily see them as they will be drowned in the bright shining light of the moon. Drat and double drat!
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The shower has been known about for at least 2000 years and has been observed throughout that time, mainly in China and the Middle East. In Catholic countries it is sometimes known as the Tears of St Lawrence as the peak of the shower coincides with the supposed date of his martyrdom.
The unfortunate saint was allegedly burned to death on a gridiron by the despotic Roman emperor, Valerian.
However, all did not go well for Valerian as he subsequently became the only Roman emperor ever to be captured by the barbarians. Their leader, Shapur, is said to have used him as a human footstool to assist him in mounting his horse – which may have given him pause to consider his many evil misdeeds.
The moon is new tomorrow so look for its slender crescent by Wednesday. First quarter arrives next Saturday at 11.08UT. It will be full, as already mentioned, at 6.58pm on August 13. Last quarter will occur at 9.54pm on Sunday the 21st and new moon returns again on Monday, August 29, at 3.04UT.
CAITHNESS Astronomy Group will be holding an open day in Caithness Horizons in Thurso on August 13 and, in September, the local branch of the British Science Association will be holding a two-day event at which you can become a rocket scientist for the day. Yes, for an entry fee of only £15, you can enter a two-person team to learn about, build and fly your very own rocket.
Neither you, nor your rocket, will get into orbit from Castlehill Heritage Centre, or the nearby Castletown airfield, but some of these rockets will reach quite a height. The event is spread across September 10
Meanwhile, next Friday, from the Kennedy Space Centre, NASA is launching an even more impressive rocket than any likely to be constructed in Caithness in the near future. The payload is Juno, named after the Roman goddess, consort of Jupiter, and, like its namesake, is intended to view the giant planet with an all-seeing eye.
The journey will be a long one but, all being well, Juno will arrive in Jove’s vicinity in five years’ time and commence its examination of his turbulent atmosphere while orbiting from pole to pole.
The principle intention is to use a variety of instruments to penetrate Jupiter’s atmosphere and determine something of its inner structure but the cross-polar orbit will also enable a very close look at Jupiter’s mighty aurora.
Like the Earth’s aurora, these are believed to be created by the interaction of the planet’s magnetic field with the solar wind. However, the origin of Jupiter’s magnetic field remains a mystery given that, unlike Earth, it is not believed to have a molten metal core which would allow the giant dynamo effect which Earth enjoys to start up.
Maybe hydrogen, under immense pressure, acts like a metal and gives rise to this phenomenon. It is hoped Juno will find out.
When its work is done, the £600 million spacecraft will be crashed into Jupiter’s atmosphere, a deliberate move to ensure it will never collide with Europa.
This monster moon is believed to have, under its icy exterior, a salty ocean many miles deep. Where there is water there may be life and NASA does not wish to risk contaminating it with our bacteria. An exciting thought in itself.
Jim A. Johnston would welcome suggestions for future topics for the Skywatching column. He may be contacted by e-mail at jimajo@btinternet.com