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Wed, 31 May 2017 - Looking Forward to June and July

June and July brings us the arrival of summer for the Northern Hemisphere, the arrival of Saturn at opposition and brilliant Jupiter on view in the early evening sky. At the end of the period the ice-giants will also be nicely on view.


Mercury, the winged messenger of mythology, starts June well separated from the Sun, some 21 degrees west of it in the early morning sky, but is very hard to glimpse from UK latitudes because it is so low against the horizon. It rises around 0315 ut on the first of the month but the sky is already brightening at mid-UK latitudes. It reaches just 5 degrees of altitude by sunrise at around 0355 ut so it might, just, be glimpsed from the south-coast in the first 15 minutes after rising. It is pretty much lost to the dawn for the rest of the UK but will be much better placed for equatorial and southern observers who should get good views in early June. Mercury then moves rapidly closer to the Sun, reaching superior conjunction behind it on the 21st of June and becoming an early evening object into July.


On the 30th of July it reaches its greatest evening (eastern) elongation from the Sun, separated by 27 degrees, but is still poorly placed for UK observers being just 5 degrees above the horizon at sunset, close by the star Regulus in the constellation of Leo; once again the southern hemisphere wins out when it comes to observing Mercury in this period.


Venus, on the other hand, is relatively easy to observe in this period and its shear brightness makes it easy to find it from the UK. At the start of June Venus rises, slightly north of due-east, around 0230 ut, 46 degrees ahead of the Sun, shining at magnitude -4.3 and showing a face some 25 arcseconds in apparent size from pole to pole, and with a 48% illuminated phase. Once found Venus can easily be followed into broad daylight and many imagers prefer to do this, waiting until the planet is high in the sky before taking pictures. Venus will pass due-south around 0900 ut and will then be at or above 45 degrees of elevation. I must remind any observer who wishes to follow a planet into daylight to take all precautions to ensure the Sun never enters the field of view of either the telescope or the finderscope to avoid permanent damage to eyes or equipment.


As the period progresses Venus moves slowly closer to the Sun and falls in both apparent size and brightness while increasing in visible phase. By the start of July Venus rises around 0135 ut and is still 43 degrees west of the Sun, reaching more than 20 degrees of elevation by sunrise. The planet shines at magnitude -4.3 and is 18 arcseconds in size with a 63% phase on view. By the end of the month these figures fall to -4.0 and 14.6 arcseconds but Venus is still visible at sunrise, around 0430 ut, and is due east at more than 25 degrees of elevation.


At the start of June Mars is setting deep in the north-western sky around 1 hour and twenty minutes after the Sun but will be too low to observe effectively in the long summer twilight. For the rest of the period Mars draws closer to the Sun and reaches conjunction with it on the 27th of July. On the other hand Jupiter is very observable, especially from the beginning of the period. It will be found due south at sunset, shining at magnitude -2.2 some 34 degrees above the horizon and showing a banded disc nearly 41 arcseconds across. As the night progresses Jupiter will sink slowly to the west and doesn’t set until around 0200 ut, just as the sky begins to lighten; this gives an extended period in which to observe the bands, zones, storms and spots visible on its ever changing face.


Moving towards July Jupiter will be found lower and lower towards the west at each sunset, on the first of the month it can be found forming a near-perfect equilateral triangle with the Moon and the star Spica in the constellation of Virgo then going on to set nearly due-west just minutes before midnight ut. By the end of July Jupiter is setting an hour earlier and will be quite low by the time it becomes obvious in the south-western sky; it is certainly best to observe it early in the period but it can be followed throughout and is another target that can be found and observed in broad daylight with suitable precautions.


Magnificent Saturn reaches opposition, due south at midnight universal time, on the 15th of June and it, too, can be observed throughout the period. With the ring system tilted towards us by 26 degrees and spreading to some 41 arcseconds across in apparent size the view should be breathtaking; unfortunately there is a problem! The path followed by the Sun across our skies is called the ecliptic and the other planets also follow a similar but not identical path; whether they are visible above or below the ecliptic depends on the plane of their orbital path and the angle it makes with the ecliptic. In June and July this year Saturn sits nearly 22 degrees below the celestial equator which explains why the rings appear tilted toward us and why we can see the north pole of the planet and the famous hexagonal cloud system that surrounds it. However, this southerly declination also makes Saturn very low in the sky when seen from the UK; along with this comes the problem of atmospheric dispersion, the bending of light from objects close to the horizon as it passes through our atmosphere. With red and blue light split at differing angles the resultant telescopic image can look very blurred and appear defocussed.


The answer, for both visual and photographic observers, is to view through an Atmospheric Dispersion Corrector or ADC. These little devices fit between the telescope and the eyepiece, or planetary camera, and correct for the atmospheric dispersion, bringing all light from the target back to a common focus and dramatically improving the view of low altitude targets. Saturn and certain other planets are going to be low in UK skies for a few years yet so, while not essential, it may be worth considering buying an ADC; good ones are now available for around £120.


The outer ice-giant planets are not readily observable at the start of the period. Neptune rises around 0110 ut in early June and may be found some 12 degrees up in the south-east, around 0230 ut, against the background stars of Aquarius, but at magnitude +7.9 and at only 2.3 arcseconds in size it is distinctly telescopic and it will be worth waiting until later in the period to get a much improved view. By late July Neptune rises around 2110 ut and transits, due south around 0235 ut at just over 30 degrees of elevation. Large amateur telescopes and cameras fitted with infrared filters may reveal cloud patterns in its azure disc.


The story is similar for Uranus, in early June it is too close to the Sun to be observed in full darkness ‘though it does lie very close to Venus and in the first week of the month and you may catch it in the early hours within some 2.5 degrees of that planet; on the 4th it sits directly above Venus which can be used as a guide to find Uranus which is itself on the edge of naked eye visibility at magnitude +5.88 and should be readily found with binoculars. The planet improves in visibility, rising at midnight ut in early July and reaching 20 degrees of elevation, a little south of due-east around 0230 ut, before the sky lightens too much to see it. By the end of July it can be followed from rise at 2230 ut until at least 0300 ut, for mid UK latitudes, when it will be nearly 40 degrees up in the south-east. Its apparent size will be small, at 3.6 arcseconds, but like Neptune it may reveal cloud details to large telescopes and cameras with infrared filters.

Alan Clitherow


Added by: Alan Clitherow