Looking Forward; The Planets in August and September.

Mercury is between the Earth and the Sun, inferior conjunction, on the 9th of August but then moves into the dawn sky to give its best morning apparition of the year for the Northern Hemisphere. Greatest western elongation from the Sun (18 degrees) occurs on the 26th and from around the 22nd Mercury can clearly be seen rising on a compass bearing of 070 degrees around 90 minutes before sunrise. On the 26th it will reach nearly 15 degrees of elevation at sunrise, for mid-UK latitudes, and with great care it can then be followed into daylight. After the 26th the planet’s elevation will fall with each sunrise but it can be observed into September and even by the 8th of the month Mercury still has 10 degrees of elevation at sunrise. Its phase, close to 50% in this period, will be clearly visible and surface albedo features have been seen and imaged under excellent seeing conditions.

Venus is in the evening sky and reaches its greatest elongation to the east of the Sun (46 degrees) on the 17th of August. Sadly the position of the ecliptic in summer evening skies means Venus is only seen at low altitudes from the UK. On that date the planet should become visible from around 1930 Universal Time (UT), 20 degrees south of due-west but at only 10 degrees of elevation. Visibility is helped by the great brightness of the planet, at magnitude -3.6 even after atmospheric extinction, and it can then be followed for around an hour until it sets. By early September Venus has only 5 degrees of elevation at sunset so it is best observed in early to mid-August.

Mars was at opposition in late July so is visible for much of the hours of darkness however it also suffers from low elevation as seen from the UK. It is best observed around the time of transit, due-south, when it attains its maximum elevation. In early August this will be shortly before midnight UT with 12 degrees of elevation; mid-period transit is at 2130 UT, still at 12 degrees up and by late September Mars transits at 2010UT with 15 degrees of elevation. In the period Mars’s brightness will fall from magnitude -2.8 to -1.3 and its apparent size from 24.3 to 15.8 arcseconds however this is more than large enough to track the progress of the global dust storm currently encircling the planet.

Jupiter will start the period low to the south-western horizon for UK observers and sinking lower as the period progresses, so is best seen in early to mid-August when detailed observation is still possible. On the first of August look to the south-west half an hour after sunset to see Jupiter shining at magnitude -2.7 around 15 degrees above the horizon; it will then appear lower night-on-night as the period progresses. With good setting-circles or an accurate go-to system Jupiter can be found in full daylight and may then reveal more detail than in twilight skies.

Like Mars, Saturn is best seen close to its time of southerly transit. In early August this is around 2140 UT when Saturn achieves 15 degrees of elevation. By September 1st transit is at 1935 UT, still at 15 degrees, but this occurs in early twilight; Saturn can then be followed into darkness, sinking below 10 degrees of elevation by 2140 UT. By the end of the period look for Saturn from 1900 UT, now some 20 degrees past transit and sinking in the darkening evening sky.

The ice giants of Uranus and Neptune are the only major planets with significant night-time elevation in the period. Neptune comes to opposition on the 7th of September so can be followed for most of the hours of darkness for in this period, just over 3 degrees east of the star Hydor in Aquarius. At Magnitude +7.8 and 2.4 arcseconds in size Neptune is a telescopic object only but apertures of around 10 inches (250 mm) or more with suitable planetary cameras and infra-red filters may reveal cloud features on this distant world. At more than 30 degrees of elevation around the hours of transit, Neptune is above most of the atmospheric distortions that plague low-altitude observation.

Uranus gains even more altitude than Neptune but doesn’t reach opposition until October 24th so is best observed in the early hours and from later in the period. In early September it transits around 0330 UT at nearly 50 degrees of elevation; transit is at 0130UT by the end of the month. At magnitude +5.7 and 3.7 arcseconds in size it may be seen by the naked eye from a dark site, close to the border between Cetus and Pisces, but is best seen telescopically. It can be distinguished from the background stars as it appears distinctly blue-green, a very odd colour for a star, and at low magnification will look slightly unfocussed compared with the true stars surrounding it; higher magnification will reveal its disc. Uranus responds well to the same cameras and filters used to image Neptune and will show subtle shading from equator to pole and may also reveal storm patches in the infra-red.

Alan Clitherow