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Mon, 31 Jul 2017 - Looking Forward: August and September 2017

All the major planets can be observed in this period but the observing opportunities may be short or inconveniently early for many. This is, perhaps, a period for the dedicated planetary observer!

Mercury may just be visible at the start August in the bright evening twilight, very low on the western horizon, but is rapidly lost, moving into inferior conjunction, between the Earth and the Sun, on the 26th of August. It emerges into the morning sky in early September and puts on a reasonable show, for Northern Hemisphere observers, during the middle of that month. It reaches its greatest western (morning) elongation from the Sun on the 12th of September when it rises 18 degrees ahead of the Sun at around 0400 UT. It will be found a little north of due-east, below and lagging a little behind the far more obvious Venus and keeping close company with distant Mars. On that date Mercury can be followed telescopically until at least 0500 UT. The same is true of a few days either side of greatest elongation; look for mercury rising close by the star Regulus, little more than half a degree apart on the 10th of September, and again on the 17th when Mercury and Mars rise one-third of a degree apart.

As mentioned Venus will be bright and obvious in the morning sky throughout this period, rising more than 3 hours before the Sun in early August, around 0125 UT, and only a little less than three hours before the Sun in early September, around 0220 UT. The planet will shine at a brilliant magnitude -4.0 for much of the period, only falling to -3.9 from mid-September while the illuminated phase will grow from 75% to 90% in the period. This will be an excellent time for both visual and photographic observation of Venus, particularly for imaging the cloud patterns visible in ultra-violet light using specialist UV or deep-blue filters.

While Mars is too close to the Sun to observe in August, it does, as mentioned, start to make an appearance from early September and is visible in a fully-dark sky by the end of the month. In late September Mars is close to Venus, rising around 0400 UT and shining at magnitude 1.8 a little north of due-east. At less than 4 arc-seconds in diameter it is comparable in size to Uranus however observers have already proven that it is possible to glimpse detail at even this early a stage in its apparition.

Jupiter, the king of the planets, is coming to the end of its period of rule in the night-time sky. In early August Jupiter will still be visible, quite low in the south-western sky, from around 2030 UT, and may be followed until it sets around 90 minutes later. This period of visibility reduces sharply as the year rolls on such that by early September Jupiter is setting at 2000 UT, and at 1820 UT by the end of the period. Observers should catch it in August or alternatively follow this planet in daylight, taking all suitable precautions to avoid equipment or eye damage when searching for an object relatively close to the Sun.

Saturn is slightly better placed in this period. In early August Saturn may be found due south at around 2050 UT and followed for nearly four hours as the planet moves lower and lower into the western sky. Shining at magnitude +0.3 Saturn sits against the background stars at the lower end of Ophiuchus, between Sagittarius and Scorpius. Its low elevation, just 16 degrees or so as it transits due south, may hinder very detailed observation however use of an Atmospheric Dispersion Corrector, if available, will greatly improve the view and the rings are beautifully presented, slanted towards us by nearly 27 degrees and are really not to be missed. By early September Saturn transits in daylight and will only become visible after around 2000 UT; it can then be followed for at least 2 hours but the period of visibility shrinks as the period moves on. By the end of September look for it on a bearing of around 210 degrees as darkness falls, some 10 degrees above the horizon.

For observers of the outer ‘ice-giant’ planets, things are well placed in this period. Neptune comes to opposition, due south at midnight UT, on the 5th of September and from early August can be found due-south, around 30 degrees high in elevation, at around 0230 UT against the background stars of Aquarius. By the end of September Neptune transits at 2230 UT, in full darkness and can still be followed above 20 degrees of elevation for around 3 hours. Although distant and tiny, distinctly telescopic at magnitude +7.8 and less than 2.5 arcseconds across in apparent size, photographic observers have caught major storm features on the face of Neptune using modern planetary cameras, larger aperture telescopes above 250 mm (10 inches) and near-infrared filters.

The same is true of Uranus which lags behind Neptune but rises even higher in the sky giving even better opportunities for steady seeing conditions. In early August Uranus rises around 2215 UT, reaching over 40 degrees of elevation before early twilight starts to hinder its visibility the next morning. In early September Uranus transits in full darkness from around 0310 UT at around 48 degrees of elevation and by the end of the period it transits at 0115 UT, still at this excellent elevation. Shining at magnitude +5.7 Uranus is on the edge of naked-eye visibility, though it has to be said you will need a dark sky and a moonless night to find it this way. Telescopically it shows a disc of 3.7 arcseconds in apparent size and it also responds very well to photographic observation, using the same equipment and IR filters as used on Neptune. If you have not bothered to find and observe these two planets before then this observing period is the best time to capture them both in one night.


Added by: Alan Clitherow