Mercury
Mercury reaches greatest eastern elongation on September 4th and apart from the first few days is visible as an "evening star" throughout August from the tropics and Southern Hemisphere. This also happens to be their most favourable evening apparition of 2015, representing the best chance to catch a glimpse of this small world after sunset. Unfortunately, from northern temperate latitudes the angle of the ecliptic is unfavourable and the planet remains low down and unsuitably placed for observation throughout the month.
On August 7th, Mercury (mag. -0.6) passes 0.6 degrees north of more brilliant Jupiter (mag -1.7). Even brighter Venus is in the same part of the sky but about 6 degrees further south. The waxing crescent Moon passes 2 degrees south of Mercury on August 16th.
The diagram below shows the evening apparition of Mercury from latitude 35S (approx. equal to Sydney, Cape Town and Santiago). Positions are displayed 45 minutes after sunset. It should be noted that Mercury fades from magnitude -1.0 to +0.1 during August.
Venus
Venus, mag. -4.2, is now lost to the bright evening twilight from northern temperate latitudes although those living further south the planet may be seen for about the first 10 days of the month. The planet appears low down after sunset above the western horizon. Jupiter (mag -1.7) and Mercury (mag. -0.8) are located nearby with Mercury passing 8 degrees north of Venus on August 5th. When viewed through a telescope or binoculars (keen eyed observers), Venus appears as a very thin crescent only a few percent illuminated.
The planet then speeds through inferior conjunction on August 15th. A few days later it re-emerges in the eastern sky just before sunrise for Southern Hemisphere observers and by the last week of the month should also visible from northern temperate latitudes.
On August 29th, Venus (mag. -4.2) passes 9 degrees south of much fainter Mars (mag +1.7).
Mars
Mars reappears this month as a morning object in Cancer. Even though it shines at magnitude +1.8 and is brighter than any of the surrounding stars it's not particularly prominent, tending to struggle against the bright dawn twilight sky. The best time to look for the famous "Red planet" is towards the end of the month when it rises two hours before the Sun from northern temperate latitudes but only about half this time from the Southern Hemisphere. On August 29th, much more brilliant Venus (mag. -4.2) passes 9 degrees south of Mars with the second planet from the Sun outshining the fourth by 100 times.
With the Earth now slowly closing in on Mars the planet's visibility period will gradually start to improve over the coming months culminating on May 22nd, 2016 the date of opposition. Mars will then shine at magnitude -2.1 almost 40 times brighter than it appears now! Observers will also be able to study the planets surface, something that's currently not possible due the planets faintness, position in the sky and small apparent size of just 3.7 arc seconds.
On August 20th, Mars passes 0.5 degrees south of the Beehive Cluster (M44) in Cancer.
Jupiter
Jupiter, mag -1.7, long evening apparition finally ends in August. The giant planet has been illuminating the evening skies for months but is now no longer visible from northern temperate latitudes and likely to be glimpsed only for about the first 10 days of the month from tropical and southern latitudes. During this time it can be seen very low down towards the west-northwest horizon shortly after sunset. As previously mentioned, nearby is more brilliant Venus and elusive Mercury passes close by Jupiter on the evening of August 7th.
Jupiter passes through solar conjunction on August 26th and therefore is positioned too close to the Sun to be safely observed for the remainder of the month.
Saturn
Saturn is located in Libra reaching its second stationary point on August 2nd; thereafter direct motion is once again resumed. This is often regarded as signaling the end of the opposition period. From mid-northern hemisphere latitudes the beautifully "Ringed planet" is visible as soon as it's dark towards the southwest (NH) / northeast (SH) before setting around or just after midnight. Due to its current southerly declination, Saturn appears low above the horizon. However from more southerly latitudes it's much better placed, higher in the sky with a longer period of visibility.
The Solar System's second largest planet fades slightly from magnitude +0.4 to +0.6 with its apparent diameter shrinking from 17.3 to 16.5 arc seconds as the month progresses.
At the beginning of the month a good opportunity exists to spot bizarre moon Iapetus. This world is famous for its "two-tone" colouration with one side being much darker in colour than the other. As a result, Iapetus when positioned on the western side of Saturn (when viewed from Earth) appears brighter than from the opposite side. On August 6th, Iapetus reaches greatest western elongation and therefore at it's brightest. At magnitude +10.1, it can be seen with a small scope of 80mm (3.1-inch) aperture.
On August 22th, the first quarter Moon passes 2 degrees north of Saturn.
Saturn during August 2015 - pdf format
Uranus
Uranus, mag. +5.8, is an evening object moving retrograde amongst the stars of Pisces. At the start of the month, the distant planet rises in the east before midnight and a little earlier each day as the month progresses. It then remains visible for the remainder of the night.
Uranus is positioned 15 degrees south, 20 degrees east of the centre of the "Great Square of Pegasus" and 3 degrees southwest of star ε Psc (mag. +4.3). The planet is bright enough to be easily spotted with binoculars or a small telescope. It's also just visible to the naked eye although this is a challenging task, requiring dark skies.
A small telescope at high magnification will show the planet as a small green disk, obviously non-stellar (apparent diameter 3.6 arc seconds). However, even when viewed through the largest amateur scopes it's difficult to make out any surface details.
On August 5th, the waning gibbous Moon passes 1 degree south of Uranus with an occultation visible from South America and the Falkland Islands (8:44 UT).
Uranus during August 2015 - pdf format
Neptune
Neptune continues to move retrograde amongst the stars of the sprawling constellation of Aquarius. With now only days to go before opposition (Sep. 1st) the distant planet is superbly placed for observation, rising above the eastern horizon not long after sunset and remains visible for the rest of the evening. With a declination of -9 degrees, Neptune is currently better seen from tropical or southern temperate latitudes, appearing high in the sky.
The planet shines at magnitude +7.8. It's the only planet that's never visible to the naked eye and requires optical aid to be seen, although Uranus requires dark skies to be glimpsed. However, provided you know exactly where to look, Neptune is a relatively easy binocular or small telescope target.
Neptune is located almost exactly in the middle of Aquarius and about halfway along an imaginary line connecting lambda (λ) Aqr (mag. +3.7) with Sigma Aqr (σ Aqr - mag. +4.8). Thirty degrees to the northeast is the "Great Square of Pegasus". The brightest star in the surrounding region of sky Fomalhaut (α Psc - mag. +1.2) is located about 20 degrees to the south of the planet.
On August 2nd, the almost full Moon passes 3 degrees north of Neptune mag +7.8 and again on August 30th.
Neptune during August 2015 - pdf format
Solar System Data Table August 2015
| Date | Right Ascension | Declination | Apparent Magnitude | Apparent Size | Illum. (%) | Distance from Earth (AU) | Constellation | |
|---|---|---|---|---|---|---|---|---|
| Sun | 5th Aug 2015 | 08h 58m 08.2s | 17d 10m 20.6s | -26.7 | 31.5' | 100 | 1.015 | Cancer |
| Sun | 15th Aug 2015 | 09h 36m 08.9s | 14d 16m 59.8s | -26.7 | 31.6' | 100 | 1.013 | Leo |
| Sun | 25th Aug 2015 | 10h 13m 16.2s | 11d 00m 58.5s | -26.7 | 31.6' | 100 | 1.011 | Leo |
| Mercury | 5th Aug 2015 | 09h 50m 13.9s | 14d 44m 38.3s | -0.8 | 05.2" | 91 | 1.306 | Leo |
| Mercury | 15th Aug 2015 | 10h 53m 54.1s | 07d 39m 40.3s | -0.3 | 05.5" | 80 | 1.213 | Leo |
| Mercury | 25th Aug 2015 | 11h 45m 25.3s | 00d 41m 26.6s | 0.0 | 06.2" | 69 | 1.091 | Virgo |
| Venus | 5th Aug 2015 | 09h 53m 33.1s | 06d 19m 57.5s | -4.3 | 54.5" | 05 | 0.306 | Sextans |
| Venus | 15th Aug 2015 | 09h 30m 56.6s | 06d 31m 41.7s | -4.1 | 57.8" | 01 | 0.289 | Leo |
| Venus | 25th Aug 2015 | 09h 08m 37.8s | 07d 54m 39.0s | -4.2 | 55.6" | 04 | 0.300 | Cancer |
| Mars | 5th Aug 2015 | 07h 57m 51.8s | 21d 41m 08.5s | 1.7 | 03.6" | 99 | 2.570 | Gemini |
| Mars | 15th Aug 2015 | 08h 25m 00.3s | 20d 20m 47.1s | 1.7 | 03.7" | 99 | 2.553 | Cancer |
| Mars | 25th Aug 2015 | 08h 51m 26.5s | 18d 46m 23.1s | 1.8 | 03.7" | 99 | 2.530 | Cancer |
| Jupiter | 5th Aug 2015 | 10h 03m 27.5s | 12d 50m 03.9s | -1.7 | 31.1" | 100 | 6.349 | Leo |
| Jupiter | 15th Aug 2015 | 10h 11m 42.2s | 12d 05m 08.0s | -1.7 | 30.9" | 100 | 6.384 | Leo |
| Jupiter | 25th Aug 2015 | 10h 19m 59.7s | 11d 19m 00.8s | -1.7 | 30.8" | 100 | 6.398 | Leo |
| Saturn | 5th Aug 2015 | 15h 45m 06.1s | -17d 48m 40.6s | 0.4 | 17.2" | 100 | 9.663 | Libra |
| Saturn | 15th Aug 2015 | 15h 45m 34.8s | -17d 52m 44.5s | 0.4 | 16.9" | 100 | 9.825 | Libra |
| Saturn | 25th Aug 2015 | 15h 46m 43.0s | -17d 58m 52.0s | 0.5 | 16.6" | 100 | 9.991 | Libra |
| Uranus | 5th Aug 2015 | 01h 15m 46.8s | 07d 18m 12.9s | 5.8 | 03.6" | 100 | 19.589 | Pisces |
| Uranus | 15th Aug 2015 | 01h 15m 20.9s | 07d 15m 20.3s | 5.8 | 03.6" | 100 | 19.439 | Pisces |
| Uranus | 25th Aug 2015 | 01h 14m 38.2s | 07d 10m 47.8s | 5.8 | 03.7" | 100 | 19.304 | Pisces |
| Neptune | 5th Aug 2015 | 22h 43m 13.1s | -08d 59m 01.2s | 7.8 | 02.4" | 100 | 29.398 | Aquarius |
| Neptune | 15th Aug 2015 | 22h 42m 17.2s | -09d 04m 49.9s | 7.8 | 02.4" | 100 | 29.265 | Aquarius |
| Neptune | 25th Aug 2015 | 22h 41m 17.3s | -09d 10m 59.1s | 7.8 | 02.4" | 100 | 29.152 | Aquarius |