Mercury passes through inferior conjunction on August 8th. The fast moving planet then moves into the morning sky, reaching greatest elongation west on August 26th. On this day, it will shine at magnitude -0.1 and be positioned 18 degrees from the Sun. From northern temperate latitudes, Mercury will rise 90 minutes before sunrise and be visible very low down above the eastern horizon.
From southern locations, Mercury is not suitably placed for observation this month.
Venus remains a brilliant early evening object, visible above the western horizon as soon as it's dark enough. On the first day of August, the planet moves from Leo into Virgo where it remains for the rest of the month.
Venus increases in magnitude -4.3 to -4.6, with its illuminated phase decreasing from 57% to 41%. During this time, the planet's apparent diameter increases from 20 to 29 arc seconds. On August 14th, the waxing crescent Moon passes 6 degrees north of Venus offering pleasant early evening viewing. On the last day of the month, Venus passes within 2 degrees of first magnitude Spica (α Vir - mag. 1.0), the brightest star in Virgo.
Mars remains superbly placed for observation during August. The red planet passed its nearest opposition for 15 years last month, and this month is visible just after sunset and for most of the remainder of the night.
Mars begins August at magnitude -2.8, with an apparent diameter exceeding 24 arc seconds. It's moving retrograde in Capricornus. However, since its distance from the Earth is now increasing, it will fade to magnitude -2.2 with its apparent diameter shrinking to 21 arc seconds by month's end. During the 3rd week of August, the planet moves close to the Sagittarius-Capricornus constellation border. On August 28th, Mars reaches its second stationary point and after that direct motion is resumed. This event is widely regarded as signalling the end of the current opposition period.
With a small telescope, of the order of 80mm (3.1-inch) aperture, and providing recent dust storms have cleared it should be possible to spot details, such as Syrtis Major, on its pink surface.
Although fading, Jupiter remains a brilliant evening object moving direct in Libra. It's visible towards the southwest (NH) / north (SH) as soon as it's dark enough. However, by months end it sets well before midnight from northern temperate latitudes, although the visibility is considerably longer for those located further south.
As the month progresses, the giant planet fades from magnitude -2.1 to -1.9 with its apparent size decreasing from 38 to 35 arc seconds. On August 17th, the waxing crescent Moon passes 5 degrees north of Jupiter. On the same day, Jupiter moves to 0.5 degrees of double star, Zubenelgenubi (α Lib - mag 2.7).
Saturn remains a well-placed evening object moving retrograde in Sagittarius. From mid-latitude northern locations, the ringed planet sets around midnight by months end but over two hours later from those located further south. During August, its brightness decreases slightly from magnitude +0.2 to +0.4 with its apparent size decreasing marginally from 18.0 to 17.3 arc seconds.
The waxing gibbous Moon passes 2 degrees north of Saturn on August 21st.
Uranus, mag. +5.8, is a late evening object in Aries. At the beginning of the month, from northern temperate locations, the distant ice giant rises about midnight, improving to a couple of hours before midnight by months end. For those living further south it rises about 30 minutes later than this. On August 7th, the planet reaches its first stationary point, which signals the beginning of this year's opposition period. It then commences retrograde motion.
Although just visible to the naked eye, Uranus is much easier to spot with optical aid. Popular 7x50 or 10x50 binoculars will reveal a "star" like point of light. By observing the planet over a number of nights its slow movement relative to the "fixed" background stars can be noticed.
The waning gibbous Moon passes 5 degrees south of Uranus on August 3rd and 31st.
Neptune, mag. +7.8, continues to move retrograde among the stars of Aquarius. The outermost planet reaches opposition on September 7th and is now superbly placed for observation. It rises above the eastern horizon just after sunset, remaining visible for the rest of the evening.
Neptune is always too faint to be spotted with the naked eye. However, it's a relatively easy binocular and small telescope target. The planet can be found about a third of the way along an imaginary line connecting phi Aquarii (φ Aqr - mag. +4.2) with lambda Aquarii (λ Aqr - mag. +3.7). First magnitude star, Fomalhaut (α Psc - mag. +1.2), is about 23 degrees south of Neptune.
Even though its apparent diameter is only 2.4 arc seconds, an 80mm (3.1-inch) refractor at medium to high magnifications will reveal a small featureless blue disk. However, even large amateur reflectors don't significantly improve the view. A scope of at least 300mm (12 inches) aperture is recommended to spot Neptune's largest moon, Triton (mag. +14.0).
On August 27th, the waning gibbous Moon passes three degrees south of the planet.
Solar System Data Table - August 2018
|Date||Right Ascension||Declination||Mag.||App. Size||Illum. (%)||Dist. (AU)||Constellation|
|Sun||Aug 01||08h 43m 33.6s||18d 08m 40.8s||-26.7||31.5'||100||1.015||Cancer|
|Sun||Aug 15||09h 37m 01.3s||14d 12m 40.1s||-26.7||31.6'||100||1.013||Leo|
|Sun||Aug 31||10h 36m 01.1s||08d 49m 43.3s||-26.7||31.7'||100||1.010||Leo|
|Mercury||Aug 01||09h 30m 45.9s||09d 56m 26.9s||2.9||11.0"||8||0.610||Leo|
|Mercury||Aug 15||08h 54m 36.5s||13d 26m 57.6s||3.3||10.2"||6||0.659||Cancer|
|Mercury||Aug 31||09h 29m 51.8s||15d 19m 48.0s||-0.8||6.5"||61||1.029||Leo|
|Venus||Aug 01||11h 36m 26.0s||02d 36m 40.3s||-4.3||20.4"||57||0.819||Leo|
|Venus||Aug 15||12h 26m 34.0s||-04d 14m 22.2s||-4.4||23.6"||50||0.707||Virgo|
|Venus||Aug 31||13h 18m 12.7s||-11d 35m 07.6s||-4.6||28.8"||41||0.580||Virgo|
|Mars||Aug 01||20h 26m 03.9s||-25d 57m 49.0s||-2.8||24.3"||100||0.385||Capricornus|
|Mars||Aug 15||20h 12m 12.7s||-26d 33m 01.4s||-2.5||23.4"||98||0.400||Capricornus|
|Mars||Aug 31||20h 07m 18.2s||-26d 04m 19.9s||-2.1||21.0"||94||0.445||Sagittarius|
|Jupiter||Aug 01||14h 46m 25.2s||-15d 02m 34.8s||-2.1||37.9"||99||5.201||Libra|
|Jupiter||Aug 15||14h 50m 50.4s||-15d 25m 51.0s||-2.0||36.4"||99||5.416||Libra|
|Jupiter||Aug 31||14h 58m 15.1s||-16d 01m 40.2s||-1.9||34.9"||99||5.654||Libra|
|Saturn||Aug 01||18h 14m 18.2s||-22d 36m 21.6s||0.2||18.0"||100||9.218||Sagittarius|
|Saturn||Aug 15||18h 11m 37.4s||-22d 39m 06.5s||0.3||17.7"||100||9.374||Sagittarius|
|Saturn||Aug 31||18h 10m 02.7s||-22d 41m 50.1s||0.4||17.3"||100||9.596||Sagittarius|
|Uranus||Aug 01||02h 01m 10.2s||11d 45m 16.4s||5.8||3.6"||100||19.745||Aries|
|Uranus||Aug 15||02h 01m 09.3s||11d 44m 53.0s||5.8||3.6"||100||19.515||Aries|
|Uranus||Aug 31||02h 00m 24.0s||11d 40m 29.9s||5.7||3.7"||100||19.277||Aries|
|Neptune||Aug 01||23h 09m 07.9s||-06d 31m 19.4s||7.8||2.3"||100||29.128||Aquarius|
|Neptune||Aug 15||23h 07m 57.7s||-06d 39m 03.9s||7.8||2.4"||100||29.010||Aquarius|
|Neptune||Aug 31||23h 06m 24.9s||-06d 49m 04.0s||7.8||2.4"||100||28.940||Aquarius|