Mercury reaches greatest elongation west on May 17th when it's positioned 26 degrees from the Sun. The planet is not well placed for observation at northern temperate locations, but from southern latitudes this happens to be the most favourable morning apparition of the year. With an extended period of visibility it can be seen in the morning skies until mid June.
On May 1st, from latitude 35S (approx. equal to Sydney, Cape Town and Santiago), Mercury rises over an hour before the Sun. At magnitude +2.5, binoculars will be required to spot the elusive planet against the bright dawn sky. Positioned a degree from Mercury is fainter Uranus (mag. +5.9). As the month progress the visibility of Mercury improves until greatest elongation west is reached. On May 17th, the planet shines at magnitude +0.5 and appears 15 degrees above the east-northeastern horizon, 45 minutes before sunrise. For the remainder of the month it gradually sinks back towards the horizon. It should be noted that Mercury improves in brightness throughout May and on the 31st it will shine at magnitude -0.3. Brilliant Venus is positioned above Mercury throughout this time.
From mid-northern temperate latitudes, Mercury can be seen for a short while around the time of greatest elongation. However, it's always low down and at best just a few degrees above the horizon.
On May 24th, the thin waning crescent Moon passes 2 degrees south of Mercury.
Venus remains a blazing beacon of light in the eastern morning sky. It's visible for over 3 hours before sunrise from southern temperate locations although only for about half as long from the Northern Hemisphere. The planet dims slightly from magnitude -4.7 to -4.5 as the month progresses with its illuminated phase increasing from 27% to 48%. Venus is so bright that it easily outshines any other planet or night-time star.
On May 22nd, the waning crescent Moon passes 2 degrees south of Venus. At the end of the month, Venus moves to within a couple of degrees of Uranus (mag. +5.9).
The current long visibility period of Mars, mag. +1.6, finally comes to an end this month. During the first part of the month, the famous "Red planet" can be seen low down above the west-northwestern horizon just after sunset, but by month's end it will be effectively lost to the Sun's glare.
On May 7th it passes 6 degrees north of Aldebaran, which at mag. +0.9 is the brightest star in Taurus. The thin waxing crescent Moon passes 5 degrees south of Mars on May 27th.
Jupiter is now a month past opposition and remains a stunning object moving retrograde in Virgo. As soon as it's dark enough, the gas giant is visible towards the east and remains so until the early hours of the morning. It starts the month at mag. -2.4 with an apparent diameter of 44 arc seconds. By months end it has faded slightly to mag. -2.3 and shrunk to 41 arc seconds.
When viewed through a telescope a wealth of Jovian cloud details are visible including bands, twists, knots and storms. Also easily visible, but not always at the same time, are the four bright Galilean moons: Io, Europa, Ganymede and Callisto.
A lovely pairing occurs on the night of May 7th when the waxing gibbous Moon passes just a couple of degrees north of Jupiter.
Saturn continues to improve in brightness and visibility as it heads towards opposition next month. The planet is currently moving retrograde and starts the month in Sagittarius, before crossing into Ophiuchus on May 18th. It rises during the evening and remains visible for the rest of the night. However, the visibility period is considerably longer from equatorial and southern latitudes than from northern locations. Saturn also appears higher in the sky from equatorial and southern latitudes.
To the naked eye, Saturn appears creamy or off-white in colour. The planet's most famous feature is its complex ring system, which can be seen with a small refractor telescope. A medium size scope of 150mm (6-inch) or 200mm (8-inch) aperture will show a wealth of details including subtle formations, divisions in the rings and up to half a dozen satellites.
During the month, Saturn brightens slightly from magnitude +0.3 to +0.1 with its apparent size increasing marginally from 17.8 to 18.3 arc seconds. On May 13th, the waning gibbous Moon passes 3 degrees north of Saturn.
Uranus, mag. +5.9, is located in Pisces. The planet passed by solar conjunction last month and reappears in the morning sky during May. For northern temperate observers it's swamped by the bright early morning twilight sky for most of the month, but at the tail end of May can be seen with binoculars and small scopes above the eastern horizon, an hour or so before sunrise. Observers located further south have it much better with Uranus rising more than 3 hours before the Sun by months end.
On May 7th, Mercury (mag +1.4) passes 2 degrees south of Uranus. The waning crescent Moon passes 4 degrees south of Uranus on May 23rd and at the end of the month, Venus is located 2 degrees from Uranus.
Neptune, mag. +7.9, is visible as a morning object from tropical and southern locations, where it rises up to four hours before the Sun at start of the month improving to just after midnight by month's end. However from northern temperate latitudes, Neptune is not as well placed, although it may be spotted low down above the eastern horizon during the second half of the month.
Neptune is currently located in Aquarius about 2 degrees east of lambda Aquarii (λ Aqr - mag. +3.7). At the beginning of the month, much brighter Venus is positioned 15 degrees to the northeast. Although it never becomes bright enough to be seen with the naked eye, Neptune can be easily spotted with binoculars or small telescopes when the sky is dark enough.
On May 20th, the waning crescent Moon passes 0.5 degrees south of Neptune with an occultation visible from the South Atlantic at 5:31 UT.
Solar System Data Table May 2017
|Date||Right Ascension||Declination||Apparent Magnitude||Apparent Size||Illum. (%)||Distance from Earth (AU)||Constellation|
|Sun||5th May 2017||02h 48m 00.4s||16d 10m 34.2s||-26.7||31.7'||100||1.009||Aries|
|Sun||15th May 2017||03h 27m 01.0s||18d 48m 05.4s||-26.7||31.6'||100||1.011||Taurus|
|Sun||25th May 2017||04h 06m 58.7s||20d 53m 56.8s||-26.7||31.6'||100||1.013||Taurus|
|Mercury||5th May 2017||01h 32m 25.7s||07d 21m 09.7s||1.7||10.5"||17||0.640||Pisces|
|Mercury||15th May 2017||01h 51m 44.1s||07d 58m 23.9s||0.6||08.7"||35||0.775||Pisces|
|Mercury||25th May 2017||02h 32m 04.0s||11d 40m 45.2s||0.0||07.1"||52||0.942||Aries|
|Venus||5th May 2017||00h 10m 27.9s||01d 48m 47.6s||-4.7||35.5"||30||0.470||Pisces|
|Venus||15th May 2017||00h 37m 47.8s||03d 17m 47.9s||-4.6||30.6"||38||0.545||Pisces|
|Venus||25th May 2017||01h 09m 56.9s||05d 37m 14.0s||-4.5||26.7"||44||0.624||Pisces|
|Mars||5th May 2017||04h 29m 07.8s||22d 29m 33.1s||1.6||03.9"||98||2.416||Taurus|
|Mars||15th May 2017||04h 58m 22.7s||23d 26m 52.0s||1.6||03.8"||99||2.462||Taurus|
|Mars||25th May 2017||05h 27m 42.5s||24d 03m 30.3s||1.7||03.7"||99||2.505||Taurus|
|Jupiter||5th May 2017||12h 56m 55.0s||-04d 24m 53.7s||-2.4||44.2"||100||4.556||Virgo|
|Jupiter||15th May 2017||12h 53m 40.3s||-04d 06m 40.5s||-2.4||44.2"||100||4.644||Virgo|
|Jupiter||25th May 2017||12h 51m 22.3s||-03d 54m 43.0s||-2.3||43.9"||99||4.753||Virgo|
|Saturn||5th May 2017||17h 46m 36.1s||-22d 01m 59.4s||0.2||17.9"||100||9.289||Sagittarius|
|Saturn||15th May 2017||17h 44m 25.9s||-22d 01m 03.7s||0.2||18.1"||100||9.188||Sagittarius|
|Saturn||25th May 2017||17h 41m 48.0s||-22d 00m 02.8s||0.1||18.2"||100||9.111||Ophiuchus|
|Uranus||5th May 2017||01h 34m 59.4s||09d 18m 06.2s||5.9||03.4"||100||20.878||Pisces|
|Uranus||15th May 2017||01h 37m 00.6s||09d 29m 42.6s||5.9||03.4"||100||20.812||Pisces|
|Uranus||25th May 2017||01h 38m 54.4s||09d 40m 30.6s||5.9||03.4"||100||20.722||Pisces|
|Neptune||5th May 2017||23h 00m 42.5s||-07d 16m 31.8s||7.9||02.2"||100||30.426||Aquarius|
|Neptune||15th May 2017||23h 01m 26.8s||-07d 12m 20.7s||7.9||02.3"||100||30.273||Aquarius|
|Neptune||25th May 2017||23h 02m 00.0s||-07d 09m 19.3s||7.9||02.3"||100||30.110||Aquarius|