Mercury reached greatest elongation east at the end of April, and from southern and equatorial latitudes the planet remains well placed for observation in morning twilight during the first half of May. On May 1st, it rises some 2 hours before the Sun. The period of visibility then decreases slightly each subsequent morning, until the planet is lost to the bright twilight sometime during the third week of the month. Between May 1st and 15th, Mercury's brightness increases from magnitude +0.3 to -0.3. On the morning of May 14th, the thin waning crescent Moon, Mercury and Uranus (mag. +5.9) will be within 5 degrees of each other. Keen eyed observers may be able to spot all three objects in the same binocular field of view, although Uranus will be challenging since it's over 250x fainter than Mercury. Of course, please take extreme care when using binoculars at this time of day, and always ensure that the Sun is below the horizon before looking.
From northern temperate locations, Mercury is not well placed for observation but may be glimpsed low down above the western horizon during the first few days of the month.
Venus, mag. -3.9, continues this month as a brilliant evening star, visible above the western horizon as soon as it's dark enough. From northern temperature latitudes, the planet sets about 2 hours after the Sun at start of month, improving to 2.5 hours by months end. For those further south the visibility period is slightly less. On May 3rd, Venus passes 7 degrees north of orange/red star Aldebaran (mag. +0.9) in Taurus, which is surrounded by the large sprawling Hyades open cluster. The planet reaches perihelion on May 15th, when it's approximately 0.718 AU (approx. 107 million kilometres or 66.7 million miles) from the Sun. On May 17th, the thin waxing crescent Moon passes 5 degrees south of Venus, providing pleasant early evening viewing. The planet then moves into Gemini on May 19th, and on the following day passes less than a degree from open cluster, M35. A small telescope should reveal the much dimmer open cluster alongside the brilliant planet.
As the month progresses, the apparent size of Venus increases from 11.5 to 13.1 arc seconds with its illuminated disk decreasing from 88% to 81%.
Mars starts the month in Sagittarius, before moving into Capricornus on May 14th. At the beginning of the month the red planet rises about 90 minutes after midnight from mid-latitude northern temperate locations, improving to around midnight by months end. From southern locations, Mars is much better placed, rising some 3 hours earlier still. This month the planet's magnitude increases nicely from -0.4 to -1.2, with its angular size increasing from 11.1 to 15.1 arc seconds. Even with a small telescope it should now be possible to spot details, such as Syrtis Major, on its salmon-pink surface.
On May 6th, the waning gibbous Moon passes 3 degrees north of Mars. On May 14th, as it closes on the Capricornus / Sagittarius constellation border, Mars passes less than 0.5 degrees from magnitude +8.7 globular cluster, M75. Later on May 22nd, the planet reaches it's southern equinox.
The highlight of the month occurs on May 9th, when Jupiter reaches opposition. On this day, the gas giant will be located 4.400 AU (approx. 658.2 million kilometres or 409 million miles) from Earth. Jupiter is currently moving retrograde in Libra and at magnitude -2.5 is unmistakable and easily brighter than any night time star. This month, the planet is visible all night long, rising in the east at sunset, reaching its highest point in the sky around midnight before setting in the west at sunrise.
Popular 7x50 or 10x50 binoculars reveal Jupiter as a very small pale disk. Also visible are the large Galilean moons (Io, Europa, Ganymede and Callisto), which continuously change position as they orbit the planet. Through small scopes, details such as the great equatorial belts and shadow transits of the moons can be observed. Larger scopes reveal the "Great Red Spot" and other cloud details, across the 45 arc second planetary disk.
On May 27th, the almost full Moon passes 4 degrees north of Jupiter.
Saturn, moving retrograde in Sagittarius, is now well placed for observation. From temperate southern latitudes, the magnificent ringed planet rises around 9pm at the beginning of May and a full two hours earlier by month's end. Like Mars and Jupiter, Saturn is currently better seen from southern rather than northern locations. From northern temperate locations, the planet rises at around midnight on May 1st and a couple of hours earlier by month's end.
When seen through a telescope, Saturn is a stunning sight. A small 80mm (3.1-inch) refractor easily reveals the ring system, which is currently wide open at a tilt of 25 degrees to the line of sight. A 150mm (6-inch) or 200mm (8-inch) reflector shows a wealth of details, such as planetary cloud formations, ring divisions and up to half a dozen satellites. During the month, the magnitude of Saturn increases marginally from +0.3 to +0.2 with it's apparent diameter improving slightly from 17.5 to 18.1 arc seconds.
Uranus, mag. +5.9, is located in Aries. 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 3 hours before the Sun by months end.
As previously mentioned, on May 14th, the thin waning crescent Moon, Mercury and Uranus will all be within 5 degrees of each other.
Neptune, mag. +7.9, is now visible as a morning object. From southern locations it rises up to four hours before the Sun at start of the month improving by a couple of hours by month's end. Although never bright enough to be seen with the naked eye, Neptune can be spotted with binoculars or small telescopes. From northern temperate latitudes, Neptune is not as well placed but can be seen in the early morning sky above the eastern horizon before sunrise, especially during the second half of the month.
The planet is located in Aquarius and about a quarter of the way along an imaginary line connecting phi Aquarii (φ Aqr - mag. +4.2) with lambda Aquarii (λ Aqr - mag. +3.7). On May 10th, the waning crescent Moon passes 2 degrees south of Neptune.
Solar System Data Table - May 2018
|Date||Right Ascension||Declination||Mag.||App. Size||Illum. (%)||Dist. (AU)||Constellation|
|Sun||May 01||02h 31m 40.5s||14d 55m 02.7s||-26.7||31.8'||100||1.007||Aries|
|Sun||May 15||03h 26m 00.8s||18d 44m 26.3s||-26.7||31.6'||100||1.011||Taurus|
|Sun||May 31||04h 30m 19.0s||21d 50m 15.3s||-26.7||31.6'||100||1.014||Taurus|
|Mercury||May 01||00h 54m 06.9s||02d 40m 38.2s||0.3||7.8"||46||0.859||Cetus|
|Mercury||May 15||02h 03m 09.1s||09d 44m 08.2s||-0.3||6.2"||68||1.083||Pisces|
|Mercury||May 31||03h 59m 22.2s||20d 13m 31.6s||-1.6||5.2"||96||1.298||Taurus|
|Venus||May 01||04h 21m 49.3s||22d 23m 12.9s||-3.9||11.5"||88||1.450||Taurus|
|Venus||May 15||05h 34m 53.5s||24d 43m 59.0s||-3.9||12.1"||85||1.375||Taurus|
|Venus||May 31||06h 58m 59.3s||24d 37m 47.9s||-3.9||13.1"||81||1.278||Gemini|
|Mars||May 01||19h 41m 02.9s||-22d 44m 10.2s||-0.4||11.1"||88||0.845||Sagittarius|
|Mars||May 15||20h 07m 24.4s||-22d 11m 32.7s||-0.7||12.8"||89||0.734||Capricornus|
|Mars||May 31||20h 31m 34.3s||-21d 47m 26.9s||-1.2||15.1"||91||0.619||Capricornus|
|Jupiter||May 01||15h 08m 05.5s||-16d 16m 20.7s||-2.5||44.7"||100||4.413||Libra|
|Jupiter||May 15||15h 00m 58.6s||-15d 47m 55.6s||-2.5||44.8"||100||4.403||Libra|
|Jupiter||May 31||14h 53m 23.3s||-15d 17m 46.3s||-2.5||44.2"||100||4.462||Libra|
|Saturn||May 01||18h 37m 52.6s||-22d 15m 44.6s||0.3||17.5"||100||9.502||Sagittarius|
|Saturn||May 15||18h 35m 59.0s||-22d 17m 30.0s||0.3||17.8"||100||9.319||Sagittarius|
|Saturn||May 31||18h 32m 26.7s||-22d 20m 47.5s||0.2||18.1"||100||9.161||Sagittarius|
|Uranus||May 01||01h 48m 23.0s||10d 36m 31.1s||5.9||3.4"||100||20.876||Aries|
|Uranus||May 15||01h 51m 19.4s||10d 52m 50.2s||5.9||3.4"||100||20.806||Aries|
|Uranus||May 31||01h 54m 25.0s||11d 09m 46.1s||5.9||3.4"||100||20.667||Aries|
|Neptune||May 01||23h 08m 27.3s||-06d 31m 44.1s||7.9||2.2"||100||30.513||Aquarius|
|Neptune||May 15||23h 09m 35.7s||-06d 25m 08.1s||7.9||2.3"||100||30.307||Aquarius|
|Neptune||May 31||23h 10m 27.7s||-06d 20m 22.4s||7.9||2.3"||100||30.047||Aquarius|