Mercury reaches greatest elongation on New Years Day when it moves out to 22.7 degrees west of the Sun. The innermost planet can be seen just before sunrise during the first two weeks of the month, low down above the south-eastern horizon. During this period its brightness is fairly constant at magnitude -0.3, with its illuminated phase increasing from 62% to 85%.
Mercury passes less than a degree south of Saturn on January 13th. Two days later the thin waning crescent Moon passing 3 degrees north of Mercury.
Venus passes through superior conjunction on January 9th. It remains too close to the Sun to be safely observed throughout the month.
Mars is now an early morning object. Except for a few hours on the last day it spends all of the month moving south-easterly through Libra. The planet increases in brightness from magnitude +1.5 to +1.2, with its angular diameter improving slightly from 4.8 to 5.6 arc seconds as the month progresses.
Mars never strays very far away this month from much brighter Jupiter (mag. -1.9). On the morning of the 7th, the two planets are at their closest and positioned less than 0.5 degrees apart. However at magnitude +1.4, Mars is about 20 times fainter than Jupiter.
Jupiter is an early morning object in Libra. The giant planet rises up to 4 hours before the Sun at start of month, with the visibility period continually improving as the month progresses. On New Years Day it shines at magnitude -1.8, with an apparent diameter of 33 arc seconds, improving to magnitude -2.0 and an apparent diameter of 36 arc seconds by month's end.
Through binoculars, the planet appears as a small off-white coloured disk without detail. Also visible are the four large Galilean moons (Io, Europa, Ganymede and Callisto). Occasionally all can be seen at once, but often some are hidden from view as they pass behind, or in front of, the planet's disk.
When viewed through a telescope, Jupiter is a superb sight. Even a small 80mm (3.1 inch) refractor shows the main equatorial cloud belts with larger scopes revealing much more detail, including smaller belts, ovals, festoons, dark regions and the Great Red Spot.
As previously mentioned, on the morning of January 7th, Mars passes a fraction of a degree south of Jupiter. On January 11th, the waning crescent Moon passes 4 degrees north of Jupiter.
Saturn, mag. +0.6, passed superior conjunction in December and reappears in the pre dawn sky this month. During the first two weeks the ringed planet remains dangerously close to the Sun, but it climbs quickly as the month progresses and in the process becomes easier to spot.
On January 13th, Mercury (mag -0.3) passes 0.6 degrees south of Saturn. Two days later, the thin waning crescent Moon will be 3 degrees north of Saturn.
Uranus, mag. +5.8, remains an evening object in Pisces not far from its constellation border with Aries. Uranus starts the month moving slowly retrograde before reaching its second stationary point on January 2nd. This event is widely regarded as signaling the end of the current opposition period, after which the planet again resumes direct motion. By month's end, the distant ice giant sets before midnight.
Uranus is easy to spot with binoculars and small telescopes. A small 80mm (3.1-inch) refractor at high magnifications shows a small greenish disk, 3.6 arc seconds in diameter.
On January 24th, the first quarter Moon passes 5 degrees south of Uranus.
Neptune, mag. +7.9, is an early evening object moving direct in Aquarius. It can be seen with binoculars and small telescopes for a few hours after dark. This month offers the last realistic opportunity to catch a glimpse of the outermost planet before it reaches solar conjunction in early March.
Neptune is positioned about 30 degrees southwest of the centre of the Great Square of Pegasus and just southeast of lambda Aqr (λ Aqr - mag. +3.7). On January 20th, the waxing crescent Moon passes 1.4 degrees south of the planet.
Solar System Data Table January 2018
|Date||Right Ascension||Declination||Mag.||App. Size||Illum. (%)||Dist. (AU)||Constellation|
|Sun||Jan 05||19h 02m 16.8s||-22d 39m 54.7s||-26.8||32.5'||100||0.983||Sagittarius|
|Sun||Jan 15||19h 45m 47.0s||-21d 12m 47.4s||-26.8||32.5'||100||0.984||Sagittarius|
|Sun||Jan 25||20h 28m 12.6s||-19d 05m 04.2s||-26.8||32.5'||100||0.984||Capricornus|
|Mercury||Jan 05||17h 25m 24.3s||-21d 46m 05.0s||-0.3||6.2"||70||1.077||Ophiuchus|
|Mercury||Jan 15||18h 21m 28.2s||-23d 19m 49.3s||-0.3||5.4"||84||1.235||Sagittarius|
|Mercury||Jan 25||19h 25m 19.0s||-23d 08m 25.6s||-0.4||5.0"||92||1.339||Sagittarius|
|Venus||Jan 05||18h 58m 07.3s||-23d 23m 11.4s||-4.0||9.8"||100||1.710||Sagittarius|
|Venus||Jan 15||19h 52m 17.6s||-21d 54m 00.4s||-4.0||9.8"||100||1.711||Sagittarius|
|Venus||Jan 25||20h 44m 53.4s||-19d 19m 02.5s||-3.9||9.8"||100||1.707||Capricornus|
|Mars||Jan 05||14h 56m 50.2s||-15d 54m 27.5s||1.4||4.9"||93||1.920||Libra|
|Mars||Jan 15||15h 21m 45.2s||-17d 39m 12.2s||1.4||5.1"||92||1.830||Libra|
|Mars||Jan 25||15h 46m 58.5s||-19d 11m 42.5s||1.3||5.4"||91||1.737||Libra|
|Jupiter||Jan 05||15h 00m 47.4s||-15d 59m 47.6s||-1.8||33.4"||99||5.904||Libra|
|Jupiter||Jan 15||15h 06m 56.0s||-16d 24m 07.2s||-1.9||34.2"||99||5.761||Libra|
|Jupiter||Jan 25||15h 12m 17.5s||-16d 44m 21.8s||-1.9||35.2"||99||5.608||Libra|
|Saturn||Jan 05||18h 06m 59.4s||-22d 32m 01.8s||0.6||15.1"||100||11.021||Sagittarius|
|Saturn||Jan 15||18h 11m 56.8s||-22d 31m 19.1s||0.6||15.1"||100||10.972||Sagittarius|
|Saturn||Jan 25||18h 16m 42.1s||-22d 29m 59.1s||0.6||15.3"||100||10.897||Sagittarius|
|Uranus||Jan 05||01h 30m 56.6s||08d 54m 02.3s||5.8||3.6"||100||19.708||Pisces|
|Uranus||Jan 15||01h 31m 10.6s||08d 55m 47.4s||5.8||3.5"||100||19.879||Pisces|
|Uranus||Jan 25||01h 31m 44.2s||08d 59m 25.9s||5.8||3.5"||100||20.050||Pisces|
|Neptune||Jan 05||22h 54m 04.1s||-07d 59m 53.3s||7.9||2.2"||100||30.463||Aquarius|
|Neptune||Jan 15||22h 55m 01.7s||-07d 53m 49.9s||7.9||2.2"||100||30.600||Aquarius|
|Neptune||Jan 25||22h 56m 08.3s||-07d 46m 53.1s||8.0||2.2"||100||30.718||Aquarius|