Solar System Phenomena - Mercury in 2018

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The upper chart shows the path of Mercury across the background stars over the course of the year. Stars to magnitude +4.5 are shown with some fainter objects included to complete constellation patterns. The white circles represent the planet on the first day of the month and are scaled according to apparent magnitude. The faint paths before the first circle and after the last circle represent the planet's positions in December of last year and January of next. In general, the planet moves from right to left except when it's in retrograde and proceding in the opposite direction. As an inferior planet, Mercury never strays far from the Sun so it always begins and ends the year near the constellation of Sagittarius, located about one quarter of the way in from the left side of the chart.

The lower charts show how the appearance of Mercury changes over the year. Below each image is listed the date, the apparent magnitude, the apparent diameter of the disk (in arc-seconds), the geocentric distance (in au), the elongation from the Sun (in degrees) and the percentage of the disk which is illuminated. Like the Moon, Mercury exhibits a complete range of phases, from new to crescent to gibbous to full and back again. Because its synodic period is around four months, Mercury completes this phase cycle three times each year. Note how Mercury's magnitude varies widely, ranging (approximately) from -2.0 to +6.0 between conjunctions.

The closest planet to the Sun is an elusive object to spot in the sky. It begins the year in the morning sky but is low to the horizon before sunrise and soon lost to view as it heads toward conjunction in February. The March apparition in the west is best seen from northern latitudes with the April apparition in the east favouring the southern hemisphere. Lost to view in June during superior conjunction, Mercury is again an evening sky object in July, this time favouring southern latitudes. Another conjunction in August brings the tiny planet back into the dawn sky for the best morning apparition this year for northern observers. Superior conjunction in late September is followed by good observing in the evening for those in southern latitudes. After the November conjunction, the northern hemisphere is again favoured for seeing Mercury in the morning sky before sunrise.

01 Januaryelongation 22.6°, illuminated fraction 61.7%, magnitude -0.3, disk diameter 6.7 arc-seconds
greatest elongation west 22.6°
08 JanuaryOphiuchusSagittarius
13 January0.6° south of Saturn
15 Januarydescending node
25 Januaryaphelion
31 JanuarySagittariusCapricornus
01 Februaryelongation 11.5°, illuminated fraction 95.3%, magnitude -0.6, disk diameter 4.8 arc-seconds
16 Februaryoccultation by the Moon - not visible
17 Februarysuperior conjunction
01 Marchelongation 10.0°, illuminated fraction 92.9%, magnitude -1.3, disk diameter 5.3 arc-seconds
02 MarchAquariusPisces
05 Marchascending node
09 March1.4° north of Venus
10 Marchperihelion
15 Marchgreatest elongation east 18.4°
22 Marchstationary point: direct → retrograde
01 Aprilelongation 3.3°, illuminated fraction 0.1%, magnitude +5.7, disk diameter 11.1 arc-seconds
inferior conjunction
13 Aprildescending node
14 Aprilstationary point: retrograde → direct
22 AprilPiscesCetus
23 Aprilaphelion
29 Aprilgreatest elongation west 27.0°
01 Mayelongation 27.0°, illuminated fraction 45.6%, magnitude +0.4, disk diameter 7.8 arc-seconds
12 May2.0° south of Uranus
13 May2.0° north of the Moon
15 MayPiscesCetus
16 MayCetusAries
26 MayAriesTaurus
01 Juneelongation 6.2°, illuminated fraction 96.9%, magnitude -1.7, disk diameter 5.1 arc-seconds
ascending node
06 Junesuperior conjunction
13 JuneTaurusGemini
15 Junemaximum declination north
27 JuneGeminiCancer
01 Julyelongation 23.6°, illuminated fraction 60.9%, magnitude +0.0, disk diameter 6.5 arc-seconds
04 July0.4° south of the open star cluster M44 (known as Praesepe or the Beehive Cluster)
10 Julydescending node
12 Julygreatest elongation east 26.4°
14 JulyCancerLeo
2.0° south of the Moon
20 Julyaphelion
25 Julystationary point: direct → retrograde
01 Augustelongation 14.1°, illuminated fraction 7.5%, magnitude +2.9, disk diameter 10.9 arc-seconds
04 AugustLeoCancer
09 Augustinferior conjunction
18 Auguststationary point: retrograde → direct
26 Augustgreatest elongation west 18.3°
28 Augustascending node
29 AugustCancerLeo
01 Septemberelongation 17.0°, illuminated fraction 64.8%, magnitude -0.8, disk diameter 6.3 arc-seconds
02 Septemberperihelion
05 September1.0° north of Regulus
08 Septemberoccultation by the Moon - not visible
18 SeptemberLeoVirgo
21 Septembersuperior conjunction
01 Octoberelongation 7.7°, illuminated fraction 97.6%, magnitude -0.9, disk diameter 4.8 arc-seconds
05 October2.0° north of Spica
06 Octoberdescending node
15 OctoberVirgoLibra
16 Octoberaphelion
31 OctoberLibraScorpius
01 Novemberelongation 22.6°, illuminated fraction 73.7%, magnitude -0.2, disk diameter 6.0 arc-seconds
06 Novembergreatest elongation east 23.3°
08 NovemberScorpiusOphiuchus
09 November1.8° north of Antares
12 Novembermaximum declination south
17 Novemberstationary point: direct → retrograde
25 NovemberOphiuchusScorpius
24 Novemberascending node
27 Novemberinferior conjunction
29 NovemberScorpiusLibra
01 Decemberelongation 8.5°, illuminated fraction 5.6%, magnitude +2.8, disk diameter 9.5 arc-seconds
05 December1.9° south of the Moon
06 Decemberstationary point: retrograde → direct
15 DecemberLibraScorpius
greatest elongation west 21.2°
20 DecemberScorpiusOphiuchus
21 December0.9° north of Jupiter