Solar System Phenomena — Mercury in 2021

The path of Mercury against the background stars in 2021

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 can be a difficult object to spot. It begins the year in the western sky after sunset but disappears in early February. After inferior conjunction it reappears in the morning sky between late February and late April, an apparition which favours observers in equatorial and southern latitudes. Mercury has a particularly close appulse with Jupiter during this time. Superior conjunction in mid-April heralds another appearance after sunset from April to mid-June. This is the best evening apparition for planet watchers in the northern hemisphere but everyone can enjoy the spectacle of Mercury and Venus only 0.4° apart at the end of May. Inferior conjunction follows, then Mercury swings back into the morning sky for the months of June and July. After superior conjunction at the beginning of August, another evening apparition begins which runs from August to early October. This appearance is almost unobservable from northern temperate latitudes but provides the best viewing opportunities for astronomers in the southern hemisphere. These observers will also be in the best place to see Mercury slip past Mars in mid-August. The final morning apparition runs from mid-October to late November which is the best dawn appearance of the planet for far northern latitudes; this is followed by the final evening apparition in December. The lunar occultation of 4 December may well be unobservable.

01 Januaryelongation 7.3°, illuminated fraction 98.1%, magnitude −1.0, disk diameter 4.8 arc-seconds
08 JanuarySagittariusCapricornus
10 Januaryplanetary conjunction: 1.6° south of Saturn
11 Januaryplanetary conjunction: 1.4° south of Jupiter
14 January2.3° north of the Moon
24 Januarygreatest elongation east: 18.6°
ascending node
29 Januaryperihelion
30 Januarystationary point in right ascension: direct → retrograde
01 Februaryelongation 14.1°, illuminated fraction 18.2%, magnitude +1.1, disk diameter 8.8 arc-seconds
07 FebruaryCapricornusAquarius
08 Februaryinferior conjunction
13 Februaryplanetary conjunction: 4.6° north of Venus
14 Februaryplanetary conjunction: 3.9° north of Jupiter
17 FebruaryAquariusCapricornus
20 Februarystationary point in right ascension: retrograde → direct
01 Marchelongation 26.5°, illuminated fraction 46.9%, magnitude +0.3, disk diameter 7.8 arc-seconds
03 Marchdescending node
05 Marchplanetary conjunction: 0.3° north of Jupiter
06 Marchgreatest elongation west: 27.3°
13 MarchCapricornusAquarius
14 Marchaphelion
29 March1.4° south of Neptune
01 Aprilelongation 17.2°, illuminated fraction 86.0%, magnitude −0.5, disk diameter 5.3 arc-seconds
02 AprilAquariusPisces
07 AprilPiscesCetus
10 AprilCetusPisces
11 April3.0° north of the Moon
19 AprilPiscesAries
superior conjunction
22 Aprilascending node
24 Aprilplanetary conjunction: 0.7° north of Uranus
25 Aprilplanetary conjunction: 1.2° north of Venus
27 Aprilperihelion
01 Mayelongation 13.4°, illuminated fraction 83.4%, magnitude −1.2, disk diameter 5.6 arc-seconds
13 May2.1° north of the Moon
17 Maygreatest elongation east: 22.0°
18 Maymaximum declination north
29 Mayplanetary conjunction: 0.4° south of Venus
30 Maystationary point in right ascension: direct → retrograde
descending node
01 Juneelongation 13.8°, illuminated fraction 7.7%, magnitude +2.9, disk diameter 11.2 arc-seconds
10 Juneaphelion
11 Juneinferior conjunction
22 June1.8° north of Aldebaran
23 Junestationary point in right ascension: retrograde → direct
01 Julyelongation 21.0°, illuminated fraction 27.0%, magnitude +1.0, disk diameter 8.7 arc-seconds
04 Julygreatest elongation west: 21.6°
10 JulyTaurusOrion
12 JulyOrionGemini
19 Julyascending node
24 Julyperihelion
27 JulyGeminiCancer
01 Augustelongation 1.8°, illuminated fraction 100.0%, magnitude −2.2, disk diameter 5.0 arc-seconds
superior conjunction
05 AugustCancerLeo
19 Augustplanetary conjunction: 0.1° south of Mars
26 AugustLeoVirgo
descending node
01 Septemberelongation 23.8°, illuminated fraction 74.0%, magnitude +0.0, disk diameter 5.8 arc-seconds
06 Septemberaphelion
14 Septembergreatest elongation east: 26.8°
21 September1.2° south of Spica
27 Septemberstationary point in right ascension: direct → retrograde
01 Octoberelongation 1.9°, illuminated fraction 0.0%, magnitude +5.7, disk diameter 10.1 arc-seconds
09 Octoberinferior conjunction
planetary conjunction: 2.5° south of Mars
15 Octoberascending node
18 Octoberstationary point in right ascension: retrograde → direct
20 Octoberperihelion
25 Octobergreatest elongation west: 18.4°
01 Novemberelongation 16.5°, illuminated fraction 79.1%, magnitude −0.8, disk diameter 5.8 arc-seconds
03 Novemberlunar occulation: 1.2° south of the Moon
10 NovemberVirgoLibra
planetary conjunction: 1.0° north of Mars
24 NovemberLibraScorpius
22 Novemberdescending node
29 NovemberScorpiusOphiuchus
superior conjunction
01 Decemberelongation 1.4°, illuminated fraction 100.0%, magnitude −1.2, disk diameter 4.6 arc-seconds
04 Decemberlunar occultation: 0.02° north of the Moon
11 DecemberOphiuchusSagittarius
16 Decembermaximum declination south
29 Decemberplanetary conjunction: 4.2° south of Venus


The dates, times and circumstances of all planetary and lunar phenomena were calculated from the JPL DE406 solar system ephemeris using the same rigorous methods that are employed in the compilation of publications such as The Astronomical Almanac.