SkyEye

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.

Mercury begins the year as a morning sky object but immediately disappears from view. Three more morning apparitions occur in 2020, with the March–April appearance an excellent one for equatorial and southern hemisphere observers and the end-of-year apparition favouring northern temperate latitudes. Mercury also appears in the west after sunset four times this year. The late spring appearance in May and June is the best chance for planet chasers in the north to see this object in the evening whilst late August through October yields splendid observing opportunities for viewers in southern latitudes.

• Apparitions of the Inferior Planets from Latitude 50° North
• Apparitions of the Inferior Planets from Latitude 10° North
• Apparitions of the Inferior Planets from Latitude 30° South

Because the orbits of the planets are tilted slightly to the plane of the ecliptic, a planet normally passes to the north or the south of the Sun at conjunction. However, if the planet is near a node (the place in the orbit where the planet crosses the ecliptic) when it reaches conjunction, the planet may appear to cross in front of or behind the disk of the Sun. This situation occurs in May when Mercury actually passes behind the Sun from the vantage point of Earth. This type of conjunction is sometimes called an anti-transit or secondary eclipse.

Sources

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.