Solar System Phenomena — Neptune in 2021

The path of Neptune against the background stars in 2021

The chart shows the path of Neptune across the background stars over the course of the year. Stars to magnitude +10.5 are shown. 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.

The lower chart shows how the appearance of Neptune changes over the year. Below each image is listed the date, the apparent magnitude, the apparent diameter of the disk (in arc-seconds) and the geocentric distance (in au). Because the relative distance of Neptune does not greatly vary throughout the year, neither does its appearance through a telescope.

Neptune is the most distant planet in the solar system from the Sun and the smallest of the four gas giants. Because of its great distance, it is not visible to the naked eye so a small telescope is always necessary to observe it. The blue ice giant remains in the constellation of Aquarius all year and is visible in the evening sky at the opening of 2021. It is lost in the glow of sunset later in February and undergoes conjunction in early March. Neptune's two encounters with Mercury and Venus immediately afterwards are unobservable in the dawn sky. Opposition occurs in mid-September when the planet is visible all night. It is primarily an evening sky object later in the year. Southern hemisphere observers are somewhat favoured for viewing this faint object.

01 Januarymaximum declination south
11 Marchconjunction
14 Marchplanetary conjunction: 0.4° north of Venus
29 Marchplanetary conjunction: 1.4° north of Mercury
13 Junewest quadrature
22 Junemaximum declination north
26 Junestationary point in right ascension: direct → retrograde
14 Septemberopposition: magnitude +7.8, apparent diameter 2.5 arc-seconds
02 Decemberstationary point in right ascension: retrograde → direct
12 Decembereast quadrature


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.