The Position of Neptune in the Night Sky:

2006 to 2023

by Martin J. Powell

From early 1998 through to March 2010, Neptune was positioned in Capricornus, the Sea-Goat. In March 2010, Neptune entered the constellation of Aquarius, the Water Carrier, spending about five months there before returning to Capricornus. In January 2011 it entered Aquarius once more, where it remained until it moved into Pisces, the Fishes, in May 2022.

Neptune reaches opposition to the Sun (when it is closest to the Earth and brightest in the sky for the year) every 367½ days on average, i.e. about 2½ days later in each successive year. The apparent magnitude of the planet varies little during the period shown in the star chart: from +7.8 (at opposition) to +8.0 (at superior conjunction). Around all opposition dates shown on the star map, Neptune is due South at local midnight in the Northern hemisphere (due North at local midnight in the Southern hemisphere).

The apparent diameter of the planet (its angular size when seen from the Earth) at opposition throughout the period covered by the star chart is 2".4 (2.4 arcseconds, where 1 arcsecond = 1/3600th of a degree).

In 2010, Neptune completed its first orbit since it was discovered on September 23rd, 1846. The planet was at its 1846 position in the sky (i.e. at the same longitude measured relative to the Earth) on three occasions: in mid-April 2009 (moving direct), in mid-July 2009 (moving retrograde) and in early February 2010 (moving direct). Neptune returned to its discovery position in its orbit (i.e. at the same longitude measured relative to the Sun) in late June 2010.

Neptune will cross the celestial equator in Pisces in April 2026 (heading North-eastwards and moving direct), in September 2026 (moving retrograde and temporarily heading South-westwards) and again in February 2027 (resuming North-easterly motion, moving direct). For the first time in over 81 years, the planet will then become visible for a longer period of time in the Northern hemisphere than in the Southern hemisphere (it last crossed the celestial equator - as it headed Southwards along the ecliptic - in Virgo in 1944).

Looking further ahead, the pale-blue ice giant will continue its Northward motion along the ecliptic until it reaches its most Northerly point in Gemini in 2067, having crossed its perihelion (i.e. its closest orbital point to the Sun) at 29.81 Astronomical Units (4.46 thousand million kms or 2.8 thousand million miles) in Aries in 2042.

 [Terms in yellow italics are explained in greater detail in an associated article describing planetary movements in the night sky.]

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Neptune Conjunctions with other Planets, 2011 to 2023

Viewed from the orbiting Earth, whenever two planets appear to pass each other in the night sky (a line-of-sight effect) the event is known as a planetary conjunction or appulse. Not all planetary conjunctions will be visible from the Earth, however, because many of them take place too close to the Sun. Furthermore, not all of them will be seen from across the world; the observers' latitude will affect the altitude (angle above the horizon) at which the two planets are seen at the time of the event and the local season will affect the sky brightness at that particular time. A flat, unobstructed horizon will normally be required to observe most of them.

The majority of conjunctions involving Neptune are not spectacular to view because the planet is never visible to the naked-eye. Twilight quickly renders the planet unobservable (even through binoculars) such that conjunctions taking place less than about 22° from the Sun are difficult or impossible to see. Since Venus is always less than 47° from the Sun and Mercury is always less than about 27° from the Sun, it follows that conjunctions of either of these inferior planets with Neptune will have a limited window of time within which they will be observable.

Because Mercury is mostly seen in twilight, the number of occasions during which it can be seen in conjunction with Neptune is inevitably limited. During the period from 2010 to 2050, there are only ten occasions when Mercury is positioned more than 22° from the Sun at the moment of its conjunction with Neptune. One such occasion takes place on April 2nd 2019, when the planets are positioned 25° West of the Sun. This conjunction is visible from Equatorial and Southern hemisphere latitudes, low down in the Eastern sky before sunrise. However, the planets' low altitude (mostly less than 10° when Neptune is still visible) is likely to prove a problem for many observers. After 2019, another opportunity to see the two planets together will be on April 3rd 2020, when they are again 25° West of the Sun; this will likewise favour Equatorial and Southern hemisphere observers. In fact, the relationship of Mercury's eccentric orbit to that of the Earth's is such that the planet's most favourable elongations are best seen from the Southern hemisphere. Hence for the most part, Northern hemisphere observers are denied the opportunity of seeing the Sun's closest and furthest planets together in the sky.

Optical aid will always be required to glimpse Neptune as a pale-blue 'star'. Even when the elongation is favourable, a further problem beckons in that the glare caused by the brighter nearby planet (Venus in particular) makes it difficult to see the much fainter planet beside it. In such instances (e.g. for the Venus-Neptune conjunction of January 13th 2017) binocular observers may find it easier to position Venus just outside the binocular field of view so that Neptune may be more comfortably viewed.

Neptune's most interesting conjunctions take place when the planet is within a few months of opposition, at which times they involve the superior planets Mars, Jupiter, Saturn and Uranus; these events are rare, however. The most recent conjunction of note was between Neptune and Jupiter on December 20th 2009, when Jupiter described its significantly larger planetary 'loop' about one apparent Full Moon-diameter to the South of the outermost gas giant. It was the last of three conjunctions which took place between these planets during that year (more details can be found here). Most conjunctions between Neptune and the superior planets, however, occur at elongations of less than 90°, when Neptune is below its brightest magnitude in any given apparition. During the nine-year period from 2011 to 2020, for example, only one visible conjunction takes place at an elongation greater than 90° (with Mars in June 2020), Neptune being almost magnitude +8 at every conjunction. Neptune's next favourable conjunctions with superior planets will be with Mars in January 2017, with Jupiter in April 2022 and with Saturn in June 2025.

In terms of their sizes, their positions within the Solar System and their constituent elements, Uranus and Neptune are often considered to be planetary 'twins'. From the viewpoint of the Earth, faster-moving Uranus 'overtook' Neptune when the two planets were in Sagittarius in July 1993, at which point they were seen in conjunction. This was the first time these planets had been in conjunction since they were discovered (Uranus in 1781, Neptune in 1846). Conjunctions between Uranus and Neptune are rare events indeed, occurring about every 172 years; the next one will be in the year 2164.

The following table lists the conjunctions involving Neptune which take place at solar elongations of greater than 20° over the period in question. In several cases, other planets and/or stars are also in the vicinity and these are detailed. Note that, because some of the conjunctions occur in twilight, the planets involved may not appear as bright as their listed magnitude suggests.

Although any given conjunction takes place at a particular instant in time, it is worth pointing out that, because of the planets' relatively slow daily motions, such events are interesting to observe for several days both before and after the actual conjunction date.

There are in fact two methods of defining a planetary conjunction date: one is measured in Right Ascension (i.e. perpendicular to the celestial equator) and the other is measured along the ecliptic, which is inclined at 23½° to the Earth's equatorial plane (this is due to the tilt of the Earth's axis in space). An animation showing how conjunction dates are determined by each method can be found on the Jupiter-Uranus 2010-11 triple conjunction page. Although conjunctions measured along the ecliptic can be significantly closer, the Right Ascension method is the more commonly used, and it is the one which is adopted here.

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Finder Chart for Neptune, 2023

During 2023 Neptune can be found several degrees South of the Circlet of Pisces. positioned on or near the border of Aquarius, the Water Carrier, with Pisces, the Fishes.

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Credits

• The star charts were created using the commercial software MegaStar by Willmann-Bell, Inc.. Star colours were based on those shown in "Patrick Moore's Colour Star Atlas" (Mitchell Beazley Publishers Ltd., London, 1973).
• Dates of constellation boundary crossings were determined using the commercial software RedShift 5 by Maris Technologies Ltd. The future conjunction date of Uranus and Neptune was found using the 'Conjunctions' option of the software's 'Sky Diary' facility.
• The date of Neptune's perihelion crossing was determined using the SkyGazer Ephemeris utility by John Biondo.
• Deep-sky objects (galaxies, gaseous nebulae and/or planetary nebulae) shown on the finder chart are those deemed visible through telescope apertures of 4 inches (100 mm) or more under reasonably dark skies, as shown in the "Interstellarum Deep Sky Atlas" by Ronald Stoyan and Stephan Schurig (Oculum-Verlag/Cambridge University Press, 2014).

Copyright  Martin J Powell  2006-2023