The Position of Neptune in the Night Sky,
2006 to 2023
by Martin J Powell
Star map showing the path of Neptune against the backdrop of stars in Capricornus and Aquarius from August 2006 to September 2023 (click on thumbnail for full-size image, 63 KB). Positions are marked for each opposition date. Neptune begins the period describing a series of shallow 'hybrid' formations (part zig-zag, part loop) having crossed the ecliptic from North to South in 2003. By the time the planet is South of the Circlet of Pisces, the 'hybrid' loops have transformed into conventional, South-facing loops (the individual 'loops' are not discernible in this chart because of the scale of the map; instead, each loop appears as an oblique line). Note that, because the angular width of Neptune's loops (about 2º.8 across) are greater than its annual orbital motion, each successive loop overlaps (in longitude) with the next.
The star map applies to observers in the Northern hemisphere (i.e. North is up); for the Southern hemisphere view, click here (66 KB). The faintest stars shown on the map have an apparent magnitude of about +4.9. Printer-friendly versions of this chart are available for Northern (27 KB) and Southern hemisphere (28 KB) views. Astronomical co-ordinates of Right Ascension (longitude, measured Eastwards in hrs:mins) and Declination (latitude, measured in degrees North or South of the celestial equator) are marked around the border of the chart.
The above chart will help in finding the general location of the planet throughout the period in question, however a detailed finder chart will be required to pinpoint the planet precisely - see below.
From August 2006 to March 2010, Neptune was positioned in Capricornus, the Sea-Goat, where it had been situated since early 1998. In March 2010, Neptune briefly 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 will remain until it moves into Pisces, the Fishes, in April 2022 (a photograph of this region of the night sky can be seen below).
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).
Neptune imaged by NASA's Voyager 2 spacecraft during its August 1989 flyby, showing the previously-unknown Great Dark Spot (click for full-size image, 3 KB). Having observed the bland globe of Uranus in 1986, scientists were initially surprised at the high level of activity in Neptune's atmosphere (Image: NASA-JPL).
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 August 2026 (moving retrograde and temporarily heading South-westwards) and again in January 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 point (i.e. its closest point to the Sun; in this case 29.81 Astronomical Units or 4.46 thousand million kms) in Aries in 2046.
[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,
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 an appulse. However, not all planetary conjunctions will be visible from the Earth 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 almost impossible to see. When one considers that Venus is always less than 47° from the Sun, whilst Mercury is always less than about 27° from the Sun, it follows that, whenever either of these planets are involved in conjunctions with Neptune, twilight will usually be a problem, the lighter sky diminishing the visual impact of the conjunction.
Because Mercury is only ever 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.
Binoculars 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.
Most conjunctions between Neptune and the other superior planets (Mars, Jupiter, Saturn and Uranus) occur at elongations of less than 90°, when Neptune is below its brightest apparent 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 most interesting conjunctions take place when the planet is within a few months of opposition, at which times they involve the much brighter planets Mars, Jupiter or Saturn; 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). 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 between 2011 and 2020 at solar elongations of greater than 20°. 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.
Neptune conjunctions with other planets from 2011 to 2020 (click for full-size table, 82 KB) Note that there are no observable planetary conjunctions involving Neptune during 2013. The column headed 'UT' is the Universal Time (equivalent to GMT) of the conjunction (in hrs : mins). The separation (column 'Sep') is the angular distance between the two planets, measured relative to Neptune, e.g. on 2017 Jan 13, Venus is positioned 0°.4 North of Neptune at the time shown. The 'Fav. Hem' column shows the Hemisphere in which the conjunction will be best observed (Northern, Southern and/or Equatorial). The expression 'Not high N Lats' indicates that observers at latitudes further North than about 50°N will find the conjunction difficult or impossible to observe because of low altitude and/or bright twilight.
In the 'When Visible' column, a distinction is made between Dawn/Morning visibility and Dusk/Evening visibility; the terms Dawn/Dusk refer specifically to the twilight period before sunrise/after sunset, whilst the terms Evening/Morning refer to the period after darkness falls/before twilight begins (some conjunctions take place in darkness, others do not, depending upon latitude). The 'Con' column shows the constellation in which the planets are positioned at the time of the conjunction.
To find the direction in which the conjunctions will be seen on any of the dates in the table, note down the constellation in which the planets are located ('Con' column) on the required date and find the constellation's rising direction (for Dawn/Morning apparitions) or setting direction (for Dusk/Evening apparitions) for your particular latitude in the Rise-Set direction table.
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 conjunction dates measured along the ecliptic are technically more accurate (separations between planets 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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Finding Neptune in Your Local Night Sky
Where in the night sky should I look for Neptune tonight? In which direction and how high up will it be?
The location of a planet (or any other celestial body) in your local night sky depends upon several factors: the constellation in which it is positioned, your geographical latitude and longitude and the date and time at which you observe. To find a planet in the night sky at any particular date and time, we must know two things: a direction in which to look along the observer's horizon (eg. Southeast, East-Southeast) and an angle to look above the horizon (known as altitude or elevation).
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Eastern Aquarius & the Circlet of Pisces A photograph showing the region of the night sky through which Neptune passes through to 2024 (click on thumbnail to see full-size photo, 201 KB). Stars in the photograph can be seen down to about magnitude +7.5. An annotated version of the photograph can be seen here (76 KB).
Finder Chart for Neptune, 2017
During 2017, Neptune can be found in the Eastern half of Aquarius, a short distance South and East of the star Lambda Aquarii ( Aqr, mag. +3.7). To find this star, first locate the 'The Steering Wheel' asterism in the Northern section of the constellation and 'star-hop' South-eastwards from it. Use the Mini-AstroViewer® Java applet above to help locate Aquarius in your local night sky.
The Path of Neptune in Eastern Aquarius during 2017, with positions marked on the first day of each month (click on thumbnail for full-size chart, 88 KB). A Southern hemisphere view can be found here (92 KB). Where the planet is too close to the Sun to be observable, the path is shown as a dashed line. Ideally, searches for Neptune should be carried out on Moonless nights, i.e. in the two-week period centred on the New Moon in any given month.
Neptune reaches opposition to the Sun on September 5th 2017 (indicated on the chart by the the symbol ) when its apparent magnitude is +7.8 and its apparent diameter is 2".4 (2.4 arcseconds). The planet is then 28.938 AU (4,329 million kms or 2,690 million miles) from Earth.
Much of the star field in the chart should be easily contained within a binocular field of view (which typically ranges from 5° to 9°). Stars are shown down to magnitude +8.5. Right Ascension and Declination co-ordinates are marked around the border, for cross-referencing in a star atlas. Printer-friendly (greyscale) versions of the chart are available for Northern (40 KB) and Southern hemisphere (41 KB) views.
Click here (71 KB) to see a 'clean' star map of the area (i.e. without planet path); observers may wish to use the 'clean' star map as an aid to plotting the planet's position on a specific night - in which case, a printable version can be found here (33 KB).
Previous finder charts for Neptune can be viewed for 2011, 2012, 2013, 2014, 2015 and 2016.
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Copyright © Martin J Powell 2006 - 2016
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