The Position of Jupiter in the Night Sky:
2019 to 2022
by Martin J. Powell
The path of Jupiter against the background stars of Scorpius, Ophiuchus, Sagittarius, Capricornus and Aquarius from November 2018 to March 2022, with positions marked on the 1st of each month (click on the thumbnail for the full-size chart, 202 KB). Periods when the planet is unobservable (i.e. when it is too close to the Sun, or passes behind it) are indicated by a dashed line; hence the planet becomes lost from view (in the evening sky) in early December 2019 and becomes visible again (in the morning sky) in mid-January 2020. The chart shows the changing shape of a planet's apparent looping formation as it moves through the zodiac. Jupiter crosses the ecliptic (the apparent path of the Sun, Moon and planets) in a Southward direction in late February 2020, following which it describes a zig-zag formation in Eastern Sagittarius. In the years on either side of ecliptic crossing the planet describes a hybrid loop (half loop, half zig-zag); Northward-facing in 2019 and Southward-facing in 2021.
The star map applies to observers in the Northern hemisphere (i.e. North is up); for the Southern hemisphere view, click here (206 KB). The faintest stars on the map have an apparent magnitude of about +4.8. Printer-friendly versions of this chart are available for Northern (95 KB) and Southern (97 KB) hemisphere views. Astronomical co-ordinates of Right Ascension (longitude, measured Eastwards in hrs:mins from the First Point of Aries) and Declination (latitude, measured in degrees North or South of the celestial equator) are marked around the border of the chart. Click here (173 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 (84 KB). Night sky photographs of the region, together with dates of the planet's passage of the brighter stars, can be seen below.
Star names shown in yellow-green were officially adopted by the International Astronomical Union (IAU) in 2017 and 2018. The six such star names shown on this chart were drawn from Chinese, Aboriginal, Polynesian, Persian and South African mythology (for more details see the IAU's Working Group on Star Names pages).
Having spent the 2017-18 apparition in the constellation of Libra, the Balance, Jupiter enters Scorpius, the Scorpion, in mid-November 2018. The planet is out of view from Earth at this time, passing through superior conjunction (positioned directly behind the Sun as seen from the Earth) later that month.
Jupiter imaged by Teruaki Kumamori (Sakai City, Osaka, Japan) on May 10th 2018 using a 14-inch (350 mm) Schmidt-Cassegrain reflector telescope and CMOS camera (click on the thumbnail for a larger image, 8 KB) (Image: Teruaki Kumamori / ALPO-Japan)
Jupiter crosses into the non-zodiacal constellation of Ophiuchus, the Serpent-Bearer, shortly before re-appearing in the dawn sky - rising just ahead of the Sun - in mid-December 2018. Jupiter reaches its Eastern stationary point in mid-April 2019 before turning retrograde (moving East to West) and describing its 2019 'hybrid' loop in South-eastern Ophiuchus. Jupiter reaches opposition (its closest and brightest orbital position in relation to the Earth for that year) in mid-June 2019, positioned a few degrees North-west of the star Oph (Theta Ophiuchi, apparent magnitude +3.2). The planet continues its retrograde motion and reaches its Western stationary point in mid-August 2019. It then resumes direct motion (West to East), crossing the border into Sagittarius, the Archer, in mid-November 2019. Jupiter heads out of view in the dusk twilight during the second week of December 2019, marking the end of the planet's 2018-2019 apparition. At this time Jupiter is positioned 1° North of the faint gaseous nebula commonly called the Lagoon Nebula (Messier 8 or NGC 6523).
Jupiter passes through superior conjunction in late December 2019, re-appearing in the dawn sky in central Sagittarius in mid-January 2020, heralding the start of its 2020-2021 apparition. Over the next few weeks Jupiter is positioned several degrees North of the asterism (star pattern) commonly called the Teapot. Specifically, the planet is positioned North of the teapot's handle, which is comprised of four moderately bright stars. Jupiter continues its Eastward (direct) motion until it reaches its Eastern stationary point in mid-May 2020, a couple of degrees from Sagittarius' Eastern border. It then turns retrograde and over the next six months describes a zig-zag formation against the faint stars of Eastern Sagittarius. The planet reaches opposition in mid-July 2020, at the centre of the zig-zag, a few degrees North of the star 52 Sgr (52 Sagittarii, mag. +4.6). Jupiter then continues Eastwards and reaches its Western stationary point in mid-September 2020. It then resumes direct motion, leaving Sagittarius and entering Capricornus, the Sea-Goat in mid-December 2020. The 2020-21 apparition draws to a close as Jupiter heads into the evening twilight in early January 2021, positioned 5° degrees SSE of the star Dabih (1 Cap or Beta-1 Capricorni, mag. +3.0v).
Jupiter remains out of view for the next five weeks, passing through superior conjunction in late January 2021 before emerging into the dawn sky in mid-February 2021. The 2021-2022 apparition begins with Jupiter passing just 2' (2 arcminutes or 0°.03, where 1 arcminute = 1/60th of a degree) North of the star Cap (Theta Capricorni, mag. +4.0) in central Capricornus. This apparition sees Jupiter ascend the ecliptic (the path of the Sun, which the Moon and planets follow very closely) by a significant amount, improving the planet's visibility for Northern hemisphere observers and slightly worsening visibility for Southern hemisphere observers (see the Jupiter Transit Altitudes section below). Now heading North-eastwards, Jupiter moves into Aquarius, the Water Carrier, in late April 2021, reaching its Eastern stationary point in the third week of June 2021. The planet then changes direction and heads South-westwards, retrograding back towards Capricornus over the next two months and re-entering that constellation in mid-August 2021. Opposition is reached less than a day after crossing the border, with Jupiter positioned just 7' (0°.11) West of the boundary with Aquarius. The planet's 2021-22 'hybrid' loop is described on the border between Capricornus and Aquarius, Jupiter reaching its Western stationary point in Eastern Capricornus in mid-October 2021. The planet then resumes direct motion once more, heading North-eastwards along the ecliptic and re-entering Aquarius in mid-December 2021. The 2021-22 apparition ends as Jupiter heads into the dusk twilight in mid-February 2022, positioned just to the South-west of the star Cap (Lambda Aquarii, mag. +3.9) and 10° SSE of the so-called Steering Wheel asterism which occupies the Northern part of the Water Carrier.
Jupiter in Leo, the Lion photographed by the writer a few days before the planet's opposition in March 2016, when it was positioned near the star Sigma Leonis (at the rear paw of the Lion) (click on the thumbnail for the full-size photo, 67 KB). An annotated version of the photo can be seen here (63 KB). Jupiter will return to Leo in 2026.
[Terms in yellow italics are explained in greater detail in an associated article describing planetary movements in the night sky.]
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Jupiter Opposition Data, 2019 to 2021
Jupiter reaches opposition to the Sun (when it is closest to the Earth and brightest in the sky for any given apparition) every 398.9 days on average, i.e. about 33½ days later in each successive year. For the period covered by the above star map, oppositions take place on June 10th 2019, July 14th 2020 and August 19th 2021. Around opposition, the planet is due South at local midnight in the Northern hemisphere (due North at local midnight in the Southern hemisphere).
The apparent magnitude of the planet at opposition during the period of the star chart is -2.5 (in 2019), -2.6 (in 2020) and -2.7 (in 2021). Jupiter's apparent size (i.e. its angular width as seen from the Earth, measured in arcseconds, where 1 arcsecond = 1/3600 of a degree) at opposition is 46".0 (in 2019) increasing to 47".6 (in 2020) and then 49".1 (in 2021).
Because of Jupiter's rapid rotation speed, its disk appears as an oblate spheroid through telescopes and high-magnification binoculars (i.e. it appears flattened at the poles and bulged at the equator). The dimension given above is the apparent equatorial diameter of the planet; its apparent polar diameter is about 6.3% less.
Superior conjunction (when Jupiter passes behind the Sun as seen from the Earth) takes place on December 27th 2019, January 29th 2021 and March 5th 2022. The planet is not visible from Earth for about two weeks on either side of these dates. At superior conjunction the magnitude fades by almost one whole magnitude to -1.7 (in 2019), -1.8 (in 2021) and -1.9 (in 2022) and the planet's apparent diameter shrinks to 31".7 (in 2019), 32".5 (in 2021) and 33".0 (in 2022).
Data relating to Jupiter's oppositions from 2019 to 2021 are provided in the table below.
Jupiter opposition data for the period 2019 to 2021 (click on thumbnail for full-size image, 32 KB). The Declination is the angle of the planet to the North (+) or South (-) of the celestial equator; on the star chart, it represents the planet's angular distance above or below the blue line. The angular diameter (or apparent size) of the planet as seen from Earth is given in arcseconds (where 1 arcsecond = 1/3600th of a degree).
Jupiter's opposition distance from Earth slowly reduces throughout the period, so that its angular diameter at opposition increases slightly year by year. This is reflected in the planet's apparent magnitude, which brightens slightly over the period. Jupiter's solar distance also reduces over the period as it heads towards perihelion - its closest orbital point to the Sun - in January 2023. The Tilt (the inclination of Jupiter's rotational axis relative to the Earth's orbital plane) is positive (+) when Jupiter's Northern hemisphere is tipped towards the Earth and negative (-) when its Southern hemisphere is tipped towards the Earth; the maximum value it can attain is ±3°.4.
The Tilt values were obtained from NASA's Jupiter Ephemeris Generator 2.6. All other data was obtained from 'Redshift' and 'SkyGazer Ephemeris' software. The Jupiter images were obtained from NASA's Solar System Simulator.
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Jupiter Conjunctions with other Planets,
January 2019 to February 2021
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 since 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.
Six observable conjunctions take place during the period, four of them in the morning sky. Due to Jupiter's high Southerly declination, all of them are more favourably observed from the Southern hemisphere. The planet Saturn, also occupying this region of the zodiac in recent years, is positioned not far away from Jupiter in all of these events.
Conjunctions between Jupiter and Venus are perhaps the most spectacular of all to view and the most photogenic. Between January 2019 and February 2021 there are three occasions when these two planets can be seen together. The morning conjunction of January 22nd 2019 is visible worldwide and takes place only two weeks after Venus' greatest Western elongation from the Sun. Although it has the widest separation of the three conjunctions, it is the only one which can be seen from the whole of the Northern hemisphere and which can also seen against a dark sky. The Jupiter-Venus conjunction of February 11th 2021 is the closest of the three, the separation between the two planets being 26' (0°.44). It is only visible from Equatorial and Southern hemisphere latitudes. With a solar elongation of just 10° it is a difficult one to observe, the two planets being positioned at most only 6° above the ESE horizon as the fainter planet (Jupiter) disappears from view in the dawn twilight.
The only evening conjunction between Jupiter and Venus takes place on November 24th 2019. With the exception of high-Northern latitudes (where it is not observable), it is a reasonably good conjunction for the Northern hemisphere and is only slightly better for the Southern hemisphere. At latitude 50° North the pair are positioned around 6° high in the South-west as Jupiter comes into view in the dusk, whilst at 30° North the pair are around 14° high, also in the South-west. At mid-Southern latitudes the pair stand around 15° to 18° high in the WSW when Jupiter appears in the twilight. The pair will only be seen against a truly dark sky South of about latitude 45° North.
When Jupiter aligns with Mars a planetary conjunction takes place (click on the thumbnail for a larger image [123 KB] and click here [44 KB] for a closer view). This event was photographed by the writer in the dawn sky on January 7th 2018, when both planets were positioned in Libra and separated by just 0°.2.
Conjunctions between Jupiter and Mars provide a good opportunity to compare their differing colours with the naked-eye. The Jupiter-Mars conjunction of March 20th 2020 is a particularly good one for Southern hemisphere observers, the two planets being positioned almost 70° away from the Sun in the constellation of Sagittarius. At magnitude +0.9 Mars is however relatively dim, being several months away from its closest and brightest position in relation to the Earth (at opposition) in October 2020. Together with Saturn to the East, the three planets form a slender triangle which points Eastwards towards Capricornus. At latitude 15° South Jupiter and Mars reach a significant 59° above the ESE horizon as the Red Planet disappears from view in the twilight. Even at 45° South they are 53° above the ENE horizon when Mars disappears from view. For Northern hemisphere observers the conjunction is much lower in altitude but nonetheless easily observable. At 30° North the pair reach a respectable 29° above the South-east horizon as Mars fades from view, whilst at 50° North they are 11° high in the South-east.
The rarest of the conjunctions during the period is that between the giant planets Jupiter and Saturn on December 21st 2020. Known historically as Great Conjunctions, Jupiter-Saturn conjunctions take place about every twenty years, the last one having been in May 2000. However, they are not always best placed for viewing, sometimes taking place at narrow solar elongations. The most spectacular conjunctions between these two planets occur when they are both within days of opposition, at which time they are particularly bright and visible throughout the night. Such events are very rare however, taking place about every 139 years or so (the next will be in the year 2238). Perhaps the best-known conjunction between Jupiter and Saturn was that in the year 7 BC, in the constellation of Pisces, the Fishes. In the early seventeenth century the German astronomer Johannes Kepler (1571-1630) suggested that this event might have been the origin of the Star of Bethlehem, referred to in St Matthew's Gospel of the Bible. Specifically, this was a triple conjunction - a series of three conjunctions which took place between May and December of that year - which, it is argued, was such an unusual chain of events that the Magi (the 'wise men' or astrologers) gave it a special significance. Critics of this theory say, among other things, that the two planets were too far apart to attract any particular attention, their angular separation at best having been about 1° (about two apparent Full Moon diameters).
The 'Great Conjunction' of December 2020 takes place on the day of Earth's winter solstice, the planets being only 30° away from the Sun in the evening sky. Whilst not ideally placed for viewing and being well past their opposition dates, Jupiter and Saturn are separated by only 6 arcminutes (0°.1), making them excellent photographic targets through the eyepiece of a telescope. After 2020, they will next meet in November 2040 in the constellation of Virgo, the Virgin.
Finally, a morning conjunction between Jupiter and Mercury takes place on March 5th 2021, only one day ahead of the latter planet's greatest Western elongation. In other words, this is about as far away from the Sun that a planetary conjunction with Mercury can take place. However, the visibility of Mercury is heavily dependant on latitude and local season, and in this case the conjunction cannot be viewed from higher-Northern latitudes. Southern Tropical latitudes are best placed to view it, the pair reaching up to 20° in altitude above the Eastern horizon before Mercury disappears from view in the dawn twilight.
The following table lists the observable conjunctions involving Jupiter which take place during 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.
Jupiter conjunctions with other planets from January 2019 to February 2021 (click on thumbnail for full-size table, 37 KB). 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 Jupiter, e.g. on 2020 Mar 20, Mars is positioned 0°.7 South of Jupiter 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 45°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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Constellations of the Zodiac: Photographs
Cap & Aqr
Sgr & S. Zodiac
Sco & S. Oph
Scorpius, Southern Ophiuchus, Sagittarius, Capricornus and Aquarius Three photographs showing the region of the night sky through which Jupiter passes from late 2018 to early 2022 (click on the thumbnails for their full-size versions: 93 KB, 206 KB and 111 KB). The regions of the star chart which are covered by each photograph are shown on the overlay chart above (72 KB). Dashed lines indicate that the photograph extends beyond the boundaries of the chart. The faintest stars visible in each photo are about magnitude +8.0. Note that the photographs do not have the same scale because of the varying camera lens settings and image resolutions.
As it slowly moves along the 'celestial highway' known as the ecliptic (the apparent path along which the Sun, Moon and planets move through zodiac) Jupiter passes numerous bright stars; these are listed below, in chronological order:
Jupiter reached opposition to the Sun in central Taurus in late 2012 (click on thumbnail for full-size photo, 339 KB). The picture shows the planet in the Western sky before dawn, when it began to sink into the suburban skyglow. Orion is seen at the left of the picture and Auriga is at the upper right. An annotated version of the photo can be seen here (180 KB).
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Martin J Powell is a participant in the Amazon Europe S.à r.l. Associates Programme, an affiliate advertising programme designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.co.uk, Amazon.de and Amazon.fr
Jupiter Transit Altitudes, 2019 to 2021
Jupiter is the largest of the Solar System planets and it can show considerable detail even through modest-sized telescopes. A major factor determining the likelihood of seeing a clear telescopic image is the altitude (angle above the horizon) of a planet at the time of observation. For the naked-eye observer, apart from the increased likelihood of obstruction from trees and buildings, a planet's low altitude is generally of little consequence, however for the telescopic observer high altitude is essential in order to minimise the effects of turbulence, atmospheric dimming and light pollution (skyglow) which prevails near the horizon. Consequently, telescopic observers consider high altitude transits (when a celestial body crosses the observer's meridian, reaching its highest point in the sky) as more favourable than low altitude transits. As a general rule, telescopic observation is best done when a celestial body's altitude is greater than about 30°; hence observation in the couple of hours after rising or before setting is best avoided, unless there is no other alternative.
Jupiter's meridian transit altitude (as seen from any given point on Earth) varies from one year to the next in the course of its 11.8-year journey through the zodiac constellations. Its most Northerly point is attained in Gemini (around 23½° North of the celestial equator) then - some six years later - its most Southerly point is attained in Sagittarius (around 23½° South of the celestial equator). In the intervening years, the planet lies somewhere between these two extremes.
The meridian transit altitude at which an observer sees a planet depends not only upon the constellation in which the planet is positioned at the time, but also upon the observer's latitude. As a result, certain apparitions are more favourable to observers in one hemisphere than to observers in the opposite hemisphere.
In the 2012-14 period, observers at mid-Northern latitudes saw Jupiter at its highest meridian transit altitude for some twelve years, as the planet traversed the Northernmost constellations of the zodiac. From 2015 to 2018, however, observers here saw Jupiter's transit altitude reduce significantly year by year as the planet headed Southward through Leo, Virgo and Libra, the planet crossing the celestial equator in September 2016.
For Southern hemisphere observers, the 2012-14 period saw Jupiter at relatively low altitudes when it reached meridian transit (due North in the Southern hemisphere) providing less-than-optimal viewing conditions for telescopic observers. Since 2015, viewing circumstances from these latitudes have improved year-on-year, as the planet heads towards its most Southerly declination in Sagittarius in December 2019.
Transit altitudes of Jupiter at successive oppositions from 2019 to 2021, as seen from a variety of latitudes (click on thumbnail for full-size table, 23 KB). The Declination (Dec.) is the angle of the planet to the North (+) or South (-) of the celestial equator at the time of the planet's opposition. The Altitude Range is the approximate altitude variation over the course of the apparition, e.g. for the 2018/19 apparition at latitude 40° North, the transit altitude of Jupiter ranges from (27°.6 - 1°.1) = 26°.5 to (27°.6 + 1°.1) = 28°.7. The table demonstrates that, after 2020, Jovian transit altitudes improve significantly for Northern hemisphere observers but worsen for Southern hemisphere observers.
What are the best and worst case scenarios regarding Jupiter's transiting altitude? Northern hemisphere observers witnessed their best case scenario (and Southern hemisphere observers witnessed their worst) in the 2013-14 observing season, when Jupiter was positioned at its most Northerly point in Gemini (see table on the 2011-14 page). Observers at mid-Northern latitudes then saw the planet transit at around 70° high in the sky. Mid-Southern hemisphere observers fared rather worse, the planet transiting at only 30° high (worst case scenario). The next optimal observing times for Southern hemisphere observers will be in the 2018-19 and 2020-21 apparitions, when the planet will be transiting at altitudes of around 70° to 80° at mid-hemispheric latitudes (best case scenario). Observers at mid-Northern latitudes will then see Jupiter transiting at altitudes of less than 30° (worst case scenario). The Northern hemisphere's next best viewing times will be in the mid-2020s.
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Moon near Jupiter Dates, 2018
The Moon is easy to find, and on one or two days in each month, it passes Jupiter in the sky. Use the following table to see on which dates the Moon is in the vicinity of the planet:
Moon near Jupiter dates for 2018 (click on thumbnail for full-size table, 37 KB). No dates are shown for December because Jupiter is too close to the Sun to view during this month. The Date Range shows the range of dates worldwide (allowing for Time Zone differences across East and West hemispheres). Note that the dates, times and separations at conjunction (i.e. when the two bodies are at the same Right Ascension) are measured from the Earth's centre (geocentric) and not from the Earth's surface (times are Universal Time [UT], equivalent to GMT). The Sep. & Dir. column gives the angular distance (separation) and direction of the planet relative to the Moon, e.g. on May 27th at 17:39 UT, Jupiter is 3°.9 South of the Moon's centre. The Moon Phase shows whether the Moon is waxing (between New Moon and Full Moon), waning (between Full Moon and New Moon), at crescent phase (less than half of the lunar disk illuminated) or gibbous phase (more than half but less than fully illuminated).
On December 1st, 2008, Jupiter, Venus and the four-day-old Moon formed an impressive celestial grouping in the evening sky (click thumbnail for full-size image, 15 KB). This is the writer's simulation of how the event appeared to residents of Cairo, Egypt, at the end of evening twilight (around 1810 Local Time), when the group was situated low down in the South-western sky. Venus (at left of picture) was an 'Evening Star' at magnitude -4.0 and Jupiter was at magnitude -1.8 (closing in on the Sun, heading towards superior conjunction) at the time of the event. On the same day, observers in Europe and North-west Africa saw the Moon pass in front of Venus - in an event called a lunar occultation - around local sunset/dusk. The next lunar occultation involving Jupiter will take place in late 2019.
The Moon moves relatively quickly against the background stars in an Eastward direction, at about its own angular width (0º.5) each hour (about 12º.2 per day). Because it is relatively close to the Earth, an effect called parallax causes it to appear in a slightly different position (against the background stars) when seen from any two locations on the globe at any given instant; the further apart the locations, the greater the Moon's apparent displacement against the background stars. Therefore, for any given date and time listed in the table, the Moon will appear closer to Jupiter when seen from some locations than from others. For this reason, the dates shown in the table should be used only for general guidance.
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Position of Jupiter's Four Brightest Moons
Jupiter's four brightest moons (satellites) - namely Ganymede (magnitude +4.6 at opposition), Io (+5.0), Europa (+5.3) and Callisto (+5.6) - can readily be seen through telescopes or steadily-held binoculars. The moons are seen to change their position in relation to each other, along the planet's equatorial plane, from one night to the next. In fact, their motion can be detected in the space of just a few hours.
Because of their low magnification, binoculars may have some difficulty detecting Io since it is the closest of the four moons to the planet; it never lies more than three Jupiter-diameters away. Europa is easier, but Ganymede is the easiest of the four to see. Callisto moves furthest away from the planet but it is also the faintest of the four.
Due to Jupiter's shallow axial tilt (3º.1 to the plane of its orbit), the Jovian moons appear to present a more-or-less linear motion when seen from the Earth. This is in contrast to, say, Saturn with its relatively high axial tilt (26º.7), which causes its moons to mostly follow apparent elliptical paths around the planet when viewed from the Earth (see Saturn's moon positions). Approximately every six years, when the Earth passes through Jupiter's equatorial plane, the Jovian moons are seen to become involved in mutual occultations (where the moons pass in front of each other) and mutual eclipses (where a moon's shadow falls upon another moon). Numerous mutual events took place during the 2014-15 observing period and the next events will take place between January 2021 and March 2022.
The following Flash program shows the current position of Jupiter's four brightest moons (based on your computer's clock and Time Zone settings):
The Positions of Jupiter's four brightest satellites in relation to the planet (click on thumbnail to access Flash program, width 655 pixels, 108 KB; the graphic requires the Adobe Flash Player plug-in to display correctly). Binocular and terrestrial telescope users in the Northern hemisphere should use the default 'Erect Image' (North up, East to the left) setting; Southern hemisphere observers using this equipment will need to click on the 'Inverted' (North down, West to the left) button.
Users of astronomical telescopes in the Northern hemisphere will need to use the 'Inverted' option to match the view in their telescope, whilst those in the Southern hemisphere should use the default ('Erect Image') setting. The 'Mirror Reversed' button applies to astronomical telescopes with a star diagonal attached.
Enter the required values for Date (in the form mm/dd/yyyy) and Time and click on 'Recalculate' to see the position of the moons for any date and time between January 1st 1900 AD and December 31, 2100 AD. The Timezone offset from UT is determined by the settings in your web browser.
Other details shown are the planet's apparent magnitude, its angular size (in arcseconds), its distance from the Sun (in Astronomical Units) and the planet's System II Longitude (the Jovian longitude of the central meridian, i.e. the imaginary line through the centre of the planet's disk from pole to pole). Since Jupiter's outer layers are gaseous, the planet does not rotate as a solid body; in fact the equatorial region (known as System I ) makes one rotation in 9h 50m 30s whilst the rest of the planet (System II) rotates once in 9h 55m 40s. The Great Red Spot is located in System II, at a latitude of about 22° South.
Pressing the 'Display' button generates a list of Jovian satellite phenomena for the selected date - namely transits (when a moon or its shadow passes across the planet's disk), occultations (when a moon passes behind the planet's disk) and eclipses (when a moon enters Jupiter's shadow). All of these events can be observed in telescopes.
The next three transit times of the Great Red Spot (GRS) - i.e. when it crosses the planet's central meridian - are also listed, the GRS itself being displayed on the graphic. Note that the accuracy of these times is dependant upon the Jovian longitude of the GRS, which slowly drifts over time. By default, the program uses a longitude of 98°, however this is now incorrect and the value must be updated in order to provide accurate transit times. As of September 2018, the longitude of the GRS was approximately 292°, so this value should be entered in the 'GRS Longitude' box and the timings recalculated by pressing the 'Display' button. The current longitude of the GRS can be determined from the graph displayed at the JUPOS website.
Times of all events in the program are given in Universal Time (UT) which is equivalent to Greenwich Mean Time (GMT).
The 'Jupiter's Moons' program by John Bartucci is available as a standalone, executable (exe) file which can be downloaded from the The Wilderness Center Astronomy Club website.
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Copyright Martin J Powell October 2018
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