The never ending dance of all the four main satellites is of constant interest to the visual observer, especially as the changing configurations are very different from night to night to hour to hour. All four moons display discernible small disks that average about 1 arcsec across. These satellite disks are readily visible in 10.5cm telescopes, and this can be quickly confirmed by various stellar occultations or by satellite shadows cast onto the upper atmosphere of Jupiter when in transit. More often than not, all four satellites are labelled by either Roman numerals I = Io, II = Europa, III = Ganymede and IV = Callisto. Others simply label the moons from 1 to 4. All the four Galilean moons always move in their orbits from east to west across the disk of Jupiter. I.e. During transits, and move west to east when undergoing occultations with Jupiter itself. Both Io and Europa due to their proximity do more frequently experience many transits and occultations. These are visible at least every one or two days from any same fixed location. Further out, Ganymede and Callisto have much longer orbital periods, and so experience events regularly but less periodic. As the orbital orientation ( i ; inclination) of the satellite orbits also follows the same paths of the planets — the ecliptic. Except occasionally for Callisto, events never pass above or below of the main bright disk of Jupiter. Ganymede will undergo one transit and occultation about once per week, and Callisto roughly twice per per month.

When viewed from the Earth, these events also depends on where Jupiter is placed in its orbit around the Sun. This finds that the darkened shadow projected behind Jupiter into space is not always exactly perpendicular to the observer — except at the times of opposition and at the unseen conjunction. It means the moons can be occulted by the shadow well away from the apparent disk of Jupiter. Even transits can appear away from the main disk of Jupiter, even though the projected shadow of the moons are seen upon the upper Jovian cloud-tops. Differences for Io are not as significant as they are for Europa, Ganymede and Callisto.

Times for all these mutual events are easily predictable and are commonly published in an annual ephemeris or when using various astronomical computer software. Calculations of such mutual satellite phenomena begin in the 1750s, that was slowly improved in accuracy in time. Observing the predicted mutual phenomena was usefully employed for the local longitude determination of ships sailing to distant shores. Predictions were made with an dedicated ephemerides, first issued by the H.M. Nautical Almanac Office in England. Such observations soon proved not to be easily made from the moving deck of the ship. These really had to be made from the foreshore — not very useful if placed mid-ocean or away from land. Timing of these regular Jovian satellite transits and occultations can be made by amateurs, and these collected over time can slightly improve our understanding of their orbits and perturbations.


The number and possibilities of the various phenomena is complicated when listed, the events are usually shortened and abbreviated. Common abbreviations include; Ec or E for Eclipse; Oc or O for Occultation; Tr or T for Transits, Sh or S for Shadow Transit. The interactions with the Jovian disk or shadow are similar to lunar occultations or planetary transits of Venus and Mercury, and these are given as; D for Disappearance, R for Reappearance, and for satellite interactions with the Jovian disk as I for Ingress and E for Egress. Hence, EcD or ED, means eclipse disappearance or ShE means Shadow transit eclipse, etc. Most of the times for phenomena are calculated for the time of mid-eclipse and not for ingress and egress times — the start and beginning of phenomena. This is similar for the times when the shadow intersects the planetary disk or the shadow is eclipsed by Jupiter disk or projected shadow.

In general, in the time after conjunction with the Sun and then leading up to the opposition of Jupiter. Also the mutual shadow phenomena will be on the western side of the planet, where any observed eclipses are before the occultation, and any shadow transit with be before the transit of the actual moon. Then from after opposition and before any solar conjunction, the mutual shadow phenomena will intersect with the shadow on the eastern side, whilst the occultations will occur before eclipses, and transits will be before shadow transits. However, it is important to not that the behaviour of each moon varies significantly through the yearly Jovian events, which is made more varied by the interactions between occultations and eclipses with the position of the planet shadow.

The maximum positions of the Jovian shadow compared to the beginning of transits or eclipses against the Jovian edge of the oval disk vary roughly at maximum at quadrature, when Jupiter is at right angles to earth-sun alignment — when either east of west of the planetary disk. A maximum co-ordinate separation is measured from the shadow intersection of the actual centre of the moon in question, whose diameter varies for each main Galilean moon. This is roughly about, expressed in Jovian radii, 2.2 RJup for Io, 2.8RJup for Europa, 3.7RJup for Ganymede and 4.1RJup for Callisto (assuming the moon does no pass above or below Jupiter.) The amount can be for either east (E) of west (W) of the planet, and varies slightly due to smaller factors like the distance of Jupiter and where it lies in its geocentric position.

As the orbits of Jupiter and the satellites are slightly eccentric or slightly above or below the ecliptic, eclipses and transits do vary at the points of contact with the disk. This produces a slight displacement where the mutual phenomena intersects the disk. For Io, the maximum displacement is about 0.3RJup north or south of the equator of Jupiter, 0.5RJup north or south for Europa, 0.8RJup north or south for Ganymede, and 1.2RJup for Callisto.

Callisto happens to be a special case, as the moon is furthermost out can passes just above or below the Jovian disk. The latter cases are very rare, though, and may happen only twice every twelve years or so depending on where Jupiter is in orbit around the Sun, and its closeness to perijove — the point closest to the Sun in the planetary orbit. Visual observers should note these are maximum values, and that they are generally less than these. They also do not correspond to the exact opposite values before and after opposition nor for the exact times of disappearance and reappearance.

Most of these values are not necessary to be presented in an ephemeris, because the differences are usually less than several minutes. This allows observers to sometimes make unbiassed observations of the times of these phenomena for historical records of mutual events. Organisation of most information is made to assist observers being derived from the preparation of the yearly and formal USNO Astronomical Ephemeris.


Io (I) and Europa (II) Phenomena

Io (I) before opposition has its eclipses prior to touching the preceding or western planetary limb but will emerge if occulted from the following or eastern limb. After opposition this arrangement is the exact opposite order.

A similar thing happens nearly always for Europa (II), though on very rare occasions both the eclipse and occultations will be seen as separate phenomena that will happen only for a few weeks around quadrature and has the maximum inclination at the similar time.

Ganymede (III) and Callisto (IV) Phenomena

Both Ganymede (III) and Callisto (IV) display similar phenomena to Io (I) except when near the time of opposition where the mutual events of transit or occultation with the shadow are viewed simultaneously. For a few weeks, the times between the ingress and ingress of the moon coincide roughly with the event. Either side of this period, find prior to opposition, the moon will disappear into eclipse and reappear from eclipse on the preceding or western limb, and then will be occulted. Then after oppositions, III and IV will first remerge from the occultation event this time instead on the following or eastern limb, and then have the eclipse in the shadow away from the observed limb of Jupiter.

Unique Callisto (IV) Phenomena

Callisto (IV) displays similar phenomena to the other three moons, though on rare occasions when the alignments are right, can find both the shadow and the position of the moon can pass above or below Jupiter. This occurs when the inclination is above 2°.

Multiple phenomena

Based on the relationships between the orbital periods and mean motion it is impossible for Io, Europa and Ganymede to be in transit at the same time. It is, however, quite possible that two of these three can display transits or eclipses with Callisto (IV).

Very rare times will find that Io, Europa and Ganymede will be completely invisible in the background sky, where any of them may be transiting across the disk with the third undergoing eclipse or occultation. Conversely, two may be in eclipse at one time, while the third may be transiting the Jovian disk.

Even rarer is that all four satellites are not visible at all.

Mutual Phenomena Between the Main Jovian Moons

Very rarely, the four main moons may experience mutual eclipses or occultations between one another. Occultations occur when the disk of one moon passes behind another, while the moon is front will transit. This event can be like solar eclipses, being either as partial (P), annular (A) or total (T).

Such events are rare because they relies on the orbital inclinations of the satellites must coincide with the orbital plane of the line-of-sight of the observer. The duration here these mutual events occur in several short periods of up to six weeks. Each event can last up to about twenty minutes, though the majority are less than ten.

The times when this occurred are twice for each orbit of Jupiter around the Sun. If the orbital period is 11.8626 years, then the time between them is nearly six years (5.93 years.) Previous series were during 1978-1979, 1984-1985, 1990-1991 and 1996-1997. The last was in 2003-2004, while the next series of events will again start in the beginning 2010.

Sometimes these events are of the other main Jovian satellites, which in the earlier times were deemed as important because they allowed knowledge of moon sizes and orbits. There can be some problems with observing these events due to the bright glare of Jupiter or from poor seeing. Magnitudes of the merging objects tend to slightly brighten, or if in eclipse, may drop by several magnitudes

Further Comments on Observed Phenomena

At the time of the mutual event some discrepancies may amount to several minutes. If the eclipse or transit does occur, there maybe an apparent slight fading of the brightness of the moon as it interacts with the brightness of Jupiter itself.

Graphical Representations of the Jovian Satellites

Many references do not give the times of the mutual phenomena, but instead express the relative movements of the moons by their monthly graphical representation of all four moons together. The result in four sinusoidal curves of varying periods being very useful as it shows which side the moon appear in the telescope either east or west of the planet. It is useful as it mainly identifies which moon is which. This graphic cannot be reproduced continuously as the light-time between Jupiter and the Earth causes slightly differing delays in the time of the said mutual phenomena.

Disclaimer : The user applying this data for any purpose forgoes any liability against the author. None of the information should be used for either legal or medical purposes. Although the data is accurate as possible some errors might be present. Onus of its use is placed solely with the user.


Last Update : 29th March 2018

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