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It happens only for a few months every six years, that we can see mutual occultations and eclipses of the Galilean moons of Jupiter. We are presently in such a period but for only a few months more. During a mutual eclipse one satellite passes through the shadow, cast by the Sun, of a second satellite. At an occultation one satellite passes in front of another as seen from the Earth. The precise times of the resulting minimum brightness and the measurement of the reduction in brightness provide useful information of the relative positions of the satellites involved.
Interested? Before you grab a pair of binoculars or a small telescope, first read this whole article, written by the RASNZ occultation specialist.

Conditions for mutual events to occur
For a series of mutual events of the satellites of a planet such as Jupiter and Saturn to occur, the satellites must have orbits which are close to coplanar. The plane of their orbits will also need to be very close to the planet's equatorial plane. The four Galilean satellites of Jupiter, Io, Europa, Ganymede and Callisto, do orbit Jupiter close to the plane of Jupiter's equator. The largest orbital inclination is Callisto's at 0.51°. Jupiter's equator is itself inclined at just over 3° to its orbit (unlike the Earth's at nearly 23.5°). This results in the plane of Jupiter's equator and consequently the planes of the satellite orbits, being tilted towards or away from the Sun for most of the Jovian year.

Jupiter's orbital period is 11.86 years: its equator becomes edge on to the Sun twice in each orbital period, the two occurrences being separated by close to 6 years. Hence series of mutual events occur at 6 year intervals. Between these times the small inclination of Jupiter's equator to its orbit is sufficient to prevent mutual events occurring. A series of mutual events are now occurring. The previous set were in 2009.

Mutual eclipses will occur within a few months of the date when the equatorial plane of Jupiter passes through the Sun, while mutual occultations are centred on the date when the Earth passes through Jupiter's equatorial plane. The periods during which the two types of mutual event occur will overlap. The passage of Jupiter's equatorial plane through the Sun marks the time of a Jovian equinox. This year the equatorial plane was edge on to the Sun on February 4. This marked Jupiter's southern spring equinox, so since February 4 Jupiter's south pole has been slightly inclined towards the Sun. By chance, this year the date almost coincided with that for Jupiter being at opposition, February 6.

jupnewmoonseditAs a result of the coincidence of dates the periods over which mutual occultations and eclipses occur are almost identical, principally from September 2014 to August 2015. Few of the events in July and August will be observable as Jupiter is in conjunction with the Sun at the end of August. Before conjunction Jupiter will set quite early in the evening, that is it will be visible for only a short time, and even then be low in the western sky.

What gear to use?
Binoculars of power 10x50 or 12x50 should show the four Galilean moons most for the time, but they may get lost in Jupiter's glare when close to the planet. For a telescope the most useful thing is the highest magnification usable in the conditions to separate the Jupiter and its moons. There's no problem with the amount of light as everything is very bright - in many ways too bright.

occulteclipseMutual occultations
At a mutual occultation one satellite will pass in front of another as viewed from the Earth. So at the time of the occultation the Earth and the two satellites are in line. An occultation visible from the Earth will not, in general, be visible from other parts of the solar system.

The most obvious visual effect is that over a period of an hour or so the two satellites involved will be seen to get closer and closer until their images merge, followed by their separation some time later. Normally during the occultation satellites in their orbits round Jupiter will pass one another. Almost all occultations are partial events, that is the occulted satellite is only partly hidden to view, the line up is not perfect.

Occultations can last for just a few minutes up to half an hour or so. If the satellites are moving in opposite directions, the occultation will be quite short, but if it is a case of an inner satellite overtaking a slower moving satellite further from Jupiter, the occultation will be longer..

It would be necessary to be watching for a considerable time before and after the predicted time of the event to see the whole sequence. How long would depend on the instrument and magnification used. It also is a function of the duration of the occultation. A mutual occultation should be visible in a reasonable pair of binoculars, provided the two moons are not too close to Jupiter itself. A small telescope will give a better view.

While the obvious change during an occultation is the merging and separation of the two satellites, there is also a dip in the brightness of their combined image as the occulted satellite is partly hidden to view. The drop in brightness is usually not noticeable, but can be detected by making a video and measuring the brightness of the combined satellites frame by frame.

Mutual eclipses
At a mutual eclipse the eclipsed satellite passes through the shadow of another satellite as cast by the Sun. Thus at a mutual occultation the Sun and the two satellites are in line, but normally not in line with the Earth. As viewed from the Sun this would be an occultation. But from the Earth we are usually viewing from an angle due to our position in our orbit round the Sun. So at an eclipse we usually see the two satellites involved separated from one another.

The exception is close to the time of Jupiter's opposition when the Sun, Earth and Jupiter are also in line. If a mutual eclipse occurs at or close to the time of Jupiter's opposition, then an occultation will be seen from the Earth more or less hiding the eclipse. From another part of the Solar system, e.g. Mars, the eclipse would be visible, but there would be no occultation.

It should be noted that unlike occultations, mutual eclipses will be visible from other parts of the Solar system and will occur at the same time (allowing for differences in light time from Jupiter) as seen from any suitable vantage point in the solar system.

Mutual eclipses are normally partial or occasionally annular. Total mutual eclipses are rare. There are none during the current series, in 2009 only one total eclipse was predicted, when Ganymede eclipsed Europa . In many cases the change in brightness of the eclipsed satellite is small and not easily visible to the eye. A few are much deeper, with magnitude changes easily detected by eye. Some eclipses last for several hours depending on the relative motions of the two satellites.

Some forthcoming mutual events visible from New Zealand (2015).
See our Monthly Reports here
Code (i) = Io (ii) = Europa (iii) = Ganymede (iv) = Callisto
Moons & Event Date Mid Time Duration Mag Change
(iv) Occs (iii) 2 April 10:48 pm 23 min 0.7
(i) Ecls (ii) 3 April 11:09 pm 297 sec 0.7
(iii) Occs (iv) 8 April 08:08 pm 390 sec 0.1
(i) Ecls (ii) 11 April 12:23 am 305 sec 0.3
(ii) Occs (i) 23 April 08:18 pm 196 sec 0.1
*(ii) Occs (iv) 26 April 07:39 pm 228 sec 0.1
*(ii) Ecls (iii) 26 April 08:20 pm 506 sec 0.5
(iii) Occs (iv) 27 April 10:13 pm 506 sec 0.6
(ii) Occs (i) 30 April 10:32 pm 190 sec 0.1 *)
(iv) Occs (iii) 3 May 10:39 pm 406 sec 0.1 *)
*(i) Ecls (ii) 5 May 08:16 pm 319 sec 1.3
(i) Occs (iii) 17 May 08:11 pm 295 sec 0.3
(iii) Occs (i) 20 May 09:05 pm 382 sec 0.3
(iii) Ecls (i) 21 May 06:51 pm 93 min 0.2
(iii) Ecls (i) 21 May 08:07 pm 97 min 0.1
*Click on link above for screenshot   *) Low altitude

Visually mutual eclipses tend to be a little disappointing. The two satellites will normally be in view but of course not in contact, sometimes quite a distance apart. The only change will be that the eclipsed satellite, over the period of the eclipse, gradually dims slightly and then brightens again. Often the change will not be detectable by eye, unless another moon of similar brightness is close enough to make comparisons. In only a few eclipses is the drop in brightness is sufficient to be obvious.

As for occultations, an eclipse can be recorded by video and then suitable software used to measure the brightness of the eclipsed satellite frame by frame to obtain a light curve of the event. By inserting a time stamp on the video at the time it is made it is possible to measure the time at which the brightness is least which gives the mid time of the event. This and the maximum change in magnitude can give very precise information as to the relative positions of the two moons in their orbits.

OccultWatcher is a Windows program that helps astronomical occultation observers to stay tuned with the incoming events by tracking the asteroidal occultation predictions published and regularly updated in various sources called feeds.
The OccultWatcher programme can be downloaded here.

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