![\"\"](\"https:\/\/spaceengine.org\/wp-content\/uploads\/2023\/06\/scr01447-scaled.jpg\")
<\/a><\/p>\nTo start, you add objects of interest to the table in the left-bottom side of the tool. Addition is straightforward: you select an object with any method available in SE (left click, search by name, click in Solar system browser, etc), and press the \u201cAdd\u201d button on the tool.<\/p>\n
The tool displays some help text informing the user to add at least 3 objects in the same planetary system. The system is set automatically to that of the currently selected object when you open the tool.<\/p>\n
The first object in the list is the \u201cviewpoint\u201d, i.e. the planet\/moon\/sun from which you want to observe the event. In this example it is Jupiter's moon, Callisto, with the other objects being Ganymede, Europa, and Jupiter itself.<\/p>\n
Note that the \u201cOrdered alignment\u201d mode takes into account the order of the objects: it searches for only those events in which Ganymede is closer to Callisto (viewpoint) than Europa, and Europa is closer to Callisto than Jupiter. In other words, Ganymede and Europa are in front of Jupiter as observed from Callisto.<\/p>\n
If you swap Europa and Jupiter in the list (using the arrow buttons next to the \u201cAdd\u201d button), the tool will search for other events. In this example, Ganymede is in front of Jupiter, and Europa is behind Jupiter:<\/p>\n
Of course, just because you can arrange objects a certain way in the list doesn\u2019t mean you can find such an ordered alignment. For example, you will never find a \u201cCallisto - Europa - Ganymede - Jupiter\u201d ordered alignment because such an orbital configuration is physically impossible; in the case of the Jovian moons, you can\u2019t even find the alignment \u201cGanymede - Europa - Io - Jupiter.\u201d These three moons never align on one side of the gas giant due to the Laplace resonance<\/a>, but configurations like \u201cGanymede - Europa - Jupiter - Io\u201d are possible.<\/p>\n Some procedural systems also have similar resonances which prevent finding alignments like \u201call big moons at one side of a planet\u201d, but you can increase the \u201cprecision\u201d parameter to several degrees to find a decent looking picture:<\/p>\n Another example of an ordered alignment in a procedural system:<\/p>\n This mode is very similar to ordered alignment, but it ignores the order of objects in space. In the case of a planet and its moons, it will find all alignments of those objects, even if some moons are hiding behind the planet. It is more suitable for finding alignments of planets in the sky, when you don\u2019t know or care about their real arrangement in space. Such alignments are sometimes called \u201cconjunctions\u201d, though this is not exactly what the tool finds. The exact conjunction of 3 or more planets is impossible, but the tool looks for \u201capproximate conjunctions.\u201d In the following example I found all the upcoming conjunctions of Venus and Mercury, ignoring distance to each (in this screenshot Venus is farther than Mercury):<\/p>\n Note that the tool has limitations: it may find multiple dates of the same event. This is because the tool is not \u201csmart\u201d (yet); it does simple time iterations with a constant step, so if the step is too small or if the angular precision is too large, it will detect the same event several times. And it does not detect the exact \u201ccenter\u201d of the event, you must do it yourself, if you want to. The opposite can also occur: the tool may miss some events if the time step is too large or the angular precision is too small. Future updates may improve this, but for now it is important to keep these limitations in mind.<\/p>\n Below is a close conjunction of Jupiter and Saturn as viewed from Earth, from December 2020. The tool also detected future conjunctions in 2080 and 2147. Note that I edited the Precision manually to 8 arc minutes (the default value for alignment mode is 1\u00b0).<\/p>\n A triple conjunction (better to say \u201ctriple alignment\u201d) of Venus, Mars, and Jupiter:<\/p>\n The next type of event is transit. Probably the most famous transit events are of Venus transiting over the Sun. In this mode, the first object in the list must be the observation point (Earth in this example), the next one is the object which serves as a background for the transit (the Sun), and the next is the object which is doing the transiting (Venus). Note that the angular precision is automatically set to the angular radius of the Sun (more exactly, to the sum of the angular radii of the Sun and Venus at the current date<\/i>), and the time step is also calculated automatically. I manually set the start date to the year 1800.<\/p>\n Some transits are detected several times, due to the limitations described above. Note that Venus transits (and all other events in the Solar system) are accurate only in the validity range of the analytical orbit models used by all involved objects. By default, SE uses the DE436 model for the major planets and the Sun, which is valid only between the years 1550 and 2649.<\/p>\n Another limitation is with the automatic calculation of time step and angular precision, though it is more or less valid in the case of almost-circular orbits. If you look for transits of a procedural planet over its star\u2019s disk and the planet\u2019s orbit is highly elliptical, or if the planet orbits a close binary star, these calculations of angular precision on the current date may not satisfy all possible orbital configurations, so you may need to adjust the values manually. The curved arrow button next to the input field re-calculates the value at the current date<\/i> using the actual orbital position of the objects.<\/p>\n You may also add more than one transiting object. For example: a triple transit of Io, Ganymede and Callisto over Jupiter (as observed from Earth). Note that SE does not take into account light travel delay, so the event time in real life would be some 50 minutes later (at a distance of 6 AU).<\/p>\n Interestingly, you can find extreme cases, such as the transit of Venus over Jupiter. One such event happened in 1818. Note the difference in surface brightness between Venus and Jupiter (this screenshot was taken in auto exposure mode):<\/p>\n An occultation is technically the same as a transit, but where the transiting object is larger than the background object. For example, when Jupiter transits its moon Io (covers it from the observer\u2019s perspective), this is called an occultation of Io by Jupiter. Like in other modes, the event finder allows you to find multiple occultations by the same occulting object. Shown below is an example of a simultaneous occultation of Titan and Rhea by Saturn (position of Rhea is highlighted by the selection pointer):<\/p>\n Such events are not that interesting, but you may search for more extreme cases. For example, the occultation of Neptune by Jupiter! One such event happened on January 3-4, 1613. At that time, Neptune was not discovered yet. Galileo Galilei observed Jupiter and its moons on December 28, 1612, and labeled Neptune as a star! Despite Galileo having observed Neptune, he is still not considered its discoverer because he never identified it as being a planet.<\/p>\n Eclipses are transits or occultations as observed from the sun. The first object in the list must be the sun (or another light source, as the tool allows one to find eclipses of one moon by another as viewed from the planet). In the case of a single-star system, the tool adds the sun automatically; but in multi-star systems you must add the star yourself. The next object is the one which is being eclipsed (ie. receiving the shadow), and the last one is the object creating the eclipse (that is, casting the shadow). The event finder calculates the angular precision and time step for this configuration, but again, this works well only for orbits which are not very elongated.<\/p>\n For multiple eclipses, Jupiter is a great example. Here is a famous triple eclipse (and double transit) on March 28, 2004:<\/p>\n You can also find eclipses of moons by planets (in the example below, Charon is being eclipsed by Pluto):<\/p>\n Moons can similarly be eclipsed by other moons (In this example, Rhea is being eclipsed by Titan):<\/p>\n You can also observe the eclipse of a distant planet by a hot jupiter in a red dwarf system (technically it is a transit, but it has a similar effect):<\/p>\n Or you can see mutual eclipses of stars in a binary system:<\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\nAlignment<\/h4>\n
<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\nTransit<\/h4>\n
<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\nOccultation<\/h4>\n
<\/a><\/p>\n<\/a><\/p>\nEclipse<\/h4>\n
<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n<\/a><\/p>\n