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    I’m preparing a meeting at my local society on transits and occultations. I’m struggling to get nice tight definitions for the terms eclipse and occultation, and transit to an extent. My working definitions are:

    Eclipse – an obscuring of the light from one celestial body by the passage of another between it and the observer or between it and its source of illumination

    Transit – when a celestial body passes directly between a larger body and the observer

    Occultation – when one object is hidden by an apparently larger object that passes between it and the observer

    My questions with these definitions are:
    1. why do we not just call all occultations “eclipses”; does the definition of eclipse need some reference to foreground or distant object size?
    2. does the transit definition need to stipulate the background object is “apparently larger” or is the background object ALWAYS absolutely larger anyway?
    3. in the occultation definition, is the foreground object always apparently larger, are there examples when it is absolutely larger?

    Thanks for any clarification.

    David Basey


    FWIW, here are my thoughts/answers.

    1.  An eclipse is a shadow related phenomenon where one body is cut off from its primary light source by a third. So in a solar eclipse the Sun is hidden by the passage of the Moon’s shadow. The same principle applies to lunar eclipses and eclipses of Jupiter’s satellites by either Jupiter or each other. Occultations refer to bodies passing in front of each other where one is not illuminated by the other. Occultations of stars by the moon or asteroids for example.
    2.  I would suggest the background object has to be absolutely larger. Were it smaller then this would be an occultation.
    3. An example where the foreground object can be absolutely larger might be found in mutual occultations of Jupiter’s satellites. Consider an occultation of Io by Ganymede for example.

    There is one other question to consider, when does a transit become an eclipse? After all, in November there is a transit of Mercury not a solar eclipse by Mercury!


    John O’Neill

    Hello James & David.

    My thoughts

    1. An eclipse is when we see a body in the sky passing into the shadow of another body. Examples are lunar eclipses and eclipses of Jupiter’s satellites. This does not include Solar ‘Eclipses’ as the Earth is not in the sky.

    2. Yes, transits occur when a smaller body is silhouetted against a larger body in apparent size. Examples include the Transit of Mercury and Annular ‘Eclipses’ of the Sun.

    3.  Occultations occur when a larger body covers a body with smaller apparent diameter. Examples are Total ‘Eclipses’ of the Sun and Asteroids occulting stars.


    Robin Leadbeater

    Though it does not mean I am right !.  “Solar eclipses” appear to be a special case and should really be called transits or occultations

    For me an eclipse involves observing the shadow in the light from a luminous body cast by one body on another body eg as observed from earth between Jupiter and its moons. The observer may be on the body which casts the shadow eg a lunar eclipse. A “solar eclipse” (total or partial) as observed on earth is an occultation or transit and is only an eclipse when observed from a location not on the earth eg from the moon or the ISS when even a “total eclipse” appears partial


    EDIT: I suppose you could call observing the shadow of the moon racing across the landscape  towards you or even noting the darkness of the ground at your feet during a “Total Solar Eclipse” observing an eclipse but for me the sight of the moon in front of the sun is definitely an occultation


    Thanks all. Interesting discussion. This has helped.


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