Is there a term or a name for the darkness caused when a planet eclipses its moon?

For example; if people lived on Titan, a small portion of the near sides lunar day would be eclipsed or partially eclipsed by Saturn. What would that night be called?

  • $\begingroup$ This is quite an interesting question; Welcome to Stack Exchange! $\endgroup$
    – uhoh
    Oct 7, 2022 at 21:29
  • $\begingroup$ Even without an eclipse, you can't really see the Sun on Titan. From en.wikipedia.org/wiki/Titan_(moon)#Atmosphere Opaque haze layers block most visible light from the Sun and other sources and obscure Titan's surface features. $\endgroup$
    – PM 2Ring
    Oct 8, 2022 at 3:12
  • $\begingroup$ I've voted to close. Language follows need. When people are living on Titan, they might invent a common name for this. But as nobody lives on a moon nobody has a word. And answer would therefore be speculation. $\endgroup$
    – James K
    Oct 9, 2022 at 7:03
  • $\begingroup$ Note: because Titan's orbit is tilted, most of the time Saturn doesn't eclipse Sun for Titan. The eclipse seasons happen about every 15 years. See astronomy.stackexchange.com/questions/48585/… $\endgroup$
    – Heopps
    Nov 9, 2022 at 12:14

1 Answer 1


tl:dr: we would really call that darkness "totality" or perhaps less likely say that we are "in the umbra".

Traditionally "night" refers to darkness on a given point on its surface caused by a body's rotation around it's own axis, bringing the observer's location to the "back side" facing away from the Sun (in our own solar system) or star in another solar system.

That gets a little tricky when it's a multiple star system of course.

The phenomenon where one astronomical body blocks sunlight from reaching another body (as opposed to the body blocking it's own light) is generally called either an eclipse or occultation, with "eclipse" used for bodies that orbit around each other (one directly above the other in an orbital hierarchy) and "occultation" for all the other cases ("sideways" relationship in an orbital hierarchy), like one of Jupiter's Galilean moons occulting the other. For more on that see:

So I think we can safely call this event an eclipse.

Let's see how long it is. Titan orbits a circumference of 7.67 million kilometers every 16 days, giving it a speed of about 480,000 km per day or 20,000 km per hour. Saturn's equatorial diameter is about 121,000 kilometers, so the maximum duration of eclipse could be about 6 hours, roughly similar to an "Earth night" and perhaps roughly ten times longer than the totality of a lunar eclipse in our Earth-Moon system.

So as astronomers (amateur or pro) we would really call that darkness "totality" or perhaps less likely say that we are "in the umbra".

  • $\begingroup$ This is what we call the darkness on Earth when the moon eclipses the sun. Have you any evidence that we would have used the same word for eclipses of the sun from a moon, and in particular any actual use of "totality" from any object that has been eclipsed by Jupiter. $\endgroup$
    – James K
    Oct 7, 2022 at 22:13
  • $\begingroup$ @JamesK the lack of any other word, combined with astronomers (amateur or pro) preponderance to talk about astronomy leads me to this inexorable conclusion. I hold this truth to be self-evident, but there's always a chance that I'm wrong. Therefore in this particular case the proof or disproof is left as an exercise for the reader. $\endgroup$
    – uhoh
    Oct 7, 2022 at 23:14

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