I was finding the definition of the lunar eclipse (in which the earth blocks the sunlight that comes to the moon), and I have found that it could be defined differently.

Definitions of lunar eclipse that differ:

  1. A phenomenon in which the moon enters the earth's shadow when viewed from the earth and cannot be seen or is completely obscured.

  2. An eclipse in which the moon appears darkened as it passes into the earth's shadow.

I think the second one is right, because even if the moon is blocked by earth, sunlight that has a long wavelength could reach the moon. And that's the cause of red moon while total lunar eclipse.

If the first definition is also right, in what situation can the moon be invisible from earth during lunar eclipse?

  • 4
    $\begingroup$ You should provide the references for the definitions. Some sources are more accurate than other. I agree that a lunar eclipse will remain visible, even if faint. $\endgroup$
    – JohnHoltz
    May 3, 2022 at 17:17
  • $\begingroup$ Partially quoting moon.nasa.gov/news/172/… "In a total lunar eclipse, the entire Moon falls within the darkest part of Earth’s shadow, called the umbra. When the Moon is within the umbra, it will turn a reddish hue", so total obscuration is NOT required, and definition 2 is correct $\endgroup$ May 5, 2022 at 13:57
  • $\begingroup$ @JohnHoltz thanks for answering my question, and giving me advice on how to write questions here. I'll cite the source of information in a few days to make this question clear. $\endgroup$ May 8, 2022 at 14:14
  • $\begingroup$ Also thanks @barrycarter, for giving me additional information. $\endgroup$ May 8, 2022 at 14:14

2 Answers 2


A lunar eclipse occurs when the Earth's shadow falls onto the moon.

Shadows have two parts, a penumbra around the edge of the shadow in which the light source is partially obscured, and an umbra in which the light source is fully obscured. When the moon is in the umbra of the Earth, it is still lit by light (mostly red light) that has passed through the Earth's atmosphere.

The moon is never "completely obscured".

The definition is "the moon is in the Earth's shadow". It matters not to the definition whether the moon is completely dark, or partially lit. In reality it is always partially lit, because the Earth has an atmosphere.


The exact definition of X

can be a challenge, because any definition can be by itself "exact" even if wrong or disputed, and the premise of using the word "The" is that there can be only one which, really, there are probably more.

But we can certainly propose a clear and unambiguous definition that will work for most people and most cases.

Eclipses can be thought of as geometrical phenomena involving three points.

I suppose you can leave it at that and have a mathematical eclipse when they're co-linear but I think we need a definition that involves realistic sizes for the light source and the occluding/eclipsing body.

Let's call them A and B (e.g. Sun and Earth)

For the location where the eclipse "happens" i.e. point C, it could be a point or tiny object like a spacecraft, or it could be an extended object like the Moon.

No matter, the eclipse is a geometrical effect where B comes between C and A and at least partially blocks C's view of A. In other words, there are some lines drawn from the surface of A to point C or to areas on C's surface that intersect B.

That will stop most of the light, but as you point out B can refract or scatter, so that light has other paths available to it besides straight lines in vacuum.

Refraction and scattering do not interfere with it being a geometrical eclipse, it just affects the experience and effects.


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