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A lunar eclipse occurs when the Earth is in between Sun and Moon. OK, I understand this (see diagram from Nasa).

A new moon is also when the Earth is in between Sun and Moon. So, why are they not the same? What am I missing?

I can sense something is different because, a Lunar Eclipse only occurs during a Full Moon and not a New Moon.

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    $\begingroup$ Search for "why isn't every new moon an eclipse". You'll find lots of explanations with diagrams that make it easier to understand. $\endgroup$ Jun 1 at 13:54
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    $\begingroup$ Lunar eclipses occur during full moons. Did you mean a solar eclipse? $\endgroup$
    – dan04
    Jun 1 at 16:44
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    $\begingroup$ A new moon occurs when the moon is between the earth and the sun. The lineup goes Sun-Moon-Earth - the side of the moon facing earth is not illuminated. A full moon occurs when the earth is between the moon and the sun - the lineup is Sun-Earth-Moon, the side facing earth is also illuminated by the sun. $\endgroup$ Jun 1 at 18:23

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A new moon is also when the Earth is in between Sun and Moon.

You got this one wrong. The description above applies to full moon. Correct is:

A new moon is when the Moon is (roughly) in between Sun and Earth. Then we look at the side of Moon that doesn't get sunlight, making Moon appear very dark (invisible to the human eye).

  • New moon is Sun - Moon - Earth.
  • Full moon is Sun - Earth - Moon.

At new moon, solar eclipses can happen if Moon gets located exactly between Sun and Earth, thus blocking the view from Earth to Sun. This only rarely happens because Moon's orbit around Earth is tilted a bit against Earth's orbit around Sun, meaning that in the "new moon" situation, most of the time Moon passes a bit "above" or "below" the Sun.

Similar for full moon and lunar eclipses. Full moon happens when, looking from Earth, Moon is opposite to Sun (Earth roughly between Sun and Moon). Then we look at Moon's side that gets full sunlight. Only if Earth is exactly between Sun and Moon, then Moon enters into the shadow cast by Earth, thus becoming very dark - what we call a lunar eclipse.

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    $\begingroup$ I had to roll back an edit of my post, as the editing user didn't recognize that OP mixed up the ordering of Sun, Moon and Earth in the "new moon" situation. $\endgroup$ Jun 2 at 8:11
  • $\begingroup$ Note that lunar eclipses are more common than solar eclipses, because the Earth casts a much bigger shadow than the Moon does. $\endgroup$
    – Mark
    Jun 3 at 3:27
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The orbit of the moon is tilted relative to the ecliptic by about 5 degrees.

This means that when the moon passes the sun, it might be up to five degrees above or below it in the sky. Since the sun is only ½ a degree across, that is a big miss. Usually the moon misses the sun.

But as the orbit is tilted, there are two points (called nodes) where the orbit of the moon crosses over the path of the sun. So twice a month, the moon is in the same plane as the sun. If this happens at the same time as New Moon there is an eclipse.

A new moon is when the moon passes between the Sun and the Earth. A solar eclipse is when the moon is in a straight line between the Sun and the Earth. Solar eclipses can only happen at New Moon, but most New Moons are not eclipses.

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The Moon is in the "New" phase when it is between the Earth and the Sun. The side of the Moon illuminated by the Sun is the side facing away from the Earth, and the Moon is (barely) visible on the "dayside" of the Earth.

The Moon is in the "Full" phase when it is on the far side of the Earth from the Sun. The side of the Moon illuminated by the Sun is the side facing the Earth, and the Moon is visible on the "nightside" of the Earth.

The Moon during a Lunar Eclipse is in the "Full" phase, but also requires that the Moon pass through the shadow of the Earth. While within the shadow, the illumination of the Moon is significantly reduced.

The question's assumption about a lunar eclipse happening at the same point of the Moon's orbit around the Earth as a "New" Moon, is in error.

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