Earth and Space · Grade 6
If the Moon goes round us every month, why is there not an eclipse every month?
Because the Moon's orbit is tilted by about five degrees compared with the Earth's path around the Sun. In most months the Moon passes a little above the line or a little below it, and the shadow misses. Only when it crosses the plane at exactly new or full Moon do the three really line up.
Learning objectives
- Say which body casts the shadow in a solar eclipse and which casts it in a lunar eclipse
- State the phase the Moon must be in for each of the two kinds of eclipse
- Explain why an eclipse does not happen every month, using the tilt of the Moon's orbit
- Explain why a solar eclipse is visible only from a narrow strip of the Earth while a lunar eclipse is visible from a whole hemisphere
Two eclipses, two shadows
In a solar eclipse the Moon stands between the Earth and the Sun and throws its shadow onto us. It hides the Sun from us, so it must happen at new Moon.
In a lunar eclipse the Earth is in the middle and throws its shadow onto the Moon. The Moon enters our shadow and turns red, so it must happen at full Moon.
Notice the symmetry: in either eclipse it is the body in the middle that casts the shadow. Remembering who is in the middle settles 'who shadows whom' every time.
Why they are rare
If the Moon's orbit lay in exactly the same plane as the Earth's, there would be a solar eclipse at every new Moon and a lunar eclipse at every full Moon — twice a month.
The orbit is tilted by about five degrees. Five degrees sounds small, but across nearly 400,000 kilometres it is enough for the shadow to pass entirely above or below the ball.
Only about twice a year does the Moon cross the plane at the right moment. So eclipses come in seasons rather than monthly.
A narrow strip against half a world
The Moon is far smaller than the Earth, so the shadow it throws on us is narrow — a band only a few hundred kilometres wide, sweeping across the surface.
Anyone inside that band sees a total solar eclipse. Anyone a few hundred kilometres away sees a partial one, or nothing.
A lunar eclipse is the reverse: the shadow the Earth casts is larger than the whole Moon. So everyone who can see the Moon at that moment — an entire hemisphere — sees the same eclipse at the same time.
Worked examples
There is a lunar eclipse. What phase is the Moon in?
- In a lunar eclipse the Earth is in the middle
- So the Moon is on the opposite side from the Sun
- That is exactly the arrangement of a full Moon
Answer: Full Moon
Who casts the shadow in a solar eclipse?
- In a solar eclipse the Moon is in the middle
- The body in the middle casts the shadow
Answer: The Moon, onto the Earth
A friend in another country saw the eclipse and you did not. Which kind was it?
- A lunar eclipse is visible from a whole hemisphere at once
- A solar eclipse is visible only from a narrow strip
Answer: Solar — the strip did not pass over you
Common mistakes
- Mixing up who casts the shadow on whom
- The body in the middle always casts it. In a solar eclipse the Moon is in the middle and shadows us; in a lunar eclipse we are in the middle and shadow it.
- Expecting an eclipse every month
- The Moon's orbit is tilted about five degrees, so in most months the shadow misses above or below. An eclipse needs the right phase and a crossing of the plane.
- Thinking a solar eclipse is visible everywhere
- The Moon's shadow on us is very narrow. It is the lunar eclipse that a whole hemisphere sees, because our shadow is larger than the Moon.
What to remember
- The body in the middle casts the shadow.
- Solar at new Moon, lunar at full Moon.
- The five-degree tilt is why they are rare.
- Solar: a narrow strip. Lunar: half a world.