الأسبوع 14 · الدرس الثاني «خسوف القمر»: أطوار القمر والظل وشبه الظل والخسوف الكلي والجزئي ولماذا لا يحدث كل شهر · 3 دروس
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Lunar eclipse: why not every month?
Science Prep 1 Term 1, Unit 4: pp. 88–93
GoalExplain a lunar eclipse, say the difference between total and partial, and say why there is not one every month.
New words
lunar eclipse · when the Moon moves into the Earth’s shadowtotal eclipse · the whole Moon enters the darkest part of the shadowpartial eclipse · only part of the Moon enters the shadow
Explanation
A lunar eclipse happens when the Earth lies between the Sun and a full Moon, so the Earth blocks some or all of the sunlight that would reach the Moon. If the whole Moon enters the darkest part of the shadow it is a total eclipse, and the Moon often looks dim red or copper. If only part enters, it is a partial eclipse. There is not an eclipse at every full moon because the Moon’s orbit is tilted by about 5° to the Earth’s orbit around the Sun, so most full moons pass above or below the shadow. There are several lunar eclipses in a year, counting partial ones, but few are total. A lunar eclipse can be seen from the whole night side of the Earth. When the Moon lies between the Sun and the Earth we have a solar eclipse; name it, but never look at the Sun.
Worked examples
Eclipse
Order in a line
lunar
Sun, Earth, Moon
solar
Sun, Moon, Earth
Line-up for a lunar eclipse. Sun, Earth, Moon, in a straight line, in that order. The Moon is full. Line-up for a solar eclipse: Sun, Moon, Earth.
The Moon’s orbit is tilted by about 5°. At most full moons the Moon passes a little above or below the Earth’s shadow and receives full sunlight, so no eclipse is seen.
Total or partial? If a dark bite covers only the edge of the Moon, it is partial. If the whole disc turns dim red or dark, it is total.
Class activity
Make an eclipse model, with an adult. In a dim room, use a small lamp for the Sun, a large ball for the Earth and a small ball for the Moon. Move the small ball into the big ball’s shadow, then slightly above it. When does the small ball go dark? Draw both positions and write a sentence under each.
Practice
1
When does a lunar eclipse happen?
When the Earth is between the Sun and a full Moon.
2
What is the difference between a total and a partial lunar eclipse?
In a total one the whole Moon enters the dark shadow; in a partial one only part does.
3
Why is there not a lunar eclipse every month?
The Moon’s orbit is tilted by about 5°, so most full moons miss the Earth’s shadow.
