Week 14 · Lesson 2 Lunar Eclipse: the Moon’s phases, shadow and penumbra, total and partial lunar eclipses, why there is not one every month, a report on solar eclipses · 3 lessons
1
The Moon’s phases
Science Prep 1 Term 1, Unit 4: pp. 88–93
GoalExplain why the Moon seems to change shape and put its eight phases in order.
New words
moon phase · the shape of the lit part of the Moon that we seenew moon · the phase when we cannot see the lit sidefull moon · the phase when the whole face we see is lit
Explanation
The Moon is a dark body. It does not make light; it shines by reflecting sunlight. It goes round the Earth and always shows us the same face, because it turns once on its axis during each trip round the Earth. Half of the Moon is always lit by the Sun, but from the Earth we see different amounts of that lit half, so the shape seems to change. The phases in order are: new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter and waning crescent. From one new moon to the next takes about 29½ days, one lunar month. The Moon’s own trip round the Earth, and its turning, take a little less, about 27.3 days. Seen against the stars, the Moon moves from west to east. Never look at the Sun; we study the Moon at night.
Worked examples
full moon→waning gibbous→last quarter→waning crescent→new moon
Put the phases in order. After a full moon the lit part shrinks: waning gibbous, last quarter, waning crescent, then new moon again.
A lunar month is about 29½ days. The Moon shows eight main phases in it. So the main phases come about 29.5 ÷ 8, a little under 4 days, apart.
A friend says the Moon has a bright side and a dark side that never changes. Not so: every part of the Moon gets sunlight in turn. We see the same face, but the lit part of it changes.
Class activity
Moon diary, with an adult. For a month, look at the Moon on clear evenings and draw its shape each time. Put your drawings in order and name the phases. Count how many days passed between two full moons.
Practice
1
Why does the Moon shine?
It reflects sunlight.
2
Which phase comes just after the first quarter?
The waxing gibbous.
3
Why do we always see the same face of the Moon?
It turns once on its axis for each trip round the Earth.
