The Earth in Relation to the Universe
Senior 1 · Geography
The Earth in Relation to the Universe
Lesson 2 of 4
22 min
In Kinyarwanda the same word, ukwezi, means both "the Moon" and "a month". That is no accident. For thousands of years people have counted time by the Moon, because its shape changes in a regular cycle that takes about a month. In this lesson you will find out why the Moon changes shape, and why it sometimes disappears in the middle of the day or turns red at night.
Definition
The Moon: the Earth's only natural satellite. It is about 3,474 km across (a little over a quarter of the Earth's width) and orbits the Earth at an average distance of about 384,400 km. It makes no light of its own: it shines by reflecting sunlight.
The Sun always lights up half of the Moon, just as it lights up half of the Earth. As the Moon travels round the Earth, we see different amounts of that lit half. When the Moon is between the Earth and the Sun, its lit side faces away from us and we see nothing: a new Moon. A week later we see half of the lit side (first quarter); after two weeks the whole lit side faces us (full Moon); then the lit part we see shrinks again.
Figure 1. Why the Moon has phases: the Moon is always half lit, but we see different amounts of the lit half as it goes round the Earth.
While the lit part is growing the Moon is waxing; while it is shrinking it is waning. A crescent is less than half lit, and a gibbous Moon is more than half lit. From Rwanda, close to the Equator, a young crescent Moon after sunset often looks like a boat or a smile, lit from below by the Sun that has just set.
Definition
Phases of the Moon: the changing shapes of the lit part of the Moon as seen from the Earth, repeating every 29.5 days: new Moon, waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, last quarter and waning crescent.
Worked example 1: predicting the next full Moon
A full Moon is seen on the night of 7 September. When will the next full Moon be, and when is the new Moon in between?
Step 1 The cycle of phases takes 29.5 days, so the next full Moon comes about 29.5 days later: 7 September + 29.5 days. September has 30 days, so 23 days take you to 30 September, and 6.5 more days bring you to 6 or 7 October.
Step 2 The new Moon comes halfway through the cycle: days after the full Moon, around 22 September.
Warning
Phases are not caused by the Earth's shadow. A common mistake is to think the dark part of a crescent Moon is the Earth's shadow. It is simply the Moon's own night side, turned away from the Sun. The Earth's shadow falls on the Moon only during a lunar eclipse.
An eclipse happens when the Sun, the Earth and the Moon line up exactly, so that one body's shadow falls on another.
Figure 2. Solar and lunar eclipses. The umbra is the full shadow; the penumbra is the partial shadow around it.
Definition
Solar eclipse: a darkening of the Sun seen from the Earth, when the Moon passes directly between the Sun and the Earth and its shadow falls on the Earth. It can only happen at new Moon. It is total where the umbra falls and partial in the wider penumbra.
Definition
Lunar eclipse: a darkening of the Moon, when the Earth passes directly between the Sun and the Moon and the Earth's shadow falls on the Moon. It can only happen at full Moon, and it can be seen from the whole night side of the Earth.
Worked example 2: why is there not an eclipse every month?
New Moon and full Moon happen every month, so why are eclipses rare? The Moon's orbit is tilted by about to the Earth's path round the Sun. Most months the new Moon passes a little above or below the line from the Sun to the Earth, and its shadow misses us; the full Moon passes above or below the Earth's shadow. Only when a new or full Moon happens just where the two paths cross do the three bodies line up, which occurs only a few times a year.
Important
Case study: the long red Moon over Rwanda, 27 July 2018
On the evening of 27 July 2018 people across Rwanda watched the longest total lunar eclipse of the twenty-first century. The full Moon rose in the east, then slid into the Earth's shadow, and for 1 hour 43 minutes it glowed a dull copper-red. The middle of the eclipse came at about 22:21 Rwanda time. The same eclipse was seen at the same moment across Africa, the Middle East, Asia and Australia, because a lunar eclipse is visible from everywhere on the night side of the Earth.
Why red and not black? The Earth's atmosphere bends a little sunlight into the shadow. On the way it scatters away the blue light (which is why our sky is blue) and lets the red through, the same red we see at sunset.
Moon Phases: Crash Course Astronomy #4, by CrashCourse (10 min). Watch the demonstration with a lamp and a ball: it shows why we always see the same face of the Moon and why its phases change.
A solar eclipse can only be seen from a narrow strip of the Earth, but a lunar eclipse can be seen by half the world at once. Use Figure 2 to explain why.
Tip
Solar eclipse = Sun hidden = Moon in the middle. Lunar eclipse = Moon hidden = Earth in the middle. The body in the middle casts the shadow. And "DOC": a waxing Moon grows like the letter D, a waning Moon shrinks like the letter C (in the northern view of Figure 1).
Practice
Remember
Sources and credits
The lesson text, phase and eclipse diagrams, worked examples and practice are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
