Fig. 3.1 shows the positions of the Sun, Earth and Moon during a lunar eclipse. The Earth is between the Sun and the Moon, and the Earth's shadow falls on t...
Question 1 Report
Fig. 3.1 shows the positions of the Sun, Earth and Moon during a lunar eclipse. The Earth is between the Sun and the Moon, and the Earth's shadow falls on the Moon. The Moon takes approximately 27.3 days to complete one orbit of the Earth. The average distance from the Earth to the Moon is 3.84 × 10⁵ km.
(a) State what the Moon is an example of in terms of bodies that orbit planets. [1]
(b) Explain why we can sometimes see the Moon from Earth, even though the Moon does not produce its own light. [1]
(c) State why a lunar eclipse does not occur every month. [1]
(d) Describe the appearance of the Moon to an observer on Earth during a total lunar eclipse. [1]
(e) Calculate the approximate orbital speed of the Moon around the Earth. Assume the orbit is circular. [2]
Answer Details
(a) The Moon is an example of a natural satellite. [1]
A satellite is any object that orbits a planet. The Moon orbits Earth, but unlike artificial satellites (which are built and launched by humans), the Moon formed naturally billions of years ago. This makes it a natural satellite.
(b) The Moon reflects sunlight. [1]
The Moon has no internal source of light. We see it because light from the Sun strikes the Moon's surface and bounces (reflects) towards Earth. The changing angle between the Sun, Moon and Earth during the Moon's orbit is why we see different phases (crescent, half, full).
(c) The Moon's orbit is tilted (inclined) relative to the Earth's orbit around the Sun, so the three bodies do not align exactly every month. [1]
The Moon's orbital plane is tilted by about 5 degrees to the plane of the Earth's orbit (the ecliptic). Most months the Moon passes slightly above or below the Earth's shadow, so no eclipse occurs. A lunar eclipse only happens when the Moon crosses the ecliptic at the same time as it is on the opposite side of the Earth from the Sun.
(d) During a total lunar eclipse, the Moon appears dark or reddish (copper-coloured). [1]
The Earth blocks direct sunlight from reaching the Moon. However, some sunlight is refracted (bent) through Earth's atmosphere onto the Moon. Blue light is scattered away (the same reason the sky appears blue), so mainly red light reaches the Moon, giving it a reddish tint.
(e) The orbit is assumed circular, so the distance travelled in one orbit equals the circumference:
\( C = 2\pi r = 2 \times 3.14 \times 3.84 \times 10^5 \text{ km} = 2.41 \times 10^6 \text{ km} \)
Convert the orbital period to hours:
\( t = 27.3 \times 24 = 655.2 \text{ h} \) [1]
\( v = \frac{d}{t} = \frac{2.41 \times 10^6}{655.2} \approx 3680 \text{ km/h} \approx 1.02 \text{ km/s} \) [1]
The Moon travels roughly 3680 km every hour along its orbit. This is fast compared to everyday speeds, but relatively slow for an astronomical body because the Moon orbits at a large distance where Earth's gravitational pull is weaker.
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