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Earth and Space - The movement of the Moon relative to the Earth

Grade 5Cambridge (IGCSE)

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Moon is a natural satellite of the Earth, held in place by the Earth's gravitational pull.

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The Moon orbits the Earth in a roughly circular path (slightly elliptical) at an average distance of approximately 3.84×1053.84 \times 10^5 km.

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The Moon takes approximately 27.327.3 days to complete one full orbit around the Earth relative to the stars, but about 29.529.5 days to complete a full cycle of phases (New Moon to New Moon).

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The Moon exhibits synchronous rotation, meaning its rotational period is equal to its orbital period (Trotation=TorbitT_{rotation} = T_{orbit}). This is why we always see the same side of the Moon from Earth.

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The Moon does not emit its own light; it reflects light from the Sun. The amount of the illuminated side visible from Earth changes as the Moon moves, creating the Lunar Phases.

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The gravitational pull of the Moon on the Earth's oceans is the primary cause of tides.

📐Formulae

Torbit≈27.3 daysT_{orbit} \approx 27.3 \text{ days}

Tlunar_cycle≈29.5 daysT_{lunar\_cycle} \approx 29.5 \text{ days}

daverage≈384,400 kmd_{average} \approx 384,400 \text{ km}

v=2πrTv = \frac{2\pi r}{T}

💡Examples

Problem 1:

If the Moon takes about 2828 days to orbit the Earth, approximately how many degrees does it travel in its orbit each day?

Solution:

The Moon travels approximately 12.86∘12.86^{\circ} per day.

Explanation:

A full circle is 360∘360^{\circ}. To find the daily movement, we divide the total degrees by the number of days: 360∘28 days≈12.86∘\frac{360^{\circ}}{28 \text{ days}} \approx 12.86^{\circ} per day.

Problem 2:

Explain why we always see the 'Man in the Moon' (the same face) rather than the 'dark side' of the Moon.

Solution:

This is due to synchronous rotation: Trotation=TorbitT_{rotation} = T_{orbit}.

Explanation:

Because the Moon rotates on its own axis at the exact same rate that it revolves around the Earth, the same hemisphere is always facing toward us.

Problem 3:

If a New Moon is observed on the 1st1^{st} of the month, on approximately what date would you expect to see the next Full Moon?

Solution:

Approximately the 15th15^{th} or 16th16^{th} of the month.

Explanation:

A full lunar cycle is about 29.529.5 days. The Full Moon occurs halfway through this cycle. 29.52≈14.75\frac{29.5}{2} \approx 14.75 days. Therefore, the Full Moon occurs about 1515 days after the New Moon.