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Keeping Time with the Skies - How Does the Moon's Appearance Change?

Grade 8CBSE

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

🔑Concepts

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The Moon is the Earth's only natural satellite and is a non-luminous body, meaning it does not produce its own light; it is visible because it reflects sunlight.

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Phases of the Moon: The change in the shape of the visible part of the Moon as seen from Earth is caused by the Moon's revolution around the Earth. The cycle starts from New Moon (Amavasya) to Full Moon (Purnima).

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The period between one Full Moon and the next Full Moon is approximately 29.529.5 days. This is often referred to as a lunar month.

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Waxing: This refers to the period during which the illuminated portion of the Moon visible from Earth is increasing (from New Moon to Full Moon).

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Waning: This refers to the period during which the illuminated portion of the Moon visible from Earth is decreasing (from Full Moon to New Moon).

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The Moon takes approximately 27.327.3 days to complete one revolution around the Earth and exactly the same time to complete one rotation on its axis. This is why only one side of the Moon is ever visible from Earth.

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The Moon's surface is dusty and barren, covered with many craters of different sizes and high mountains, some as tall as the highest mountains on Earth (8848 m8848 \text{ m}).

📐Formulae

Distance between Earth and Moon≈3.84×105 km\text{Distance between Earth and Moon} \approx 3.84 \times 10^5 \text{ km}

Duration of one Synodic Month (Full Moon to Full Moon)≈29.5 days\text{Duration of one Synodic Month (Full Moon to Full Moon)} \approx 29.5 \text{ days}

Speed of Light(c)≈3×108 m/s\text{Speed of Light} (c) \approx 3 \times 10^8 \text{ m/s}

Time taken by reflected moonlight to reach Earth=DistanceSpeed≈1.28 seconds\text{Time taken by reflected moonlight to reach Earth} = \frac{\text{Distance}}{\text{Speed}} \approx 1.28 \text{ seconds}

💡Examples

Problem 1:

Calculate the difference in days between a standard Solar Year of 365365 days and a Lunar Year consisting of 1212 lunar months (each 29.529.5 days).

Solution:

First, calculate the total days in a Lunar Year: 12×29.5=354 days12 \times 29.5 = 354 \text{ days} Now, subtract the Lunar Year from the Solar Year: 365−35411\begin{array}{r} 365 \\ - 354 \\ \hline 11 \end{array}

Explanation:

A Solar Year is 365365 days, while 1212 cycles of the Moon's phases equal 354354 days. The difference of 1111 days is the reason why lunar-based calendars (like the Hijri calendar or some Hindu calendar calculations) shift relative to the Gregorian calendar every year.

Problem 2:

If the Moon is approximately 3,84,000 km3,84,000 \text{ km} away from Earth, how long does it take for light to travel from the Moon to Earth? (Use c=3,00,000 km/sc = 3,00,000 \text{ km/s})

Solution:

t=dvt = \frac{d}{v} t=3,84,0003,00,000t = \frac{3,84,000}{3,00,000} t=1.28 st = 1.28 \text{ s}

Explanation:

By dividing the distance by the speed of light, we find that it takes roughly 1.281.28 seconds for light reflected from the Moon's surface to reach our eyes.