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Keeping Time with the Skies - Phases of the Moon

Grade 8CBSE

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

🔑Concepts

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The Moon is a non-luminous celestial body. It becomes visible to us because it reflects the sunlight falling on it. We only see the part of the Moon that reflects sunlight towards the Earth.

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The sequence of the changing shapes of the bright part of the Moon as seen from the Earth are known as the Phases of the Moon.

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New Moon (Amavasya): This occurs when the Moon is positioned between the Earth and the Sun. The illuminated side faces away from the Earth, making the Moon invisible.

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Full Moon (Purnima): This occurs when the Earth is positioned between the Sun and the Moon. The entire illuminated side of the Moon is visible from Earth.

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Waxing Phases: After the New Moon, the visible illuminated portion of the Moon increases. The sequence is: New Moon →\rightarrow Waxing Crescent →\rightarrow First Quarter →\rightarrow Waxing Gibbous →\rightarrow Full Moon.

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Waning Phases: After the Full Moon, the visible illuminated portion decreases. The sequence is: Full Moon →\rightarrow Waning Gibbous →\rightarrow Last Quarter →\rightarrow Waning Crescent →\rightarrow New Moon.

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The Moon takes the same amount of time to complete one rotation on its axis as it takes to complete one revolution around the Earth (27.327.3 days). Because of this, only one side of the Moon is ever visible from Earth.

📐Formulae

Duration of one Lunar Month (Synodic)≈29.5 days\text{Duration of one Lunar Month (Synodic)} \approx 29.5 \text{ days}

Time for one Revolution (Sidereal)≈27.3 days\text{Time for one Revolution (Sidereal)} \approx 27.3 \text{ days}

Interval between New Moon and Full Moon≈29.52≈14.75 days\text{Interval between New Moon and Full Moon} \approx \frac{29.5}{2} \approx 14.75 \text{ days}

💡Examples

Problem 1:

If a Full Moon is observed on the 1st1^{st} of October, approximately on which date will the next New Moon occur?

Solution:

The next New Moon will occur approximately on October 15th15^{th} or 16th16^{th}.

Explanation:

The time interval between a Full Moon and a New Moon is roughly half of a lunar month. Since a full cycle is ≈29.5\approx 29.5 days, the half cycle is: 29.52=14.75 days\frac{29.5}{2} = 14.75 \text{ days}. Adding 14.7514.75 days to October 11 gives approximately October 15.7515.75.

Problem 2:

Calculate the approximate number of Full Moons that can occur in a standard calendar year of 365365 days.

Solution:

Approximately 1212 Full Moons.

Explanation:

To find the number of lunar cycles, we divide the total days in a year by the length of one synodic month: Number of Moons=36529.5≈12.37\text{Number of Moons} = \frac{365}{29.5} \approx 12.37. This means there are usually 1212 Full Moons, and occasionally a 13th13^{th} one (often called a Blue Moon).

Problem 3:

A student observes the Moon and notices that the visible bright portion is increasing every night. Is the Moon in the 'Waxing' or 'Waning' phase?

Solution:

The Moon is in the Waxing phase.

Explanation:

The term Waxing refers to the period where the illuminated part of the Moon visible from Earth is growing in size, moving from New Moon toward Full Moon.