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Keeping Time with the Skies - Making sense of our observations

Grade 8CBSE

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

🔑Concepts

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Celestial objects like the stars, planets, moon, and many other objects in the sky are known as celestial bodies.

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The Moon is the brightest object in the night sky. It does not have its own light; it reflects the light of the Sun. The various shapes of the bright part of the moon as seen during a month are called phases of the moon.

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The time period between one full moon and the next is slightly longer than 2929 days (specifically about 29.529.5 days).

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Stars emit their own light. The Sun is also a star. Stars appear to move from East to West because the Earth rotates on its axis from West to East.

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The Pole Star (PolarisPolaris) appears to be stationary because it is situated in the direction of the Earth's axis of rotation.

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A Light Year is the distance travelled by light in one year. It is a unit of astronomical distance. The speed of light is approximately 3×108 m/s3 \times 10^8 \text{ m/s}.

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Constellations are groups of stars that form a recognizable pattern. Examples include Ursa Major (SaptarishiSaptarishi), Orion (theHunterthe Hunter), Cassiopeia, and Leo Major.

📐Formulae

Distance=Speed×Time\text{Distance} = \text{Speed} \times \text{Time}

1 light year≈9.46×1012 km1 \text{ light year} \approx 9.46 \times 10^{12} \text{ km}

Speed of light (c)≈3×105 km/s\text{Speed of light (c)} \approx 3 \times 10^5 \text{ km/s}

Time period of lunar cycle≈29.5 days\text{Time period of lunar cycle} \approx 29.5 \text{ days}

💡Examples

Problem 1:

Alpha Centauri is the closest star to the solar system (after the Sun). Its distance is about 4.34.3 light years. Calculate this distance in kilometers.

Solution:

Distance=4.3×(9.46×1012 km)\text{Distance} = 4.3 \times (9.46 \times 10^{12} \text{ km}) Distance≈40.678×1012 km=4.0678×1013 km\text{Distance} \approx 40.678 \times 10^{12} \text{ km} = 4.0678 \times 10^{13} \text{ km}

Explanation:

Since 11 light year is the distance light travels in a year, we multiply the given light years by the value of 11 light year in kilometers (9.46×1012 km9.46 \times 10^{12} \text{ km}).

Problem 2:

If Star A is 5050 light years away and Star B is 1818 light years away from Earth, calculate the difference in their distances from Earth in light years using vertical subtraction.

Solution:

50−1832\begin{array}{r} 50 \\ -18 \\ \hline 32 \end{array}

Explanation:

To find the difference in distance between two celestial objects relative to Earth, we subtract the smaller distance from the larger distance. The result is 3232 light years.