Measuring Distances in Light Year

Q: The movement of Earth around its axis is called

Movement of earth on its own axis is called Rotation while movement around the Sun is called Revolution.

Q: Why is the distance between stars expressed in light years? What do you understand by the statement that a star is eight light years away from the Earth?

Distance between stars are very large in magnitude. Hence a new unit is required so that the value of distance is not too big and is sensible. A light year is defined as the distance travelled by light in one year. \$1\ ly = 9.461 \times 10^{12}\ km\$\$8\ ly = 7.57 \times 10^{13}\ km\$A star is eight light years away from earth means that light from the star takes 8 years to reach the earth's surface. As shown above, its approximate distance from the earth is \$7.6 \times 10^{13}\ km\$.

Q: Do all the stars in the sky move? Explain.

No, all the stars do not move in the sky. The Pole star is located above the axis of rotation of the Earth in the north direction. Thus, it appears stationary at a point in the sky, it does not move in the sky. All other stars in the sky appear to move from East to West in the sky.

Q: Distance from Earth to Proxima Centauri, the next nearest star is:

Proxima Centauri, a part of a triple star system named Alpha Centauri, is the closest star to our solar system. It is about \$4.24\$ light years away from the Earth.

Q: Which of the following is NOT a planet of our Solar System?

Planets of solar system are : Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Sirius is not a planet of the Solar system. It is a star. It is the brightest star in the night sky and located close to Orion.

Q: Which of the following is NOT a planet of the sun ?

Sirius is a star,it is not a planet of the sun.The star Sirius is the brightest star in the sky. It is commonly known as the Dog Star.

Q: A star is ten light years away from the earth. Suppose it brightens up suddenly today. After how much time shall we see this change?

1 light year means the distance traveled by the light in one year. So, we will be able to see a star which is located 10 light years away from the Earth after ten years.

Q: Which of the following figures depicts the position of pole star correctly?

The pole star is towards the North Pole as shown in figure in option (A).

Physics

definition

Light year

A light year is defined as the distance travelled by light in one year. It is hence a measure of distance. It is often used as a unit because of the large distances between celestial bodies (like stars).