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Solar day is the time from noon on one day to the noon on the other day. I.e. 24 hours

Sidereal day is the times it takes for Earth to rotate on its axis.

A solar day is longer than a sidereal day because as earth rotates 360…

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When Earth catches up with Mars and goes past it which makes it seem like Mars is going backward.
Earth takes over Mars because it has a smaller orbit.

The amount of light a planet reflects to Earth depends on:

How close the planet is to Earth


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Refraction is a change in direction of wave as it travels from one medium to the other, due to
change in wave speed.

Speed of light varies in different mediums.

When refraction occurs, the frequency remains the same but the wavelength changes which causes
change in wave speed…

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If a lens has long focal length, it will have low power. A stronger lens has short focal length and
refracts light more.


Lenses are placed in a telescope so that the length of the telescope is the sum of their focal length.

The objective lens is…

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Stars emit all types of electromagnetic radiation not just visible light.

Radio telescopes collect radio waves coming from stars using huge parabolic reflecting dish.

There are also telescopes in space, star emits X- rays and gamma rays, telescopes in space can form
images from these radiations. These…

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Telescopes on space

High cost of maintaining

High cost of setting up

There are also size limitations.


Resolving power of a telescope measures how well it can distinguish stars that appear distinct.

A spectrum of light can be seen by passing light through a glass prism.


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Even though, some stars are really bright, when viewed from Earth, they look faint. This is because
the light spreads out further as it travels through space. The radiation given out spreads equally in all

The Rutherford-Geiger-Marsden showed the evidence for nuclear atom.


Variable stars…

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Hubble found out that the further the galaxy, the faster it is moving away from us.

Hubble constant is the ratio of recession of galaxy from Earth.

Speed of recession (km/s) = Hubble constant * distance Mpc

Hubble constant is 75

The fact that all galaxies are moving…

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Nebulas are enormous clouds of hot gas and dust.

Gravity is constantly acting to pull the molecules and dust together. As the cloud collapses, the GP
energy gets turned to kinetic energy. The particles move faster and this lead to change in
temperature, which increases pressure. Eventually…

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Once the core stops releasing energy, the balance of forces keeping the star in equilibrium is lost and
the gravity will start crushing the star.

Hertzsprung-Russell diagram is a chart that plots the luminosity of a star in the vertical axis and the
surface temperature in the horizontal…


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