Chapter 24 - The Universe
Universe is a vast space and all the heavenly bodies that lie in it.
It is hard to determine the size and limitation of the universe. The
universe consists of
countless galaxies in it and galaxies has numerous stars.
A unit of astronomical distance equivalent to the distance that
light travels in one year, which is 9.4607 × 1012 km (nearly 6
million million miles).
Light travels in space at a speed of 300,000 km each second. A light
year is the distance light travels in one year. Light travels 9.46 x
km in one year. So, one light year is equivalent to 9.46 x
1012 km nearly equals to 9.5 x 1012 km.
A solar system is a group of different heavenly bodies that revolve
around the sun including the sun itself.
It includes many smaller objects that revolve around the sun. All
eight planets and their satellites as well as asterois,
comets, meteoroids, etc revolve around the sun. Sun contains 99.8 %
of the total mass of solar system. Life on earth is made
possible due to the heat and light energy of the sun and sun mainly
contains hydrogen and helium. The distance of sun from
earth is 150,000,000 km.
contains hydrogen and helium
Motion of the earth:
The earth spins around its axis once in 24 hours is called rotation
of the earth. This is equal to one day. It goes around
the sun in fixed path or orbit i.e. once in 365 days and it is
called revolution or annual motion.
Sun, earth and moon
Moon is the nearest neighbour of the earth. It is only the satellite
of the earth. We know tha earth goes around the sun
in its orbit once in a year. Similarly, the moon also revolves
around the earth in its orbit. The revolution period of the moon
around the earth is named differently as sideral month and
The moon makes a complete revolution around the earth once in a
27.33 days, it is called sideral month. It's other name is
The duration for the moon to move from M1 position
position is called synodic month. or, The duration between two
successive phases of the moon which is 29.5 days is called
synodic month. It's other name is Chandramaas.
Umbra and Penumbra:
From above figure,
From the region 'a' of the shadow a part of the light of the lamp is
seen but from 'b' light is not seen
at all. The region of complete darkness where all the light from the
source is blocked is called umbra.
Here from figure 'b' is umbra.
The region of partial darkness, where the light rays are partially
blocked is called penumbra.
When the earth passes between sun and moon the shadow of the earth
may fall on the moon and causes the partial
or complete darkness of the moon's surface. It is called lunar
eclipse. And if the moon's surface
is completely darkened it is called total lunar eclipse and if it's
surface is darkened partly it is called
partial lunar eclipse.
When the moon passes between the sun and the earth and the sun is
block, a shadow is cast on the earth. The sun
is partly or wholly darkened.
Annual Solar eclipse:
If the umbral cone formed by the moon's shadow meets before it
reaches the surface of the earth due to the
increase in the distance between the moon and the earth, the bright
part of the sun is seen as a bright ring called
Eclipses donot occur on every full moon or new moon day.
Some questions from this lesson:
1. Difference between Solar eclipse and lunar eclipse.
2. What is the effect of diurnal motion?
3. What is annual eclipse? When does it occur?
4. How does lunar eclipse occur? Explain with diagram.
5. What is light year? How many kilometres make one light year?
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