1830 Universe
Chapter
31
Universe
Universe is the limitless expanse of space around us consisting of solar (i) The gravitational pull of the Sun on the planets control their
system. star, galaxies etc. motion.
Solar System (ii) There are other heavenly bodies (about 32) which revolve around
the planets called satellites (or moons) of the planets.
(iii) A planet does not emit light of its own.
(iv) A planet do not twinkle at night.
(v) The planets are very small in size as compared to stars or Sun.
(vi) The relative positions of planets keep on changing day by day.
(vii) Most of the planets move around the sun from west to east.
(viii) The planets are made of rocks and metals.
Solar system is a family of nine planets, satellites, asteroids, comets, (ix) The temperature of planet depends upon its distance from sun.
meteors, meteroites and dust particles orbiting around the Sun. (2) Asteroids : The small pieces of planet revolving around the sun
(1) Planets : Nine planets revolving around the sun in elliptical orbits. In between orbits of Mars and Jupiter are called Asteroids.
order of increasing distance from Sun, these are Mercury, Venus, Earth, Mars, (i) Astronomers have identified about 2000 asteroids ranging from the
Jupiter, Saturn, Uranus, Neptune and Pluto. largest 770 km diameter to bodies 1.5 km in diameter.
Table 31.1 : Some information about planets
Planet Radius Mean distance from sun Mass as compared to Time of revolution Time taken to complete one rotation Number of
R 103 km 106 km earth around the sun around its own axis satellites
Mercury 2.4 57.9 0.055 88 days 59 days —
Venus 6.1 108.2 0.815 225 days 243 days —
Earth 6.3 149.6 1 1 Year 23 hrs. 56 min. 1
Mars 3.4 227.9 0.108 1.9 Year 24 hrs. 27 min 2
Jupiter 71.4 778.3 317.9 11.8 Year 9 hrs. 50 min 14
Saturn 60.0 1427 95.2 29.5 Year 10 hrs. 14 min 10 + Ring
Uranus 23.4 2870 14.6 85 Year 10 hrs. 49 min 5 + Ring
Neptune 22.3 4594 17.2 165 Year 15 hrs. 2
Pluto 3.2 5900 0.002 248 Year 6.39 days —
(ii) The largest asteroids are called Ceres. (3) Comets : These are composed of rock like materials surrounded by
(iii) The largest asteroid complete one revolution around the sun in large masses of easily vaporisable substances like, ice, water, ammonia and
4.6 years. methane.
(i) They revolve around the Sun in highly elliptical orbits.
, Universe 1833
(ii) Their time period of revolution around the Sun is very large. maximum when earth and sun subtend an angle 90° at the planet. From
(iii) Comets appear to be having a bright head and a long tail while figure,
passing close to the Sun and when away from sun generally they show no
tail. PS r
sin ps
(iv) The tail of comet is formed when the comet is passing close to the ES 1 AU
Sun and the heat of Sun exerts a pressure on the material which gets
evaporated due to heat of Sun. where 1 AU = 1.496 10 m 11
(v) Hally comet was seen in early 1986 and is expected to be seen PE r
Thus sin rps , similarly cos pe rpe cos
again in 2062. ES 1 AU
(4) Meteors and meteorites : Meteors are the smaller pieces of stones (3) Kepler's law : According to Kepler's law the square of time period
and metals which may be produced due to the breaking up of comets while
of planet around sun is proportional to cube of semi-major axis of the orbit
approaching the Sun. When they reach earth's atmosphere due to friction
they start buring. They are also called shooting stars. a3
of planet around sun i.e. constant , if a1 and a2 are semi-major
Sometimes, the large pieces of stones (acting as meteors) do not burn T2
completely and reach the surface of the earth as stony, iron balls resulting axes of planets 1 and 2 and T and T their respective periods of revolution,
1 2
in crators on the earth surface. These are called meteorites. then
2/3
Measurement of Size of Planet a13 a3 T
2
22 or a2 2 a1
We can measure the size of a planet by measuring the angle T1 T2 T1
subtended by its diameter AB at a
For circular orbits a and a represent the radii of orbits.
point on the earth. This angle is d 1 2
A B
called angular diameter of planet. If (4) Spectroscopic method : In this method, photograph of two
d denotes diameter of planet and D different planets P and P are taken on similar photographic plates from one
1 2
its distance from the earth place of the earth. Let I and I be the intensities of the images of these two
D
1 2
planets. If R and R be the distances of these planets from the earth then
1 2
d
~– I1 R 22
D ( intensity at a point is inversely proportional to the square of
I2 R12
or d ~– D Fig. 31.1
the distance)
Earth
Measurement of Fig. 31.1 Stars
Distance of Planet From
the Earth
(1) Parallax method : The planet O is observed from two points P and 1
P on the surface of the earth. The distance between these two points, PP =
2 1 2
b, is called basis. The angle
subtended by planet at these two O
points is called parallax angle or
parallactic angle
A firey massive luminous heavenly body is called a star.
b D D
From figure ~– (1) Some features
D
(i) Stars twinkle at night.
b P1 P2 (ii) Stars are countless in number ; about 10 in a universe.
or D ~–
22
Earth
(iii) Stars are very big in size but they appear small because they are
(2) Copernicus method : The b very far off.
inferior planets (Mercury and (iv) The relative positions of stars do not change day by day.
Fig. 31.2
Venus) have nearly circular orbits.
Angle between directions of observation from earth to sun and earth to (v) Stars appear to be moving from west to east.
planet is called planet's elongation. (vi) The temperature of stars is very high.
res The distance of earth from Sun, rps The distance of planet (vii) The Sun is the nearest star to the earth. Its light reaches the
earth in 8.3 minutes.
from Sun and rpe The distance of planet from Earth
(viii) After Sun the next nearest star to earth is Alpha centuri. Its
The rps and res are fixed distances as distance is 4.3 light year from earth.
orbits have been assumed to be circular. (ix) Other bright stars in the sky are known as Spica (Chitra),
During orbital motion of the planet the Arcturus (Swati), Polaris (Dhruva), Sirius (Vydha), Canopus (Agasti) etc.
distance rpe changes. Planet's elongation is S (x) The temperature of a star is estimated from the colour of its light
rps received on earth. The blue coloured star is at higher temperature than red
90° res = 1AU coloured star.
P
rpe
E
Fig. 31.3
Chapter
31
Universe
Universe is the limitless expanse of space around us consisting of solar (i) The gravitational pull of the Sun on the planets control their
system. star, galaxies etc. motion.
Solar System (ii) There are other heavenly bodies (about 32) which revolve around
the planets called satellites (or moons) of the planets.
(iii) A planet does not emit light of its own.
(iv) A planet do not twinkle at night.
(v) The planets are very small in size as compared to stars or Sun.
(vi) The relative positions of planets keep on changing day by day.
(vii) Most of the planets move around the sun from west to east.
(viii) The planets are made of rocks and metals.
Solar system is a family of nine planets, satellites, asteroids, comets, (ix) The temperature of planet depends upon its distance from sun.
meteors, meteroites and dust particles orbiting around the Sun. (2) Asteroids : The small pieces of planet revolving around the sun
(1) Planets : Nine planets revolving around the sun in elliptical orbits. In between orbits of Mars and Jupiter are called Asteroids.
order of increasing distance from Sun, these are Mercury, Venus, Earth, Mars, (i) Astronomers have identified about 2000 asteroids ranging from the
Jupiter, Saturn, Uranus, Neptune and Pluto. largest 770 km diameter to bodies 1.5 km in diameter.
Table 31.1 : Some information about planets
Planet Radius Mean distance from sun Mass as compared to Time of revolution Time taken to complete one rotation Number of
R 103 km 106 km earth around the sun around its own axis satellites
Mercury 2.4 57.9 0.055 88 days 59 days —
Venus 6.1 108.2 0.815 225 days 243 days —
Earth 6.3 149.6 1 1 Year 23 hrs. 56 min. 1
Mars 3.4 227.9 0.108 1.9 Year 24 hrs. 27 min 2
Jupiter 71.4 778.3 317.9 11.8 Year 9 hrs. 50 min 14
Saturn 60.0 1427 95.2 29.5 Year 10 hrs. 14 min 10 + Ring
Uranus 23.4 2870 14.6 85 Year 10 hrs. 49 min 5 + Ring
Neptune 22.3 4594 17.2 165 Year 15 hrs. 2
Pluto 3.2 5900 0.002 248 Year 6.39 days —
(ii) The largest asteroids are called Ceres. (3) Comets : These are composed of rock like materials surrounded by
(iii) The largest asteroid complete one revolution around the sun in large masses of easily vaporisable substances like, ice, water, ammonia and
4.6 years. methane.
(i) They revolve around the Sun in highly elliptical orbits.
, Universe 1833
(ii) Their time period of revolution around the Sun is very large. maximum when earth and sun subtend an angle 90° at the planet. From
(iii) Comets appear to be having a bright head and a long tail while figure,
passing close to the Sun and when away from sun generally they show no
tail. PS r
sin ps
(iv) The tail of comet is formed when the comet is passing close to the ES 1 AU
Sun and the heat of Sun exerts a pressure on the material which gets
evaporated due to heat of Sun. where 1 AU = 1.496 10 m 11
(v) Hally comet was seen in early 1986 and is expected to be seen PE r
Thus sin rps , similarly cos pe rpe cos
again in 2062. ES 1 AU
(4) Meteors and meteorites : Meteors are the smaller pieces of stones (3) Kepler's law : According to Kepler's law the square of time period
and metals which may be produced due to the breaking up of comets while
of planet around sun is proportional to cube of semi-major axis of the orbit
approaching the Sun. When they reach earth's atmosphere due to friction
they start buring. They are also called shooting stars. a3
of planet around sun i.e. constant , if a1 and a2 are semi-major
Sometimes, the large pieces of stones (acting as meteors) do not burn T2
completely and reach the surface of the earth as stony, iron balls resulting axes of planets 1 and 2 and T and T their respective periods of revolution,
1 2
in crators on the earth surface. These are called meteorites. then
2/3
Measurement of Size of Planet a13 a3 T
2
22 or a2 2 a1
We can measure the size of a planet by measuring the angle T1 T2 T1
subtended by its diameter AB at a
For circular orbits a and a represent the radii of orbits.
point on the earth. This angle is d 1 2
A B
called angular diameter of planet. If (4) Spectroscopic method : In this method, photograph of two
d denotes diameter of planet and D different planets P and P are taken on similar photographic plates from one
1 2
its distance from the earth place of the earth. Let I and I be the intensities of the images of these two
D
1 2
planets. If R and R be the distances of these planets from the earth then
1 2
d
~– I1 R 22
D ( intensity at a point is inversely proportional to the square of
I2 R12
or d ~– D Fig. 31.1
the distance)
Earth
Measurement of Fig. 31.1 Stars
Distance of Planet From
the Earth
(1) Parallax method : The planet O is observed from two points P and 1
P on the surface of the earth. The distance between these two points, PP =
2 1 2
b, is called basis. The angle
subtended by planet at these two O
points is called parallax angle or
parallactic angle
A firey massive luminous heavenly body is called a star.
b D D
From figure ~– (1) Some features
D
(i) Stars twinkle at night.
b P1 P2 (ii) Stars are countless in number ; about 10 in a universe.
or D ~–
22
Earth
(iii) Stars are very big in size but they appear small because they are
(2) Copernicus method : The b very far off.
inferior planets (Mercury and (iv) The relative positions of stars do not change day by day.
Fig. 31.2
Venus) have nearly circular orbits.
Angle between directions of observation from earth to sun and earth to (v) Stars appear to be moving from west to east.
planet is called planet's elongation. (vi) The temperature of stars is very high.
res The distance of earth from Sun, rps The distance of planet (vii) The Sun is the nearest star to the earth. Its light reaches the
earth in 8.3 minutes.
from Sun and rpe The distance of planet from Earth
(viii) After Sun the next nearest star to earth is Alpha centuri. Its
The rps and res are fixed distances as distance is 4.3 light year from earth.
orbits have been assumed to be circular. (ix) Other bright stars in the sky are known as Spica (Chitra),
During orbital motion of the planet the Arcturus (Swati), Polaris (Dhruva), Sirius (Vydha), Canopus (Agasti) etc.
distance rpe changes. Planet's elongation is S (x) The temperature of a star is estimated from the colour of its light
rps received on earth. The blue coloured star is at higher temperature than red
90° res = 1AU coloured star.
P
rpe
E
Fig. 31.3
