The most important celestial bodies are as follows;
Star: massive shining sphere of hot gas as a celestial
body is called star, like our Sun in the solar system.
Planet: Planet is not the source of light. It moves
around a star. Planet is become luminous by
reflection of star’s light, like the Earth.
Satellite: Satellite, in astronomy, is a secondary
object that revolves in a closed orbit about a planet
or star, and is not a source of light. The best-known
satellite is the Earth's moon.
Comet: Comet is a relatively small, rocky, icy
celestial body revolving around the Sun. When a comet
nears the Sun, some of the ice in the comet turns into
gas. The gas and loose dust creates a long, luminous
tail that streams behind the comet and produces solar
wind. As a comet approaches the Sun, the solar heat
evaporates, or sublimates, the ices so that the comet
brightens enormously. It may develop a brilliant tail,
sometimes extending many millions of kilometers (8 to
40 million km) into space. The tail is generally
directed away from the Sun, even as the comet recedes
again. Its orbit around the Sun is ellipse and its
diameter is 16000 to 160,000km. Halley’s Comet;
comet, or ball of ice and dust orbiting the sun, that
can be seen from the Earth once about every 76 years.
Planets’ motion
Planets’ motion has characteristics as follows;
*Planets move around the Sun at similar direction.
The Planets’ movement around the Sun is called
“orbital motion”.
*Planets rotate around themselves, too. All the
planets excluding Venus and Uranus move anticlockwise.
*The orbits of all planets are ellipse, and more or
less are at the same level.
*Planets’orbital period: is a time interval
between two successive identical configuration of a
planet in the space.
*Sidereal period: The sidereal period is the time it
take a planet to come around to the same position with
respect to the fixed stars (for example, the Sun).
This is the true orbital period of the planet.
*Synodic period: The synodic period is the time
interval between two successive identical
configurations as seen from Earth, say from one
opposition to the next such occurrence.
There is a relationship between sidereal and synodic
periods. For superior and inferior planets are as
follows;
[Location of the equation]
Kepler’s Laws
First law: According to Kepler's first law, the
planets orbit the sun in elliptical paths, with the
sun at one focus of the ellipse. Therefore, the
distance between the Sun and planets varies,
permanently. “Perihelion” is the point
nearest the sun in the orbit of a planet, for example,
the Earth. “Aphelion” is the point in the
orbit of an object in the solar system that is
farthest away from the Sun (this term can be
specialized for the Earth).
Second law: The second law states that the areas
described in a planetary orbit by the straight line
joining the center of the planet and the center of the
Sun are equal for equal time intervals; that is, the
closer a planet comes to the Sun, the more rapidly it
moves. In accordance with this law, triangles ESE,
DSD, BSB have the same area (figure ). The speed
of Earth at Perihelion and Aphelion is 30.28km/s and
29.29km/s, respectively.
Third law: Kepler's third law states that the ratio of
the cube of a planet's mean distance, d, from the sun
to the square of its orbital period, t, is a
constant—that is, d3/t2 is the same for all
planets.
Astronomical Unit (AU): It is a unit of distance used
in the measurement of orbits and trajectories within
the solar system. One AU is the average distance
between the earth and the sun. Its value has been
established as, roughly, 149,600,000 km.
Galaxy
Galaxy is a massive ensemble of hundreds of millions
of stars, all gravitationally interacting, and
orbiting about a common center.
Milky Way Galaxy includes about 400 billion stars.
Among them only about 6800 stars are observable via
naked eyes. Milky Way has a diameter of 81550, its
thickness is 6500 light-year bearing a mass of 145
million times more than the Sun. The solar system
located in a distance of about 27700 light-year from
the center of Milky Way. Solar system with a speed of
250km/s moves around its center.
The Sun's completes an almost circular orbit of the
center (of the galaxy) about every 250 million years
(Cosmic-year).
