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Chapter 1 : Introduction to Astronomy



Introduction to Astronomy arrow_upward


  • Astronomy is the study of the universe.
  • It is a natural science that deals with the study of celestial objects (such as moons, planets, stars, nebulae, and galaxies).
  • Astronomy includes:
    • Atoms and molecules.
    • Stars.
    • Galaxies.
    • Clusters of galaxies.
    • The nature of the universe itself.
  • The Milky Way is the galaxy that contains our Solar System.
  • Picture of the Milky Way is shown below:

  • Why to study Astronomy? arrow_upward


  • To find answers to below questions:
    • Why there are days and nights, and seasons on the Earth?
    • Why does the Sun shine?
    • Why do stars change position at night in the sky?
    • Do stars ever die?
    • How big is the Universe?
    • Is the Universe infinite in scope and size?

    Scale arrow_upward


  • To understand Astronomy, it is necessary to understand the scale.
    • Earth is a tiny object as compared to the Sun.
    • The Sun is one star amongst over 200 billion stars gravitationally bound together to make the Milky Way Galaxy.
    • Age of the universe is known to be around  billion years.

    Light Year arrow_upward


  • A Light Year is the distance traveled by light in one year.
  • One light year = 9.46 × 1015   m
  • One light year = 9,460,730,472,580.8 km

  • Constellations arrow_upward


  • A group of fixed stars is known as Constellation.
  • Human imagination can convert a group of stars into a known figure.
  • Below picture is showing one such conversion:
  • They are designated in most cases by the name of:
    • Animals.
    • Mythological figures.

    The Celestial Sphere arrow_upward


  • The Celestial Sphere is an imaginary sphere of large radius, concentric with the Earth and rotating upon the same axis.
  • All objects in the sky can be thought of as projected upon the celestial sphere.
  • Equator and the poles are projected upward from Earth's equator and poles and are called the celestial equator and the celestial poles.
  • The celestial sphere is a very practical tool for the positional astronomy.
  • The entire sky appears to turn around imaginary points in the northern and southern sky once in 24 hours.

  • Expanding Universe arrow_upward


  • Except for a few nearby galaxies, all the galaxies are moving away from us.
    • The recession speed of a galaxy is proportional to its distance from us.
    • A galaxy that is twice as far away is moving twice as fast.

  • This implies that on large scales, all the galaxies are moving away from each other.

  • The Clockwork of the Universe arrow_upward


  • There are many familiar astronomical cycles:
    • The Day/Night cycle.
    • The phases of the Moon.
    • The seasons of the year.
    • The seven day week.

    The Day/Night Cycle arrow_upward


  • The apparent motion of the Sun defines the day.
    • Every day, the Sun rises in the east and sets in the west.
  • The rotation of the Earth affects:
    • Weather patterns
    • Ocean currents
    • The paths of missiles
  • At night, the stars move in fixed patterns.
  • The stars rotate about a fixed point in the northern sky.
  • We have the Sun rising and setting, and the stars moving in fixed patterns about a point in the sky.
  • The Earth is spinning on its axis.
  • The Earth and the other planets revolve around the Sun.

  • How long is One Month? arrow_upward


  • During the course of 27 days, the Earth moves around a substantial part of its orbit about the Sun.
  • It takes extra 2 days for the Moon to “catch up” with the Sun.
  • The full Moon always rises just after sunset.
  • The crescent Moon always points towards the Sun.
  • A crescent Moon sets shortly after sunset, or rises just before sunrise.
  • The Moon is illuminated by reflecting the sunlight.
  • The lit side of the Moon always faces the Sun.
  • Because of the motion of the Moon relative to the Sun, we see different amounts of lit and dark sides over the course of a month called the Lunar Cycle.


  • Thank You from Kimavi arrow_upward


  • Please email us at Admin@Kimavi.com and help us improve this tutorial.


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