19.1

• Day


  • Starts with Sun rising from the east
  • Ends when Sun sets in the west
• Night


  • Stars are seen to move from east to west

  
• Earth moves around Sun

  With respect to the background stars
• Star trail


  • Shows the apparent motion of objects in the sky

  
• Rotation


  Circular motion of an object around a fixed line that crosses its center

  
• Earth's rotation


  Movement of the Earth on its own axis

  
• Axis of rotation


  Imaginary line that traverses a planet through which it spins around

  
• The axis is an imaginary line passing through the center of the Earth. Its low ends on the surface are called North and South poles

  
• The Earth completes one rotation in 24 hours
• The Earth's axis is vertical, meaning it runs up and down
• Earth's axial tilt


  Angle of 23.5°, perpendicular to the orbital plane

  
• Earth's current axial tilt of 23.5° resulted from the angle of the collisions that formed the moon and has stayed through time

  
• Earth's rotation


  • Eastward - from west to east
  • Prograde rotation is Earth's counter clockwise direction when viewed from above
  • Retrograde rotation is the clockwise direction of other planets, like Venus

  
• The direction of rotation is same as that of the sun which is in the counter-clockwise direction when viewed from the North pole

  
• Day and night


  • Alternating light and darkness experienced in one rotational period
  • Due to the Earth rotating on its axis, not its orbiting around the sun
• The term 'one day' is determined by the time the Earth takes to rotate once on its axis and includes both day time and night time
• Solar time


  Calculation of the passage of time based on the position of Sun in the sky
• Places on Earth experience different time zones due to Earth's rotation
• Time zone


  An area of Earth that has a specific time that all citizens can set their clocks to
• Sundial


  Earliest type of timekeeping device, which indicates the time of day by the position of the shadow of some object exposed to the sun's rays
• Insolation


  • Radiation from Sun, short for incoming solar radiation
  • The amount of solar radiation received by the surface of Earth
• The sun radiates heat and light energy in all directions and at all times. This is called solar radiation
• Angle of insolation


  • Angle at which sunlight enters Earth determined by the path of Sun
  • The closer the angle of insolation is to 90°, the greater the intensity it will receive, leading to higher temperatures
• As Earth orbits around the sun, it travels in an oblong path rather than a perfect circle, so the planet is closer to the sun at some times during the year compared to others
• Position of Sun in the sky

  • Depends on the latitude you are standing on
  • At noontime, a person near the equator observes Sun overhead while a person in the higher latitudes observes Sun lower in the sky
• Latitude


  • Factor in insolation
  • Places near the equator receive higher insolation
  • The higher latitudes receive less insolation
• Zenith

  • The peak of Sun's path
  • The highest point reached by a heavenly body as it travels around
• Meridian


  • Imaginary line connecting the poles
  • They divide the planet vertically, from the North pole to the South pole

  
• Revolution

  • Movement of an object around another object
  • Earth's orbital motion around the sun takes 365 ¼ days to complete
  • The path on which the revolving object moves is called the orbit
  • Earth revolves around Sun from west to east

  
• Perihelion


  Point at which Earth is closest to Sun
• Aphelion


  Point at which Earth is farthest from Sun
• The general shape of Earth's orbit around Sun is an ellipse or oval

  
• Earth's revolution


  Changes Earth's surface which is tilted towards Sun, affecting the insolation intensity
• Seasons


  Caused by the varying amount of insolation received by regions