UNIT 13

• Rotation is the circular motion of an object around a fixed line that crosses the center of

  this object.
• This line is called the axis of rotation. This motion is the one that your favorite

  spinning top does. Just like your top, Earth rotates as well.
• Earth’s axis is tilted at an angle of 23.5° from the perpendicular

  of its orbital plane. This is Earth’s axial tilt.
• People can tell time based on the location of the

  Sun in the sky. This is called solar time.
• The longer the Sun is present in the sky, the greater the amount of radiation received by

  Earth’s surface. This radiation is referred to as insolation,
• The angle at which sunlight enters Earth is determined by the path of the Sun in the sky.

  This is referred to as the angle of insolation.
• Revolution is the movement of an object around another object.
• The path on which the

  revolving object moves is called the orbit.
• The general shape of Earth’s orbit around the Sun is an

  ellipse or oval, as was what the astronomer Johannes Kepler stated in one of his Three Laws

  of Planetary Motion.
• Earth is closest to the Sun (called perihelion),
• Earths is farthest (called aphelion).
• The transitions of each season are called equinoxes and solstices. These

  transitions can be recognized by the length of daytime, the altitude of the sun at noon or

  the noon altitude, and the length of the shadow of a vertical object.
• The celestial sphere is an imaginary sphere wherein Earth is considered to be at

  the center and where all celestial objects are considered to lie.
• It happens on December 22 or 23. The beginning of this season is called the

  winter solstice.
• These periods are called the equinoxes. The word comes from a Latin

  word meaning “equal nights” ,stands for the day on which the duration of day and

  night is equal.
• The spring equinox marks the beginning of spring. It is also called the vernal equinox. It

  happens on March 21. It is the season after winter which lasts from March to June.
• The fall equinox marks the beginning of spring. It is also called the autumnal equinox. It

  occurs on September 23. It is the start of autumn which lasts until December.