20.1

• Angular size


  Sun and Moon have the same angular size in the sky, meaning their visible angular diameters (around 0.5°) are the same

  
• Solar eclipse


  1. Moon passes between Earth and Sun

  2. Moon casts its shadow on Earth

  
• Solar eclipse


  • Occurs when the moon "eclipses" the sun, blocking the sun and preventing sunlight from reaching us

  
• Alignment of Earth, Moon, and Sun


  • Determines the type of eclipse
  • Affected by several factors
  • Results in different shadows cast by Moon

  
• Umbra


  Shadow where no sunlight is present, the darker part of the shadow where light is totally blocked
• Penumbra


  Larger shadow surrounding the umbra where sunlight is only partially blocked, the lighter part of a shadow where light is partially blocked
• Partial solar eclipse


  • Moon only manages to block part of Sun, visible to regions of Earth under Moon's penumbra

  
• Annular eclipse


  • Moon passes directly in front of Sun but does not manage to cover all of it, leaving Sun's visible outer edges to form a "ring of fire" around the Moon

  
• Total or annular-total eclipse


  • Moon completely covers Sun's disk, an extremely rare astronomical phenomenon

  
• Node


  The exact point where a solar eclipse can occur in the sky

  
• Sun is dangerous to look directly into at all times, as it emits large amounts of radiation which damages human eyes
• Acceptable filters for eclipse observation


  • Aluminized Mylar
  • Shade 14 arc-welder's glass
• Historical solar eclipses


  • 4 ka in ancient China, where locals believed the darkening of Sun was caused by a dragon
  • May 17, 603 B.C.E., an eclipse that frightened two warring armies into forming a truce, known as the Battle of the Eclipse