20.2

• Moonlight


  Sunlight striking the moon and reflecting off its surface, which becomes visible on Earth

  
• Moon's brightness from Earth


  Depends on where the moon is in its orbit around the planet

  
• Lunar eclipse


  Occurs when a full moon passes into Earth's shadow

  
• Full moons are required for lunar eclipses since they place Earth between Sun and Moon, which is the geometry that produces a lunar eclipse

  
• Earth's shadows


  • Umbra - a dark cone that blocks all sunlight
  • Penumbra - a shadow fanning from the globe which only partially blocks sunlight
• Umbra


  The central darker part of a shadow where light is totally blocked
• Penumbra


  The lighter part of a shadow where light is partially blocked
• Penumbral lunar eclipse


  Moon is found in the penumbra of Earth, leading to a slight dimming of the moon

  
• Partial lunar eclipse


  Moon is found in the between the umbra and penumbra of Earth, leading to the full moon taking on a different shape because of the shadow

  
• Total lunar eclipse


  Moon is found in the umbra of Earth, causing the moon to disappear or take on a blood-red color

  
• In a total lunar eclipse, the sunlight passes through Earth's atmosphere which filters out most of the blue light, making the moon appear red

  
• Nodes


  The points where the moon's orbit passes through the plane of Earth's orbit, used to pinpoint possible eclipse sites

  
• Saros cycle


  A period of 18 years, 11 days and 8 hours in which the Sun, Moon and Earth return to almost exactly the same spots they were at 18 years, 11.3 days ago, used to predict when an eclipse is going to happen

  
• Lunar eclipses are much safer to observe than solar eclipses, since the moon emits no harmful radiation

  
• The main adversary of lunar eclipse watching is the weather, as cloud cover and smog can prevent viewing