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the outer part of a shadow cast by a celestial body, where only some of the light from the source is blocked
Penumbra
that part of a shadow in which all light from a given source is excluded
Umbra
position furthest away from Earth
Apogee
position closest to Earth
Perigee
position closest to the sun
Perihelion
position furthest away from the sun
Aphelion
a period of roughly 18 years, 11 days, and 8 hours, where the Earth, Moon, and Sun align nearly identically, causing a similar eclipse to occur again
Saros cycle
Lens focuses light onto the focal plane
Refracting Telescope
Concave Mirror focuses light onto the focal plane
Reflecting Telescope
in reflecting telescopes, to re-direct the light path towards the back or side of the incoming light path.
Secondary mirror
