The umbra is the darker inner shadow. Total eclipses are seen from within this shadow. When the Moon's dark umbral shadow sweeps across Earth, a total eclipse of the Sun is visible. December's eclipse will not be seen as a total eclipse anywhere on Earth. The eclipse itself is no more dangerous to view than the Sun is any day, but curiosity drives some people to stare directly at the Sun during an eclipse, which can cause permanent damage to your eyesight.
Partial eclipses are never safe to watch without taking special precautions.
Do not attempt to observe a partial eclipse with the naked eye. Use an approved solar filter designed specifically for Sun viewing or use a home-made projection box. The safest and most inexpensive method of observing eclipses is by projection. A pinhole or small opening will cast an image of the Sun on a screen placed several feet beyond the opening. Projected images of the Sun may even be seen on the ground in the small openings created by interlacing fingers, or in the sunlight beneath a leafy tree.