Only a small portion of the Earth can observe a total solar eclipse at any given time. This is because the umbra, the darkest part of the Moon's shadow, is relatively narrow and covers only a small area on the Earth's surface.

Here's a breakdown:

* Umbra: The central, darkest part of the Moon's shadow. People within the umbra witness a total solar eclipse.

* Penumbra: The outer, lighter part of the Moon's shadow. People within the penumbra see a partial solar eclipse.

Therefore, the only people who can witness a total solar eclipse are those who are within the path of totality, which is the path the umbra takes across the Earth's surface.