Final answer:
A total solar eclipse occurs when the Moon is close enough to Earth and precisely aligned for its umbra to touch the Earth's surface, as seen in August 21, 2017 across the U.S.
Step-by-step explanation:
The least total solar eclipse occurs when the Moon's umbra does not reach the surface of the Earth. In contrast, a total solar eclipse happens when the Moon is closer than its average distance to Earth and its darkest shadow, the umbra, intersects a small point on Earth's surface. A notable example was the total solar eclipse of August 21, 2017, which was visible across the United States, where observers within the path of totality experienced darkness for a few minutes as the Moon completely covered the Sun.
During a total solar eclipse, anyone within the small area covered by the tip of the Moon's shadow witnesses a total eclipse, while those in the penumbra witness a partial solar eclipse. The 2017 eclipse was special because its path of totality spanned across the continental U.S., offering a rare viewing opportunity for millions. The circumstances and geometry of total solar eclipses suggest that alignment and distance are crucial factors for an eclipse to be classified as total.
Provide in summary after explanation: A total solar eclipse requires precise alignment between the Earth, Moon, and Sun, with the Moon being close enough to Earth so its umbra can extend onto Earth's surface. The absence of this alignment or the proper distance results in a partial solar eclipse, or in cases where the Moon's shadow does not reach Earth at all, no eclipse.