Which event (s) occur because the Earth, Moon, & Sun have a 180° alignment (straight angle) ?

The last total eclipse in the U. S. was in 1979. And the last total solar eclipse that crossed the entire continental U. S. happened in 1918. But why? Why has it been 99 years, and why have the intervening partial and even total eclipses caught only parts of the country?

Not sure if its 100% correct but it's similar to that I think