The Sun is completely blocked in a solar eclipse because the Moon passes between Earth and the Sun. Even though the Moon is much smaller than the Sun, because it is just the right distance away from Earth, the Moon can fully blocks the Sun’s light from Earth’s perspective. Sometimes the Moon is far enough away that it doesn't create a total solar eclipse. However, once the Moon's growing orbit takes it approximately 14,600 miles (23,500 km) farther away from Earth, it will always be too far away to completely cover the Sun. That won't happen for a long time though.
