Why were the simple vertical markers used by early civilizations, precursors to the sundial, inherently flawed for time organization?

They were useless at night or on heavily overcast days.

The earliest methods for organizing time relied on direct observation of the cosmos, most commonly involving simple vertical markers, which functioned as rudimentary sundials by tracking shadow movement. While this observation of the sun’s predictable cycle provided a foundational structure for organizing activities like planting and hunting, the method suffered from a critical dependency on solar visibility. If the sun was obscured due to heavy cloud cover or if the period being measured extended past sunset into the night, these shadow markers provided absolutely no indication of elapsed time. This limitation necessitated the subsequent development of independent time-measuring devices that could operate regardless of atmospheric conditions.