Final answer:
The plural form of 'ciel' in French is 'cieux', an irregular form that changes the ending 'el' to 'eux'. This reflects how many words in French deviate from standard pluralization rules. Ancient stargazers, deeply interested in the night sky, conceptualized celestial patterns and movements into the idea of the celestial sphere.
Step-by-step explanation:
The plural form of the French word ciel (meaning sky or heaven) is cieux. This is an irregular plural form in French, which does not follow the typical pattern of adding an 's' to create plural forms. Instead, it changes the ending 'el' to 'eux'. In French, such irregular forms are not uncommon and they occur with other words too, especially those that end in 'al' which change to 'aux' for the plural form (e.g., 'journal' becomes 'journaux'). Observers of the celestial sphere or stargazers have been understanding and mapping the skies since ancient times, marveling at the patterns and movements of celestial bodies.
Many early civilizations, like the Egyptians and Babylonians, noticed the stars and other celestial bodies moving across what seemed like a dome over the Earth. This observation led them to conceptualize the idea of the celestial sphere. With no modern technology or advanced telescopes, the ancient watchers of the heavens were able to recognize significant patterns in the sky, and this is reflected in the way we understand the sky and its constellations even today.