The creation account was divided in the post-exilic period into six days to provide an etiology for Shabbat. This necessitated creating light on day one to distinguish between day and night. In turn, it required assigning significance to the sun and moon on day four beyond their role as sources of light.
Prof.
Christoph Berner
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Biblical authors employed the number 7 in numerous ways to express the ideas of completion, perfection, and holiness and to highlight keywords or elements within a text.
Dr.
Elaine Goodfriend
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Early biblical laws demand a cessation of labor every seven days, but that was unconnected to Shabbat, which was originally a full moon celebration.
Prof.
Jacob L. Wright
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