The term functions as a noun phrase, where "September 11th" acts as an adjectival modifier for the core noun "moon phase." For the specific, historically significant date of September 11, 2001, the Moon was in the Waning Crescent phase.
The Waning Crescent is the final phase of the lunar cycle before the New Moon. It occurs after the Third Quarter phase, when the visible illuminated portion of the Moon is shrinking. On that day, the Moon was approximately 31% illuminated. From the Northern Hemisphere, a Waning Crescent appears as a thin sliver of light on the left side, visible in the eastern sky before sunrise.
This astronomical information is a specific, verifiable data point for the historical record of that day. It holds no causal or scientific relationship to the terrestrial events that transpired. Due to the discrepancy between the Moon's synodic period of approximately 29.5 days and the length of a calendar year, the lunar phase on any given date, such as September 11th, varies annually.
