The sun travels 15 degrees across the sky each hour, but because its path — the ecliptic — is usually tilted with respect to the horizon, it takes more than six-fifteenths of an hour, or 24 minutes, to traverse each zone of twilight. The angle of the ecliptic is particularly shallow in wintertime. On December 7, 2015, the sun set at 1651 at Greenville, Pennsylvania, near the Ohio-Pennsylvania border, but civil twilight did not end until 32 minutes later, at 1723. Extrapolating, nautical twilight ended at 1755 and astronomical twilight at 1827. The moon would not rise until 0340 the next day. A nearby airport reported a 1,000-foot ceiling. It was pitch dark. Furthermore, the National Weather Service had issued a dense fog advisory for the area.