Twilight is the period just after the limb of the Sun has set below the horizon, and just before it reaches a distance of 18 degrees below the horizon. Below is a table of sunset times and end of twilight times for Houston at a latitude near 30 degrees given for 1996 in Universal Time.
Month Sunset End of Twilight Difference .......................................................... January 17:30 17:55 25 minutes February 17:42 18:04 22 March 18:01 18:25 24 April 18:21 18:45 24 May 18:38 19:04 26 June 18:58 19:25 27 July 19:05 19:32 27 August 18:52 19:18 26 September 18:20 18:44 24 October 17:45 18:09 24 November 17:12 17:37 25 December 17:00 17:26 26 ..........................................................