Based on sunset information from the U.S. Naval Observatory, Easter Vigil this year (April 11, 2009) in the Diocese of San José cannot begin any earlier than 8:00 p.m.
From the U.S. Naval Observatory Astronomical Applications Department, the following information is provided for San Jose, Santa Clara County, California (longitude W121.9, latitude N37.3):
11 April 2009, Pacific Daylight Time
- Sunset 7:39 p.m.
- End civil twilight 8:06 p.m.
If sunset is at 7:39p, why can't we begin Easter Vigil 2009 at 7:39p?
Because there is a big technical different between "sunset" and "civil twilight." The technical definition of "sunset" is when the upper edge of the sun hits the horizon. At this point (7:39p) there's still some daylight in the sky. But what we're looking for is complete darkness.
Civil twilight in the evening is technically when the center of the sun is geometrically 6 degrees below the horizon. At this time (8:06p) there's still enough light to see the horizon, but it's dark enough to see the brightest of stars in the sky. Complete darkness, however, begins sometime after the end of evening civil twilight.
So 8:00p is the earliest time we can begin the Easter Vigil in 8:00p. If you really want to start in complete darkness, wait until 8:10p.
For more information: http://aa.usno.navy.mil/faq/docs/RST_defs.php#top