Dear Abdolali,
Civil twilight is defined as the period of daylight from true sunset/sunrise to when the sun is 6 degrees below the horizon (center of the solar disk).  There are other definitions of twilight e.g. nautical (12 degrees) and astronomical (18 degrees) but these are not normally used by crop scientists.

Cheers,
Garry


Dr Garry O'Leary
South African Sugar Association Experiment Station
Private Bag X02
Mount Edgecombe, 4300 Kwa Zulu-Natal
South Africa

Email: [log in to unmask]
Telephone: +27 31 593205
Fax: +27 31 595406
Internet: http://www.sugar.org.za/sasex/about/oleary.htm