From: Ron Jones
Date: 2016 Feb 20, 14:15 -0800
The reason for the question I was looking at a very old version of the USPS Navigation course where an example of a 5 body star & planet fix, was given. The example stated it was from an actual ship's log & the DR position was Latitude 8° 29' S Longitude 14° 05' W and the zone time of the sights were taken from 19:44 to 19:53. At that position & time the sun would have been approximately 20° below the horizon.
2002 Edition, Bowditch, Page 291, Paragraph 1910. Twilight ....... In general, the most effective period for observing stars and planets occurs when the center of the Sun is between about 3° and 9° below the celestial horizon. ....
In my experience some Venus sights have been possible when the altitude of the sun was near 3 degrees of altitude; but when the sun is below about 12 degrees the horizon can not be determined with a high degree of accuracy.