At what time, exactly, is the 'sun over the yard-arm', so I can have a drink?

Early start for you davejacobs ... 11:00am, however, it relates to the time that the sun rises above the upper yards of ships that sail through the North Atlantic.
http://www.phrases.org.uk/bulletin_board/15/messages/458.html

This sounds an incredibly vague definition.
1. Upper yards are set at different heights in different ships
2. Yardarms may be set on several different masts.
3. The sun rises different heights depending on the time of year, and the latitude.
4. It is not stated from which part of the ship the yardarm is viewed. The bow, the stern, the wheelhouse, etc.