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Q. So if the stars are so clever, can we get the time by observing the positions of the stars alone? 
A.  Yes and no.  Since ancient times humans have noticed that the stars in the northern sky appear to rotate about the Pole Star each night.  The positions and their heights and those stars visible also appear to change slowly throughout the year with the seasons.  E.g., a line through the pointer stars Merak and Dube in the Plough constellation appears to form the hour hand of a one handed clock rotating slowly clockwise about the Pole Star throughout the night.  This was useful to the monks and priests of old, because they had a need to pray at certain intervals during the hours of darkness.  Suitable instruments e.g. the ‘nocturnal’ (Have a photo available to show).  However, because it’s daylight in Europe when it’s dark in the pacific, the times found were local times. 

To work out position from the stars, an almanac is required to calculate their positions above the Earth at the time of a star observation.  It would be impractical to produce table for every local time around the Earth; we don’t know our longitude in any case; that’s what we’re ultimately trying to find out.  Therefore, tables are only produced for the local time of a person standing on the Greenwich Meridian, i.e. 0 degrees E/W, ‘Greenwich Time’.  This means that to calculate our position using the stars we must work in Greenwich Time not our Local Time, and we can’t get this from a nocturnal unless we’re standing on the Greenwich Meridian. 

Q.  So, how on earth are we going to get Greenwich Time?
 A.  The solution lies in the fact that whereas the stars appear to move around the earth at roughly 15degrees per hour, the Moon, which is far, far closer to the Earth and has an orbit of its own, moves at between 13.6 and 14.6 degrees per hour depending upon the time of year.  Therefore, the distance between the Moon and each star changes very slowly throughout the year.  This means that if someone were to provide us with a set of these distances, ‘Lunar Distances’, for chosen stars for every hour of every day throughout a particular year, we would only have to measure the angular distance between the Moon and the star and we could work backwards to calculate the Greenwich time.  Although the time by Lunar Distance was first published in 1524, a suitable method including the required tables didn’t become available until 1763.  Time to bring out some copies of pages from tables of lunar distances.  Here endeth the third lesson.  Easy then, just measure the distance and get the Greenwich time out of the tables.  Not quite, next lesson we’ll be looking at the corrections which must be applied.  DaveP