I suppose the reason Longitude had a relatively low RT% is because it was an A&E miniseries (4 hours long), never a theatre release. Nevertheless, it is hard to imagine any population of reviewers where 1 in 5 would not approve of this work. So I just chalk it up as an aberation of low numbers (5 reviews).
This tells the story of John Harrison and his invention of a reliable way to measure longitude -- before that time navigation was pretty much a question not of if you were going to wreck but when you were going to wreck.
Michael Gambon and Brian Cox are magnificent adversaries, science against power (not unlike today in the usa).
This is the better of the 2 greatest (non-pay-TV) miniseries, Lonesome Dove being the other. Roots was good as well but has not as yet made my list. For quality, intensity, and acting Longitude is in step with The Sopranos and Deadwood.