Indiaman

The transit of Venus, in which Venus passes between the Earth and the Sun, occurs in couplets eight years apart. These couplets don't return for another century. If you miss them, you miss them forever.
In 1760, astronomer Matthew Sangster leaves his family in Dublin to observe the transit of Venus from India. Despite vowing to return within three years, Matthew's journey is perilous. Delayed by events at sea, the deserts of Arabia, and the Seven Years' War, he misses the first transit.
To salvage his career, he decides to wait in India until Venus returns. Matthew spends the next eight years attempting to survive in a world beyond his understanding. As empires collide around him, the heavens move slowly above. All the while, he struggles with doubt, temptation, and a vengeful warlord intent on killing him.
With Venus drawing closer, Matthew's options are to succeed in his task, or return to his family with nothing.