Latitude
-When looking at a globe, lines of latitude appear horizontal and travel around the globe in an East/West or West/East direction. In other words, knowing the latitude would enable an observer to tell his position relative to the North and South Pole.1 Finding latitude presented fewer technical and mathematical challenges than calculating longitude.2 Using the equator as a reference point, one could use an instrument, such as a sextant, to calculate fairly precisely the angular height of the sun at noon.2 Moreover, the moon, planets, and stars were also acceptable targets as they cross over an observers meridians, for the very reason that their altitudes remain constant enough that there is no need to know the exact time of observation.3 Even people before the early modern period were able to accurately tell their latitudes with the use of instruments such as astrolabes and quadrants.4
Longitude
On a globe, lines of longitude run perpendicular to lines of latitude in a North/South direction. Because the Earth spins on its axis in a counterclockwise direction from West to East, an observer’s longitudinal position is constantly changing. And herein lies the difficulty in determining longitude. Thus, to calculate longitude, it first needs to be generated with respect to an astronomically sophisticated metropolitan center.1 In early modern times, this metropolitan center would have been Greenwich, England, with a longitude of 0º.1 From here, various methods would have been employed to determine the longitude of an observer’s position. Lunar occultations (or eclipses)—when the moon crosses between the observer and a celestial object—were routinely observed by astronomers to determine longitude at permanent and temporary land-based observatories.3 The circling moons of Jupiter could be used as a space-born clock.1,5 Also, the lunar distance method could be used by measuring the angular separation between the moon and other bright objects like stars, planets, and even the sun.1,3,5 All three of these required complex math which took hours to compute, as well astronomical tables to use the pre-determined Greenwich hour angle in order to determine longitude.1,5 More precise nautical almanacs wouldn’t be available until 1767, when Nevil Maskelyne developed them in the race for longitude by the lunar distance method. A last method, using a sea clock, or chronometer, is different than the others in that it simply uses time to determine longitude. The clock would be set to a metropolitan center such as Greenwich, England. By taking it on the ship, the observer could deduce his local time, and then take the difference in times and thus calculate longitude (more on this later).