In the 1700s, the Jesuits determined their longitudinal positions, or longitudes, primarily through astronomical observation and measurement techniques. Here are some of the methods they employed:
- Solar Observations: The Jesuits used instruments like quadrants, astronomical globes, and telescopes to observe the position of the Sun at noon. By comparing the local solar noon with a standard reference (usually the Greenwich Mean Time at a specific date), they could calculate the difference in longitudes.
- Lunar Observations: The position of the Moon relative to specific stars was observed and recorded. This information was then compared with known lunar ephemerides (tables predicting the Moon's position over time) to calculate longitudes.
- Celestial Navigation: Jesuit astronomers also made use of celestial navigation methods, particularly by measuring the positions of stars and planets at specific times. These observations were compared with astronomical tables like almanacs, which provided predicted positions of celestial objects, to calculate the longitudes.
- Transit of Venus: During the transit of Venus, when the planet passes in front of the Sun as viewed from Earth, the Jesuits calculated the time of the phenomenon from different locations. By comparing the timings and triangulation, they could determine the longitudes.
- Chronometers: In the latter part of the 18th century, the Jesuits began using accurate chronometers, which are precise timekeeping devices, to measure the time difference between their location and a reference point. This time difference was then converted to longitudinal distance.
- Land Surveying: In some cases, the Jesuits used land surveying techniques to estimate distances between two points and then determined the longitudes based on these distances.
It is essential to note that these methods were not exclusive to the Jesuits and were also used by other astronomers, navigators, and scientists of the time.