The Mathematics of the Longitude - Department of Mathematics

Chapter 8
Lunar Distance Method
8.1 Introduction
Though
the Jupiter's satellites eclipse method proved to be a better method
than the lunar eclipse method, it was still not a successful method to be
used at sea. This is because navigators were unable to observe the satellites
with a telescope on the rocking deck of a ship. Hence, there was a
desperate need to discover a practical method of determining longitude at
sea.
In 1475, a well-known German astronomer Regiomontanus suggested the
lunar distance method to find longitude. Lunar distance method is the
measurement of the angular distance between the moon and a star or
between the moon and the sun. While his theory was sound, the 15 th
century did not possess the instrument for measurement the lunar distance
and the lunar tables to make it work. In was not until mid 15 th century
when the cross-staff was proposed for making lunar distance measurements
(Figure 8.1).
Figure 8.1 Lunar distance from a star is being measured
with a cross-staff.