What observations did Edmond Halley make as astronomer royal to help solve the longitude problem?

As Astronomer Royal, Edmond Halley conducted observations—most notably timing the transits of the Moon across the meridian—with the specific hope that such data would eventually assist in solving the problem of determining longitude at sea.1

Prior to his tenure as Astronomer Royal, Halley also attempted to address the longitude problem by creating magnetic charts of the Atlantic Ocean and parts of the Pacific Ocean during his 1698–1700 scientific voyage. These charts mapped lines of equal compass declination, which he intended to use as a method for finding longitude. However, this approach was never widely adopted because compass declination proved difficult to measure with sufficient accuracy and was found to vary over time.1