Little is known of Keyser's life outside of his astronomical observations and East Indies voyages. After several trips, one to Brazil, and one to Nova Zembla, Keyser participated as the chief navigator and head of the steersmen for the first Dutch voyage to the East Indies, which left Texel with four ships on 2 April 1595 under Cornelis de Houtman. He had been trained by cartographer Petrus Plancius in mathematics and astronomy. Plancius, a key promoter to the Dutch East Indies expeditions, had instructed Keyser to map the skies in the southern hemisphere, which were largely uncharted at the time. When the fleet finally was able to obtain fresh supplies at Madagascar on 13 September, 71 of the 248 sailors had died, most of scurvy. The surviving crew stayed for several months on the island, to recover and make repairs, at which point Keyser probably made most of his celestial observations. He was aided in this by Frederick de Houtman and Vechter Willemsz. After leaving Madagascar, it took another four months for the ships to reach Sumatra and finally Bantam on Java. Trade negotiations went sour, perhaps caused by Portuguese instigators, perhaps by inexperience, and the crew was forced to find drinking water and other supplies on Sumatra across the Sunda Strait. Keyser apparently died during this crossing. On 14 August 1597, 81 survivors made it back to Texel, including de Houtman, who probably delivered Keyser's observations to Plancius.
Constellations
From the observations of the expedition, Plancius created a celestial globe in late 1597, published by Jodocus Hondius, showing twelve new constellations of the southern sky that have become accepted among the standard constellations codified by modern astronomers. The majority were named after various animals that 16th century explorers had encountered. Willem Janszoon Blaeu copied these constellations on a 1602 globe and created a new globe in 1603 incorporating further observations made by de Houtman on a second voyage to the East Indies. Johann Bayer included these new southern constellations in his influential 1603 star atlasUranometria, crediting them to "Petrus Theodori", but not acknowledging their earlier publication. Bayer is therefore often mistakenly credited for introducing them. Keyser is commemorated by minor planet Pietkeyser.
