Final answer:
Ptolemy erroneously underestimated the Earth's circumference, and thus the size of a degree of longitude at the equator is not approximately 90 km (56 mi) but rather about 111 km (69 miles).
Step-by-step explanation:
The statement that Ptolemy estimated the size of the degree of longitude on the equator at about 90 km (56 mi) is False. Ptolemy's estimate of the Earth's circumference was smaller than the actual circumference, which led to the underestimation of the size of the Earth, and correspondingly, each degree of longitude along the equator. The accurate value for a single degree of longitude at the equator is closer to 111 kilometers (69 miles).
Longitude is essential in determining one's position east or west of the Prime Meridian, which passes through Greenwich, England. The equator, being the largest circle of latitude, has the longest length per degree of longitude. It is important to note that Ptolemy's work was foundational in the development of astronomical and geographic understanding, despite inaccuracies in his measurements.