In societies based around agriculture, hunting, and other pursuits that involve human interaction with the natural world, time discipline is a matter governed by astronomical and biological factors. Specific times of day or seasons of the year are defined by reference to these factors, and measured, to the extent that they need measuring, by observation. Different peoples' needs with respect to these things mean sharply differing cultural perceptions of time. For example, it surprises many non-Muslims that the Islamic calendar is entirely lunar and makes no reference at all to the seasons; the desert-dwelling Arabs who devised it were nomads rather than agriculturalists, and a calendar that made no reference to the seasons was no inconvenience for most of them