Why do we measure time in the way that we do? Why is a week seven days long? At what point did minutes and seconds come into being? Why are some calendars lunar and some solar? The organisation of time into hours, days, months and years seems immutable and universal, but is actually far more artificial than most people realise. The French Revolution resulted in a restructuring of the French calendar, and the Soviet Union experimented with five and then six-day weeks. Leofranc Holford-Strevens explores these questions using a range of fascinating examples from Ancient Rome and Julius Caesar's imposition of the Leap Year, to the 1920s' project for a fixed Easter. ABOUT THE SERIES: The Very Short Introductions series from Oxford University Press contains hundreds of titles in almost every subject area. These pocket-sized books are the perfect way to get ahead in a new subject quickly. Our expert authors combine facts, analysis, perspective, new ideas, and enthusiasm to make interesting and challenging topics highly readable.

‘The Ancient Egyptian Book of the Moon: Coffin Texts Spells 154–160’ argues that Coffin Texts spells 154–160, recorded at around the beginning of the 2nd millennium bce, form the oldest composition about the moon in ancient Egypt and in the whole world. The detailed analysis of these spells, based on a new translation, reveals that the spells provide a chronologically ordered account of the phenomena that happen during a lunar month. It is argued that through a wide variety of mythological allusions, the separate texts – after an introduction which explains the origins of the month (spell 154) – describe the successive stages of the monthly cycle: the period of invisibility (spell 155), waxing (spell 156), events around the full moon (spell 157), waning (spell 158), the arrival of the last crescent at the eastern horizon (spell 159), and again the conjunction of the sun and the moon when a solar eclipse can occur (spell 160). After highlighting the possible lunar connotations of each spell, further chapters in the book investigate the origins of the composition, its different manuscripts preserved on coffins coming from Hermopolis and Asyut, and the survival of the spells in the later mortuary collection known as the Book of Going Forth by Day.