Today, as many of us eagerly await the coming solar eclipse, is a good time to think about how peoples of the past experienced the sky. While eclipses were rare and sometimes frightening events for ancient cultures, living day to day with the rhythms of the sun, moon, and stars shaped how different peoples measured and thought about time.

Peoples whose way of life depended on mobility, such as hunter-gatherers and pastoralists, had good reason to care about the cycles of the moon. A bright full moon could provide enough light to travel long into the night while at the same time bringing out both prey animals that hunters depended on and

nocturnal predators that could be a danger to pastoralists and their flocks. Timing movement, hunting, and watchful nights around the cycles of the full moon was advantageous to peoples whose lives and livelihoods were shaped by forces like these. The lunar cycle—the period of just over 29.5 days between one full moon and the next—formed the basis for dating systems organized around months alternating between 29 and 30 days.

Agriculturalists, on the other hand, depended more on the sun, whose light and warmth they needed for their crops. Timing planting and harvest to the cycle of the seasons was crucial—plant too early and the crops could die from cold or drought; plant too late and the harvest would not mature before the growing season ended. Since the lunar cycle does not match up with conveniently with the solar year of just under 365.25 days, early farmers had to either adjust their calendars by fudging with the number of months in each year (a process called “intercalation”) or else track the time by counting the days in the year.

But the solar year doesn’t come out to an even number of days, so solar calendars require adjusting as well. We do this today by adding an extra day to February in leap years, but ancient peoples often depended on the observation of astronomical phenomena, such as the solstices and equinoxes or the yearly cycle of observable stars in the night sky. Different cultures approached this problem in different ways. In some places, people built megalithic monuments, like Stonehenge or the aboriginal Australian stone arrangement at Wurdi Youang, to mark critical astronomical alignments. Other peoples, such as the Maya, Babylonians, and ancient Chinese, developed sophisticated mathematical methods of observing and predicting astronomical events. Hindu sages in the fourth through tenth centuries CE calculated the length of the year to an accuracy within a few minutes of modern observations. Pre-modern people pictured familiar images in the constellations of the stars not just out of an impulse to explain the visible world around them but as a memory aid for tracking the movements of important markers of the cycle of the year.

The various lunar, solar, and astronomical ways of calculating time have all left their mark on the cultures who used them. Some important modern holidays, like Christmas and the midsummer holidays celebrated in many parts of the world, fall on or near important days in the solar calendar. Others, like Ramadan, are determined by a lunar calendar and move through the seasons year by year. Still others, like Passover and Diwali, represent a compromise between lunar and solar systems—tied to a season, but falling each year on a day determined by lunar cycles. The tracking of the year through the movement of constellations across the sky, once vital to the survival of agrarian societies, remains with us in the popular pseudoscience of astrology.

Thoughts for writers

In worldbuilding, calendars are useful not just for coordinating the events of your plot but as a reflection of the societies your stories take place in. If the celestial mechanics of your world is anything like ours, people will look to the sky to help them keep track of the conditions that matter in their way of life, but different societies will care about different things. Cultures that place a premium on mobility or who care about the movements of animals—whether predators or prey—will particularly pay attention to the cycles of the moon (or whatever else lights up the night). Sedentary, agricultural societies will need to mark the turning of the seasons by the movements of the sun and stars. A lot of societies will have reasons to combine solar and lunar calendars: herders and hunters need to manage the migration of animals from one season to the next, and lunar cycles are useful for farmers to coordinate market days, social events, and political gatherings. Cultures that have incorporated both pastoral and agricultural traditions are likely to reflect the history of that negotiation in how they mark the passage of the year. The sun, moon, and stars are not just beautiful parts of the natural world; they were vital tools in the lives of people all around the world in history, as they continue to be for many people today.

Image: Two diagrams with the sun and the moon via Wikimedia (currently Getty Center; late 13th c.; ink, paint, and gold leaf on parchment)

History for Writers is a weekly feature which looks at how history can be a fiction writer’s most useful tool. From worldbuilding to dialogue, history helps you write. Check out the introduction to History for Writers here.