From Sundials to Atomic Clocks How the Universe Shapes Our Understanding of Time
Clocks: In a world of booked calendars and packed schedules, it’s hard to imagine life without them. But as it turns out, we’ve been telling time long before their invention. For millennia, people have used the stars to understand and organize the movement of time.
As early as 3500 BCE, the Egyptians began building obelisks to divide their days into parts resembling the hours we know today. The moving shadows created by the Sun hitting the obelisk helped to divide morning from afternoon, while the length of the noontime shadow showed the year’s longest and shortest days. This is the same principle behind sundials, which you may be more familiar with. But watching shadows move across the Earth isn’t the only way the sky can help us keep time.
Around the same time the Egyptians were building obelisks, a 366-day calendar structured on the movements of the Sun and the moon was being developed in China. But after a few centuries of use, astronomers began noticing that the calendar became inaccurate every 300 years or so. The reason? Well, the stars, including the Sun, aren’t as “fixed” in the night sky as they appear to be.
There’s movement happening; something that we call precession. As the Earth’s rotational axis slowly moves, the stars shift in our night sky. About every 26,000 years or so, we get a new view of the stars. Today, most of us know that Polaris is the North Star. But 26,000 years ago, Thuban— a star in the ‘tail’ of the constellation Draco—was the marker of the poles! By the 5th century CE, Chinese scholars had figured out the whole precession problem and factored it into their calendar. And roughly 500 years later, one of the greatest time-keeping achievements of ancient China was unveiled: a five-story astronomical clock tower. This mechanical structure ran on a day and night time-keeping wheel that was powered by water!