If you’re lucky enough to see the solar eclipse when it passes over America on August 21, you’ll bear witness to a centuries-long legacy. That’s because total eclipses of the sun aren’t isolated incidents that occur at random. They belong to interconnected eclipse families that humans have been using to track the phenomena since long before the first telescope was invented.

In the latest installment of StarTalk on Mashable, Neil deGrasse Tyson chats with meteorologist Joe Rao about the science behind eclipse families. According to Rao, eclipses follow Saros cycles which repeat every 18 years, 11 days, 8 hours. Astronomers keep track of many different Saros cycles. The eclipse on August 21, for example, is a member of the family Solar Saros 145. Every 18 years a Saros 145 eclipse falls over a different third of the Earth. In 1999, the great American eclipse’s “cousin” appeared in the skies over Europe and south Asia, and 18 years before that another relative could be seen over modern Russia. The Solar Saros clan can be traced all the way back to 1639 and it will keep going until 3009.

Today, scientists have space-age technology that allows them to track the moment of totality down to a fraction of a second. But thousands of years ago, before such satellite-tracking equipment was invented, ancient Babylonians only knew what they could observe from Earth. Their eclipse calculations ended up serving them pretty well: They were able to predict the same 18-year cycle we know to be true today.

Saros 145 isn’t the only family of eclipses making its way around the Earth. There are enough solar eclipse cycles to make the event a fairly common occurrence. If you’re curious to see how many will happen in your lifetime, here’s where you can calculate the number.

[h/t Mashable]