There are more people on Earth today than ever before — 8.2 billion and counting — which represents around 7% of all 117 billion people estimated to have ever lived throughout the course of human history. The figure comes from the Population Reference Bureau, which released its first estimate in 1995 and has updated it occasionally in the years since. As with most math on this scale, the calculus wasn’t easy. That’s partly because our knowledge of history is ever-evolving: When the bureau initially calculated the number, modern Homo sapiens were thought to have first appeared around 50,000 BCE, but recent discoveries put the actual date much earlier.

All the cattle on Earth weigh more than all the humans.

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With a biomass of about 386 million tons, humans weigh a lot — but we don’t weigh as much as our bovine neighbors. An estimated 1.3 billion cattle share the planet with us, and their biomass comes out to an absolutely beefy 716 million tons.

Three main factors go into the math: how long humans are thought to have been walking the Earth, the average population during different eras, and the number of births per 1,000 people during said eras. As you might imagine, the growth has been astronomical. There were just 5 million humans in 8000 BCE, 300 million in 1 CE, and 450 million in 1200. And while the bureau acknowledges that this is “part science and part art,” even being off by a few billion gives us a ballpark figure to imagine all the people who came before us.

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Aside from long blond hair, horned helmets are probably the most famous Viking accessory — but that would have been a surprise to the real Scandinavian warriors who plundered Europe between the ninth and 11th centuries. The Viking horned helmet convention dates only to the 19th century: In 1876, costume designer Carl Emil Doepler introduced it in Richard Wagner’s famous opera Der Ring des Nibelungen (The Ring of the Nibelung, often called the Ring Cycle). At the time, Germans were fascinated with the story of the Vikings, so Doepler plopped the ancient headdress of the Germans — the horned helmet — on Wagner’s Viking protagonists. The opera proved so popular that by 1900 the horned helmet was inextricably entwined with Vikings themselves, appearing in art, ads, and literature.

Bluetooth technology is named after a Viking king.

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Bluetooth is named after Viking king Harald Gormsson, known for a dead (“blue”) tooth. But why? “[He] was famous for uniting Scandinavia just as we intended to unite the PC and cellular industries with a short-range wireless link,” Intel’s Jim Kardach has said.

Yet during the Viking era, Norse warriors never actually wore horned helmets — and especially not during battle, where they’d probably have gotten in the way. Some artifacts, such as a tapestry discovered with the famous Oseberg ship burial in 1904, do depict horned figures, but these “horned” occurrences happened only — if they happened at all — during rituals. To date, archaeologists have uncovered only two preserved Viking helmets: Both are made of iron, both have guards around the eyes and nose, and both are entirely without horns.

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“Thirty Days Hath September” may be a useful mnemonic device, but there are times when that poem might have led you astray. In 1752, September was only 19 days long in the U.K., due to the Calendar (New Style) Act of 1750. That parliamentary move transferred the country from the Julian calendar to the Gregorian calendar — the former having overestimated each year’s length by about 11 minutes.

Back in 1582, Pope Gregory XIII had declared that all countries under the dominion of the Catholic Church needed to adopt the Gregorian calendar, but many Protestant nations — such as England — resisted the pope’s demands. During the 18th century, as international trade and diplomacy increased, Britain and its colonies began to view the adherence to a now-antiquated Julian calendar — first implemented in Rome by Julius Caesar in the first century BCE — as more spiteful than practical. When the U.K. finally converted to the Gregorian calendar in 1752, they jumped straight from September 2 to September 14, skipping the 11 days in between to make up for the errors of the Julian calendar. Though protests against the law arose among some anti-reformers — who purportedly rallied behind the slogan “Give us back our 11 days!” — the calendar was adopted without any further delay. 

New Year’s Day used to occur in March.

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The Babylonians of ancient Mesopotamia celebrated the new year in March, because the holiday was tied to the vernal equinox. The concept of a January 1 new year was introduced by the Romans in 153 BCE, though that date wasn’t adopted globally until Pope Gregory’s reforms in 1582.

Eastern Orthodox nations, such as Russia and Greece, also initially resisted the papal decree, waiting to transition to the Gregorian calendar until 1918 and 1923, respectively. By then, so much time had passed that those two countries skipped 13 days to bring their calendars up to speed. Russia’s stubbornness also affected Alaska; upon being sold to the U.S. in 1867, the former Russian territory leapt straight from October 6 to October 18.

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