The ancient Roman origins of the leap year

How the months of the year got named

Curious about how the months got their names? This article from The Old Farmer's Almanac explores the origins of today’s Gregorian calendar. Based on the ancient Roman lunar calendar, the months were named after Roman gods until winter when the names changed to numerical titles. Read along to learn the significance and intentions behind labeling each lunar cycle.

What was the longest year in history?

In 46 BCE, Julius Caesar declared the year would last 445 days. This four-minute podcast and accompanying article explain why. Caesar wanted to make the following year start after the winter solstice, an achievement attainable due to his tinkering with the Roman calendar. But to start the year right, Caesar had to stretch the transitional year, leading to the creation of Caesar's Year of Confusion, the longest year in history.

January is named after Janus, the Roman god of transitions

In 46 BCE, Julius Caesar introduced the Julian calendar, which established Jan. 1 as the beginning of the new year. The month was named after Janus, the Roman god of transitions, who had two faces—one looking back at the past, the other forward to the future. While the Babylonians also made promises as part of their celebration of the new year, the Roman tradition became more focused on looking backwards to plan for the coming year.

The world's different calendars, explained

The calendar we use to mark the days, months, and year can feel set in stone, but the history of calendars shows these organization systems can and do change. Parse the fascinating, complex details of the Gregorian, Julian, Chinese, Hebrew, and many more calendars in this comprehensive, well-executed explainer video.

How lunisolar calendars work

Despite being called Lunar New Year, the Chinese calendar that the holiday is based on is actually lunisolar, or based on both the sun and the moon. Many religious holidays—including Easter, Ramadan, Chinese New Year, and Passover—do not have fixed dates according to the now-ubiquitous Gregorian calendar.

The American colonies recognized New Year’s Eve in March until 1752

England followed the Julian calendar, established by Julius Caesar, which had an inaccurate method of calculating leap years. Because of that, New Year’s Day eventually slipped into the spring and was linked with the Christian festival of the Annunciation, which was on March 25. That date carried over to the American colonies, which didn’t change the first day of the new year until 1752, after England adopted the newer Gregorian calendar.

The Aztecs synced their solar calendar with a complex 260-day ritual cycle

Researchers have uncovered how the Aztecs aligned their agricultural solar calendar with a symbolic 260-day ritual calendar, utilizing celestial tracking and monuments. Evidence from ruins and writings explains why their new year corresponded to February 23 on the Gregorian calendar.

Some believe Valentine's Day evolved from the Roman holiday Lupercalia

The festival, which took place in mid-February, featured Roman priests sacrificing a goat and using its hide to lightly slap women and the land, believing it would bring fertility. Some believe that the Catholic Church repurposed the pagan holiday as a Christian one by attaching it to a saint, but scholars are not certain that this is true.

How the ancients calculated Earth's size correctly

Watch this seven-minute video to hear Carl Sagan explain how ancient Greek polymath Eratosthenes correctly calculated the size of the Earth in the third century BCE. At some point, Eratosthenes noticed a discrepancy in the length of shadows cast in different cities during the summer solstice. This observation grew into a simple experiment that enabled Eratosthenes, the head librarian at the Library of Alexandria, to calculate the Earth's size precisely.

The hour originated from an ancient Egyptian astronomical system

The movements of astronomical objects, measured in a day and a year, form the foundation of our sense of time. The Egyptians divided each day and night into 12 time eras, using arbitrarily selected stars as references rather than movements of specific celestial objects.

Ancient civilizations predicted eclipses by tracking different cycles of their recurrence

The periodic nature of the orbits of the sun, moon, and Earth results in several series called saros cycles, in which the geometric configuration of these celestial bodies that produces an eclipse recurs after about 6585 days. Because eclipses are viewable from different locations on Earth, there exist many saros cycles, one for each ecliptic configuration.

Saturnalia was a Roman festival that helped shape Christmas

Ancient Romans celebrated Saturnalia in mid-December, honoring Saturn, the god of agriculture, with week-long street festivals. These celebrations featured gift exchanges, lavish meals, and gambling, which was prohibited in public at the time.

Estimate the costs of an ancient Roman vacation

Thanks to a sophisticated system of roads, Romans could travel great distances for their Roman holidays. This interactive resource from Stanford allows you to create fictional journeys around the empire, seeing approximate routes, costs, and time.

A day in the life of a Roman soldier

In 15 CE, Roman legionary Servius Felix was eight years into a 25-year career as a warrior for the Empire. Born to a poor farmer who hoped to retire and receive several acres of land, Servius served amid fights with Germanic tribes from the north. He hopes to marry a woman back home and mourns the loss of his fallen friends.

Stanford built a Google Maps for the Roman Empire, revealing how much it cost to travel in the ancient world

Bake bread like a Roman, and honor Ceres, the goddess of wheat

Grains made up about 70% of the Roman diet, and the word "cereal" stems from the goddess Ceres—the Roman equivalent of Greece's Demeter—to whom they prayed for good harvests. Cato the Elder, a famous statesman, left behind a simple home recipe for a Roman loaf.

The history of the Caesar salad

Despite what you might guess from its name, the Caesar salad wasn't invented in ancient Rome. The popular dish was created in the 1920s by two Italian brothers who'd emigrated to the United States and opened a popular restaurant in Tijuana called Caesar's Palace. This history of the popular entrée digs into that history and unpacks how it became a restaurant staple.

Earth's core is about 2.5 years younger than its surface according to relativity

Due to differences in where objects are located within gravitational fields, time will run at different rates, which has major impacts on technology that relies on synchronization. For example, GPS satellites run about 38 microseconds faster every day than clocks on Earth's surface, resulting in GPS discrepancies of about six miles, were it not for software corrections.

How ancient Romans invested and worked

The first stock exchange didn't open for business until the early 1600s, but that didn't stop the Ancient Romans from working and investing their money centuries before. While Romans are better known for their political inventions than their economic ones, understanding how they viewed money can help paint a better picture of the Roman mindset that created systems like democracy. Read this article to learn more.

The 12 tables: Rome's attempt to fairly enforce the law

It's unclear exactly why ancient Romans drafted the Laws of the Twelve Tables in the mid-400s BCE, but the result was a collection of precisely worded, written laws that could be made known to everyone. This article explores the history and importance of these 12 bronze tables, which were an attempt to make the law equally applicable to all and were destroyed when Rome was attacked in 390 BCE.

Child mortality rates since ancient Rome

One of the great achievements of modern humanity has been the sea change in child mortality rates—the chance that a newborn eventually lives past puberty. For thousands of years, children had a 50% chance of not entering adulthood. About 300 years...

The ancient Romans invented the first known artificial sweetener—it involved lead

The product was known as "sugar of lead." It was made by boiling wine in lead pots, concentrating the grapes' natural sugars and adding sweetness from the lead that leached into the grapes. The substance was used until roughly the 1880s for medicinal purposes and also to sweeten wine. (Some users may encounter a paywall.)

Enough sunlight hits Earth in just 90 minutes to power the planet for an entire year

Despite this, in 2023, only 5.1% of electricity in the US came from solar energy. Solar panel technology has nonetheless made significant advancements since the 19th century, largely due to NASA's efforts in the space race.

Galileo’s discovery of Jupiter’s moons led to multiple scientific advancements

Beyond challenging Earth-centric views upon their discovery in 1610, the Galilean moons were also later analyzed to contribute to early astrometry and astronomical interferometry. Measurements of eclipses were also used to estimate the speed of light.

Cleopatra was a rare female pharaoh who sided with Rome to preserve power

The figure is famous for her love affair with Roman leader Mark Antony and her suicide in 30 BCE to escape capture. Parse through the many versions of Cleopatra with more recent archaeological evidence, including a 2007 coin depicting the queen as less glamorous than historically believed.

One pop-culture historian fooled the nation with a fake origin story of April Fools' Day

In 1983, Joseph Boskin, a professor at Boston University, was interviewed by the Associated Press to discuss the history of April Fools'. Boskin shared a made-up story about a jester who took over Rome for a day and proclaimed that April 1 would henceforth serve as 24 hours of silliness. AP published the fib about "King Kugel," creating buzz and confusion for news outlets around the country.

The term 'algorithm' originated from the Latinized name of al-Khwārizmī, a 9th-century mathematician whose works introduced Arabic numerals and the decimal-based numeral system to Europe and whose treatise 'Al-Jabr' gave algebra its name.

Learn more about the history of fireworks, which dates back to an accidental invention during an alchemist's quest for information about immortality.

While the sun's gravity keeps the planets in orbit, their gravity also pulls on the sun, causing it and everything else in the solar system to orbit around a mutual balance point called the barycenter—the true center of the solar system.

For centuries, it was common to sleep in two shifts: a few hours at the beginning of the night, and a longer period beginning around midnight

Visualize the path of every solar eclipse from 1900 to 2099 and see what each looks like when observed from different locations on Earth.

Track the location of the Voyager probes, currently between 13 and 15 billion miles away from Earth.

The first line of 'immortal' human cells, which can divide indefinitely and are used in scientific research, was nonconsensually taken from a Black woman.

Clever Hans was a 1900s horse that was believed to be able to solve math equations, tell time, and read and comprehend German, until a psychology student discovered how he got his answers.

Apeirophobia is an overwhelming fear of living forever, perhaps stemming from the brain's inability to conceptualize an infinite life.

The idea for the Federal Reserve was conceived in secret in 1910—with bankers donning duck hunt gear for disguise while traveling to a remote country club on Georgia's Jekyll Island

Explore chaos in this simulation of two double pendulums, where small differences in the release angles produce wildly different outcomes.

Apollo astronauts left items in the moon's 'toss zone,' including golf balls, family photos, and bags of human waste, to make room for lunar rocks.

El Niño, Spanish for 'the boy,' was named after Jesus Christ because the phenomenon resembled the weather changes observed around Christmastime.

The USPS spent $3.97B in 2024, accounting for .058% of the federal budget.

Long before Major League Baseball began allowing instant replay for balls and strikes, an umpire reluctantly used the technology to determine whether a hit was a home run, a move that inadvertently ushered in a new era for the sport.

The US Constitution is widely regarded as the longest-standing written national constitution still in force—see how it has influenced rights worldwide.

20,000 letters to the advice columnist 'Dear Abby' provide insight into prevailing sex, religion, and LGBTQ fears, anxieties, and concerns from the 1950s onward.

Planarian flatworms are small, aquatic worms with exceptional regenerative capabilities. When cut, the worms can regenerate multiple bodies in a matter of weeks.

Salvador Dalí believed he was his own dead brother, who had died nine months before he was born

Sunflowers have long been a symbol of nuclear disarmament because they can extract large amounts of radioactive cesium from soil.

The Mona Lisa's celebrity status and repeated restorations have obscured what made the painting so revolutionary for its time

Take a 3D tour of the Hajj, Islam's annual pilgrimage to Mecca, undertaken by over 1.5 million Muslims.

Scientists testing how climate change affects plants found, in a years-long simulation study, that poison ivy grew 70% faster with the resulting extra carbon dioxide.

A 2019 Facebook event called on attendees to "Storm Area 51, They Can't Stop All of Us," resulting in countless memes, the creation of a music festival, and eventually the arrests of those who did try to approach the area.