How Did Ancient People Use Calendars?

Oldest Calendars

The oldest ‘calendars‘ are vast archaeological sites that aligned posts or megaliths (giant stones) with the rising of the Sun or Moon on significant dates, such as the summer or winter solsticeThe earliest structures built as calendars seem to be designed to help calculate the solar and lunar months. Priest-astronomers used these. 

Prehistoric astronomers left no user manuals for their monuments – their uses had to be rediscovered by archaeoastronomersarchaeologists with knowledge of astronomy. 

Warren Field, Scotland, found in 2004, is the earliest site found so far. It tracks events some 10,000 years ago.

Development of Accurate Calendars

Calendar development was often driven by the need to fix religious festivals and observance, an impulse that continued with the formation of new religions such as Christianity and Islam. Both put astronomy to use in this way.

Arab astronomers and engineers were zealous in their pursuit of improved methods for keeping time so that daily prayers could be received by the devout at the right time.

Time-keeping on a larger scale was essential in scheduling religious festivals.

Lunar Calendars 

Time is naturally divided astronomically by the Earth’s orbit around the Sun (a year), the Earth’s rotation (a day) and the phases of the Moon. A lunar month (a full cycle from one new or full moon to the next is approximately 29 1/2 days. A year is 365 1/4 days.

Inconveniently, a year is 12.37 lunar months long. For early societies, a lunar month was a useful and countable period of time, one that could be easily observed and checked just by looking up at the night sky.

But if you use 12  lunar months as the basis of your year, the calendar will drift out of sync quite quickly. It will be a month out after only 3 years, and 6 months out after 18 years. To avoid this, an extra (intercalary) month has to be added every few years.

Egyptian Calendars

The ancient Egyptians began their year with the rising of Sirius (which they called Sopdet) above the horizon before sun rise.

  • System dates to c. 3000 BC.
  • Divided year into 365 days.
  • Used two different calendars.
  • Sirius, a stable star, and the brightest star in the night sky, was the basis for the Egyptian calendar.
  • Ptolemaic rulers depended on calendars.

Other cultures developed independent calendars, notably China and Mesoamerican (Central American) civilizations.

Chinese Yin-Yang Li Traditional Calendar

Origins of the Chinese calendar can be traced back to 14th century BC, though legend says it was invented in 2637 BC.

  • Literally ‘heaven-Earth’ calendar.
  • Used alongside imported calendars such as the Hindu calendar.
  • Used until 1912, when China officially adopted the Western Gregorian calendar.

How the World Works: Astronomy: From Plotting the Stars to Pulsars and Black Holes
How the World Works: Astronomy: From Plotting the Stars to Pulsars and Black Holes 

Anne Rooney

Background on Anne Rooney

Why is Easter Celebrated on Different Calendar Dates From Year To Year?


Standardization of Easter Celebration Date

The Catholic Church fixes the date of Easter, its celebration to mark the resurrection of Christ, using a method set out in AD 325 by the Council of Nicaea. In the first centuries AD, Easter was celebrated on different days by different groups of Christians, the Council of Nicaea sought to standardize it.

Role of Full Moon

Easter is now celebrated on the first Sunday after the full moon occurring on or after the spring equinox. Early Christians couldn’t simply wait to find out when the full moon would fall, then quickly celebrate Easter. They had to fit in Lent – 40 days of fasting – immediately beforehand, so had to know several weeks in advance when that full moon would fall, a task that could only be achieved by keeping astronomical records and projecting into the future.

Easter Dates 

How the World Works: Astronomy: From Plotting the Stars to Pulsars and Black Holes
How the World Works: Astronomy: From Plotting the Stars to Pulsars and Black Holes
Anne Rooney

Background on Anne Rooney

Pluto – Is it a Planet?


A Planet Lurking in the Darkness?

In the mid-1800s, astronomers began wondering if there was something out there besides Neptune – perhaps another planet – that accounted for the discrepancy in Uranus’ orbit. Astronomers were keenly interested in discovering what lurked in the darkness. And they began coming up with names for the unknown planet, including ‘Hyperion’, ‘Planet X’, and  ‘Planet O’.

Teams of astronomers spent years searching for the unknown plant. Finally, in 1930, Planet X was found by Clyde Tombaugh, who was working for the Lowell Observatory at the time. Tombaugh’s story is unusual. He was an itinerant hobbyist – he had no background in astronomy, was self-educated, and built his own telescopes.

Clyde Tombaugh

Clyde Tombaugh with His Homemade Telescope (1928)

In 1928, Clyde Tombaugh built a telescope (the one pictured above) from the crankshaft of a 1910 Buick and parts from a cream separator. He also ground his own mirrors for the reflector. He used this telescope to observe Jupiter and Mars, making drawings of what he saw. He sent his drawings to the Lowell Observatory hoping to get some professional feedback. Instead, he got a job.

Clyde Tombaugh died in 1997. He later had a rather special reward for his work. In 2006, his ashes were carried to Pluto by the NASA New Horizons mission, a space probe sent to study the planet he had discovered.

From Planet X to Pluto

Having finally found Planet X, the next question was, “What should this new-found planet be called?” A worldwide competition to name ‘Planet X’ was held in 1930, and was won by Venetia Burney, an 11-year-old English girl, who proposed the name Pluto after the Greek god of the underworld, who was able to make himself invisible. Burney was rewarded with £5 (5 pounds, UK currency).

Venetia Burney (1918-2009) was an English woman. As the winner of the planet-naming competition, Clyde Tombaugh credited Burney with first suggesting the name Pluto for the planet he discovered in 1930. At the time, she was 11 years old and lived in Oxford, England. As an adult she worked as an accountant and a teacher.

Venetia Burney -age 11-1929
Venetia Burney, age 11

Pluto Becomes a Dwarf Planet

Pluto’s reign as a planet was relatively short-lived.  In 2006, the International Astronomical Union (IAU) agreed on a formal definition for a ‘planet’ for the first time, and in the process, Pluto lost its planet status.

The IAU decreed three key requirements which must be met in order for a celestial body to be designated as a planet – Pluto passed the first two criteria, but failed on the third, that of dominating the area around its orbit, since Pluto’s orbit is cluttered with asteroids and other debris. In addition, one of Pluto’s moons, Charon, is about 1/2 the size of Pluto, which violates another standard expected of a planet.

So, having lost its status as a full-fledged planet, Pluto has become a dwarf planet, designated 134340 Pluto – now it’s just one of many large objects in the Kuiper belt. Other dwarf (minor) planets include Ceres in the asteroid belt and Eris, which lies beyond Pluto’s orbit.

How the World Works: Astronomy: From Plotting the Stars to Pulsars and Black Holes
How the World Works: Astronomy: From Plotting the Stars to Pulsars and Black Holes
Anne Rooney



Ann Rooney
Anne Rooney

. Anne Rooney website
. Royal Literary Fund (RLF)
. Scholastic Magazine

Before turning to full-time writing, Anne completed a PhD at Trinity College, Cambridge, and taught medieval English and French literature at the universities of Cambridge and York. She teaches creative writing as part of the Pembroke-King’s summer program in Cambridge. She is an RLF (Royal Literary Fund) Fellow at Newnham College, Cambridge. She lives in Cambridge and has two daughters.

Anne Rooney has written extensively on modern science, technology and contemporary issues for young people. She has worked in the computer industry for about 20 years, as well as advising educational bodies on various technological matters.

Anne Rooney writes books on science, technology, engineering and the history of science for children and adults. She has published around 200 books on a variety of subjects. Before writing books full-time, she worked in the computer industry and wrote and edited educational materials, often on aspects of science and computer technology.

Books written by Anne Rooney include:

What’s a Typical Place in the Universe Like?

Looking Into Deep Space

Take an Imaginary Journey Into Deep Space 

Imagine traveling a few hundred miles up, into the sky, from the Earth’s surface. At this point, you’d be in the slightly more typical environment of space. But, you are still being heated and illuminated by the sun, and half your view is still taken up by the Earth itself. A typical location in space has none of those features.

So, travel a few trillion miles (1 light year = 5.879 x 1012 miles = 1 trillion miles) further in the same direction. You are now so far away that the sun looks like other stars. You are at a much colder, darker and emptier place. But, it is not yet typical – you are still inside the Milky Way galaxy, and most of the places in the universe are not in any galaxy.

Continue traveling until you are clear outside the galaxy – say, a 100,000 light years from Earth. At this distance you could not glimpse the Earth even if you had the most powerful telescope that humans have built. But, the  Milky Way still fills much of your sky.

To get to a typical place in the universe, you have to imagine yourself at least a 100,000 times further out, deep into intergalactic space – finally you would have arrived in a typical location.

A Typical Place in the Universe is Dark, Cold, and Empty

Deep Space

The sky would be pitch blackThe nearest star would be so far away that if it were to explode into a supernova, and you were looking directly at it when its light reached you, you would not even see a glimmer. That is how big and dark the universe is.

It’s cold. The temperature is 2.7 kelvin, which means 2.7 degrees above the coldest possible temperature, absolute zero, or about 270 degrees Celcius (518 degrees Fahrenheit) colder than the freezing point of water. That’s cold enough to freeze every known substance except helium, which is believed to remain liquid right down to absolute zero, unless highly pressurized.

It’s empty. The density of atoms out there is below 1 per cubic meter. That’s a million times sparser than atoms in the space between stars, and those atoms themselves are sparser than in the best vacuum that human technology has yet achieved.

Almost all the atoms in intergalactic space are hydrogen or helium, so there is no chemistry. No life could have evolved there, nor any intelligence. Nothing changes there. Nothing happens.

That’s the unimaginably desolate environment which is typical of the universe – it’s a measure of how untypical the Earth and its chemical soup are, in a straightforward physical sense.

The Beginning of Infinity
The Beginning of Infinity: Explanations That Transform the World

David Deutsch



David Deutsch
David Deutsch

Born 1953

. David Deutsch website
. Wikipedia
The Infinite Optimism of Physicist David Deutsch (Scientific American interview)

David Deutsch, FSR (Fellow of the Royal Society) is an Israeli-born British physicist at the University of Oxford. He is a Visiting Professor in the Department of Atomic and Laser Physics at the Centre for Quantum Computation (CQC) in the Clarendon Laboratory of the University of Oxford.

Deutsch pioneered the field of quantum computation by formulating a description for a quantum Turing machine, as well as specifying an algorithm designed to run on a quantum computer. He is a proponent of the many-worlds interpretation of quantum mechanics.

Books David Deutsch has written:

David Deutsch Interview – Which Laws of Nature Are Fundamental?


How is the Brain Our Personal Time Machine?

Back to the Future

Our Personal Time Machine

The possibility of time travel has intrigued people for centuries. Notwithstanding all the creative books and movies, as well as, serious scientific research, one thing remains constant – our brain, the best time machine of all, is already part of our everyday existence.

The human brain, the most sophisticated device in the known universe, lies hidden away within our skull. The brain, a gelatinous mass weighing on average 3 pounds, consisting of 100 billion cells, is an organ that serves as the center of the nervous system.

Four Reasons Why the Brain is a Time Machine

1. The brain is a machine that remembers the past in order to predict the future.

The brain is at its core a prediction or anticipation machine. On a moment-by-moment basis your brain is automatically attempting to predict what is about to occur. These short-term predictions, up to a few seconds into the future, are entirely automatic and unconscious.

We also continuously attempt to make long-term predictions. In order to predict the future the brain stores a vast amount of information about the past, and adds temporal labels (dates) to these memories, allowing us to review episodes of our lives organized on a timeline.

2. The brain is a machine that tells time.

Your brain performs a wide range of time-sensitive computations, including those necessary to recognize a face, or to choose the next move in a chess game. When your brain performs the computation of telling time, it not only measures the seconds, hours and days of our lives, but recognizes and generates temporal patterns, such as the intricate rhythms of a song, or the carefully timed sequence of movements that allow a gymnast to perform a round-off backflip.

From the ability to throw a spear at a moving target, to timing the punch line of a joke, to playing Beethoven’s Moonlight Sonata on the piano, to the ability to regulate daily sleep-wake cycles and monthly reproductive cycles, virtually every aspect of our behavior and cognition requires the ability to tell time.

3. The brain is a machine that creates the sense of time.

Unlike vision or hearing, we do not have a sensory organ that detects time. Time is not a form of energy or a fundamental property of matter that can be detected via physical measurements. Yet, much in the same way that we consciously perceive the color of objects (the wavelengths of reflected radiation), we consciously perceive the passage of time.

Like most subjective experiences, our sense of time undergoes many illusions and distortions. The same duration – as measured by an external clock – can seem to fly by or drag depending on a multitude of factors.

Distorted or not, the conscious perception of the passage of time, and that the world around us is in a continuous flux, is among the most familiar and undeniable experiences of all.

4. The brain allows us to mentally travel back and forth in time.

The race to predict the future was won by our ancestors when they developed the ability to understand the concept of time and mentally project themselves backward into the past and forward into the future – that is, to engage in mental time travel.

Abraham Lincoln reportedly said, “The best way to predict the future is to create it” – this is exactly what mental time travel has allowed us to do. We went from predicting nature’s capricious ways to creating the future by overruling nature itself.

We have all mentally re-experienced the joy or sorrow of past events and run alternative simulations of those episodes to explore what could have been. In the other direction we jump into the future every time we dread or daydream about what may come, and we simulate different plot lines of our future lives in the hope of determining the best course of action in the present.

Your Brain is a Time Machine: The Neuroscience and Physics of Time
Your Brain is a Time Machine: The Neuroscience and Physics of Time
Dean Buonomano


Dean Buonomano
Dean Buonomano
born 1965

. Wikipedia
. Buonomano Lab

Dean Buonomano is an American neuroscientist, psychologist and author. Buonomano is a professor at UCLA. He joined the UCLA faculty in 1998 and has worked in the department of Behavioral Neuroscience there ever since. His research focuses on neurocomputation.

Buonomano has been described as one of the “first neuroscientists to begin to ask how the human brain encodes time.” He is a proponent of the theory that timing and temporal processing are so critical to brain function that most neural circuits are capable of telling time. He developed the influential theory that the brain tells time and processes temporal information not through an internal clock as scientists worldwide had previously theorized, but instead as a result of neural dynamics.

Buonomano is one of the developers of the general neurocomputational framework that he refers to as state-dependent networks or dynamic attractors, and others refer to as liquid state machines or reservoir computing.

Buonomano Lab utilizes research methods such as computational modelingin vitroelectrophysiologyOptogenetics, and human psychophysics to conduct research observing how individual neurons and the brain as a whole perceive and respond to time.

Books written by Dean Buonomano include:

“Your Brain is a Time Machine” – Talk by Dean Buonomano at Google, 2017