Wed. Nov 29th, 2023
The astronomical origins of the calendar.

Most of us live by our calendars to keep our schedules in order.

But did you know that the calendar has astronomical origins?

While constellations were created largely to help people remember important star patterns, they have many other uses. One of them is for the formation of the calendar.

seeing stars


Many cultures use or used the skies to keep track of the time of year.

For example, the ancient Egyptians were searching for the star Sopdet, known today in Canada as Sirius. They knew that each year, when they saw the rise of Sopdet, the annual floods of the Nile would soon come upon them.

This important information would greatly affect their lives. The same thing happened to many people around the world.

The origins of the calendar

Civilizations around the world created different versions of the calendar. However, the one that is internationally recognized today is of Roman lineage.


The ancient Romans created a 12-month calendar based on the lunar months, beginning in what we call March.

The names of the ancient Roman months are as follows:

Original name Meaning
Marcio Month of Mars (god of war)
April Month of Aphrodite (known as Venus, also the Goddess of love and beauty)
Important Month of Maia (goddess of spring)
June Month of Juno (Main Goddess and wife of Jupiter)
Quintiles Fifth month
sextile Sixth month
September seventh month
October eighth month
November Ninth month
December tenth month
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As you can see, this version is two months away. Around 700 BC. C. Januarius and Februa were added. Januarius is the god of doors and Februarius represents Februa, the month in which Roman purity was celebrated.

Almost 550 years later, the start date of the year was officially changed from Martius to Januarius, as Janus was seen as the god of beginnings.

In the year 46 BC. C., Julius Caesar changed the calendar. At that time, astronomers knew that a year lasted 365 ¼ days. When Caesar was assassinated, he had the honor of having the month of Quintillis replaced by his name, July. Years later, the month Sextilis took the name of Caesar Augustus, like August.


Most sources suggest that the origin of the week comes from an ancient Jewish tradition that began between the 6th and 9th centuries BC.

The repeated 7-day cycle became widespread in ancient Persia, Greece and Rome.



The origin of the day has the most complex background.

The origins of the day are related to the belief that a planet ruled each hour of the day. The day was named after the planet that ruled at dawn.

In ancient times, the sun and the moon were considered planets. Therefore, at this time, the known planets were Saturn, Jupiter, Mars, the sun, Venus, Mercury and the moon.

Saturn began ruling the first hour of the day on Saturday. If you go through each hour from then on representing the planets in the above order, you will get 24 hours with different planets ruling them on Saturday.

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As you can see, when they ran out of planets, they started over. The days were then named after the planet that ruled at dawn.

If we continue with this pattern, we obtain the following planets that rule the day at dawn:

Day ruling planet origin of the name in English
Saturday Saturn The day of Saturn.
Sunday Sun Sunday
Monday Moon day of the moon
Tuesday Mars The day of Tire (Tyre was the Norse god of war, equivalent to the Roman god “Mars”)
Wednesday Mercury Wodan Day (Wodan is also known as Odin)
Thursday Jupiter The Day of Thor (Thor was the Norse god equivalent to the Roman Jupiter)
Friday Venus Frigg’s Day (Frigg was the Norse goddess equivalent to the Roman Venus)

The lunar calendar on the back of a turtle

Many First Nations teachings, including those of the Anishinaabe and Haudenosaunee people, use the back of a turtle’s shell as a lunar calendar.

Depending on how we measure a lunar month, the Moon orbits the Earth in approximately 28 days. This means that in one year (365 days), the moon makes 13 revolutions, which gives us 13 lunar months with 28 days each.

Conveniently, if we examine the shell of a turtle, we find the same pattern of numbers of lunar months in a year and days in a lunar month:

For more information on the tortoise shell lunar calendar, see sources such as:

For more on this month’s astronomy, visit our Eyes on the January Skies post.