Time tracking and calendar making are so fascinating to me because of the skywatching skill required for our ancient ancestors to do these activities. I have a whole spool of questions I like to unwind occasionally. Do we share the same basic concepts of time as they had? Did they feel the passage of time like we do? How much did they think about time?

The more I learn about timekeeping, the more complex it gets. The layers of knowledge go deeper than I ever realized. Answering the simplest of questions, like how long a day is, can be as simple as one sunrise to the next sunrise, or the answer can ramble through things like sidereal days, the effect of our noncircular orbit, and other bits of minutiae that would overwhelm most people.

But this is Night Sky Tourist, and we will keep the topic interesting and fun for people who aren’t obsessed with the world of horology.




the study and measurement of time

When I first started learning about calendars, I learned that the two that have dominated most cultures were solar and lunar calendars. We’re all familiar with the solar calendar because we use it daily with our Gregorian calendar. It takes into account an approximate year length of 365.25 days. But some cultures, even today, use a lunar calendar that follows the moon’s phases and has a 360-day year. 



The Egyptians were the first culture to create a solar calendar about 5,000 years ago. They used the star Sirius in the constellation Canis Major to discover that one Earth year was 365 days. In the middle of summer, their skywatchers would wake up before sunrise and watch the eastern horizon to see if they could spot Sirius right before dawn. Morning after morning, they would watch until one morning, the Dog Star, as it was nicknamed, would appear just before sunrise.

The day Sirius appeared was the most important day of the year for the ancient Egyptians. It marked the beginning of their new year and signaled the start of the Nile River’s big flooding season. The flooded Nile meant more nutrients for their crops, which meant another year of food and, thus, another year of living.

The Egyptian calendar was simple.

  • One week was ten days
  • Three weeks was one month (30 days)
  • Four months was one season
  • Three seasons and five holy days were one year

Why five holy days? Using this structure for their calendar only brought them to 360 days. They used the five extra days to celebrate the birthdays of the gods, and nobody worked.


In 45 BCE, the Romans established the Julian calendar to bring regularity to a calendar that could stay in step with the solar year. Each year had 365 days, with every fourth year having 366 days to keep up with our solar orbit. The calendar remained in use for over 1,600 years but had a significant problem. A solar year is slightly shorter than 365.25 days, so the Julian calendar got us ahead of ourselves. The calendar got one full day ahead of the solar year every 129 years, or 3.1 days every 400 years. By the time Pope Gregory reformed the calendar in 1582, the calendar was off by ten days.

The Roman Catholic Church wanted a calendar that fixed the date of Easter in relation to the Spring Equinox. They learned that the solar year was slightly shorter than the 365.25 days allotted by the Julian calendar. The new Gregorian calendar established this rule:

“Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 were not leap years, but the year 2000 was.”


It took a little time for countries to adopt the new calendar, but the British and the British Empire held out much longer. Being predominately Protestant, they viewed the new calendar as a step toward accepting the authority of Catholicism. The colonies of the United States didn’t switch to the new calendar until 1752. Turkey didn’t fully adopt the Gregorian calendar until 1926, and Saudi Arabia didn’t do so until 2016. Ethiopia, Nepal, Iran, and Afghanistan are still holdouts and have not adopted the Gregorian calendar.


Lunar calendars are much older than solar calendars and are easier to track than their solar counterparts. It’s not difficult to visually track the moon’s phases, but as with any other calendar, questions arise that require rules to ensure the calendar works.


A lunar month is about 29.5 days in length. When counting twelve lunar months, the calendar ends up with 354 days, making it eleven to twelve days shorter than a solar year. Some cultures make up for this by inserting a thirteenth month every so many years to bring it back into alignment with the seasons.

The lunar Hijri calendar used by most Muslims is purely lunar. That means they never add an extra month to keep the dates in their seasons. That’s why their important festivals, such as Ramadan, are earlier and earlier from one year to the next. The lunar calendars of the Hebrews and Buddhists add an extra month every two or three years.


When does a lunar month begin? Again, it varies from culture to culture. The Islamic and historical Hebrew calendars start a new month at the first visual sighting of the crescent moon. The Chinese lunisolar calendar starts a new month at the astronomical New Moon when the moon is hidden in the sun’s glare. Some Hindu calendars begin a new month on the day after the Full Moon.


The world’s oldest lunar calendar may be in Aberdeenshire, Scotland. Archaeologists found pits in a field that may have had wooden posts. There appears to be a midwinter alignment in the pits, too, which could have served as an astronomic correction to bring the calendar back into alignment with the seasons. 

Illustration of how the pits might have worked. Image credit: BBC

The evidence found at Aberdeenshire dates the pits to 10,000 years old, twice as old as the formal calendars found in places like Egypt. Some archaeologists suggest this could be the place where time was invented.