Celestial Time Keeping, Solstices, Atomic Clocks and the Celts

There I was thinking about the solstice – the longest day of the year  – and once again I missed it in the sense that I am a bit late with this post as well as actually missing the actual day itself. I used to love it when I was a child in Ireland. Summer time would have kicked in and it would be bright until at least 21:30 or even later. Anyway, here in Perth, in the southern hemisphere I decided to keep a lookout for it – Jun 21 in Ireland and Dec 21 here in Perth. Or so I thought.  How do you work out the longest day? By counting the hours and minutes of sunlight, I suppose and even that managed to elude me, for the most part!

As if that accurate counting of hours and minutes weren’t enough of a hurdle, Lord Kelvin first came up with the idea of using atomic transitions to measure time when he was in his mid-fifties. Born in 1824, he defined absolute zero as -273.15 Celsius or -459.67 Fahrenheit and after whom the base unit of absolute temperature in the International System of Units was named. (Probably something to do with living through Irish winters.) All I ever knew about time was that it was the instrument by which change is measured! A modern dictionary definition of time is more exacting ‘Time is the indefinite continued progress of existence and events that occur in an apparently irreversible succession from the past, through the present, to the future.’

Here’s my attempt to measure time –


Sat Dec 14

Sun Dec 15

Mon Dec 16

Tue Dec 17

Wed Dec 18

Thu Dec 19

Fri Dec 20

Sat Dec 21

Sun Dec 22

Mon Dec 23

Sun Rise










Sun Set










Hours of light









Anyway, one hundred and forty years after Kelvin came up with the idea of atomic clocks, which measure the electromagnetic signal emitted by electrons in atoms when they change energy levels, they have become our most accurate time and frequency standards today. Atomic clocks support time distribution services internationally, television broadcasts wave frequency and global navigation satellite systems.

In 1949 an atomic clock (less accurate than a quartz one) first demonstrated the physical concept before Essen and Parry built an accurate, and bulky, atomic clock to a caesium standard half a dozen years later.

By 1964, Hewlett-Packard had released much smaller rack-mountable devices with increasing accuracy which led the scientific community to redefine the second in terms of a specific atomic frequency in 1967.

Smart phones and us, presumably, have benefitted from these advances in technology as accurate, battery-driven atomic clocks became commercially available in 2011, most easily seen in Google or Apple Maps on our phones and in our cars. Four years later, NASA launched Deep Space Atomic Clock (DSAC), a miniaturised, ultra-precise, mercury-ion atomic clock more accurate then mere terrestrial clocks, which are maintained by national standards agencies, synchronised to an accuracy of 1 part in 1014 seconds per day! How did we ever survive without such pinpoint accuracy? How did I miss marking the longest day, the summer solstice, here in the Southern Hemisphere?

So, how was time measured before pendulums, wind-up watches, quartz timepieces, grandfather clocks and smart devices became ubiquitous? What served as units of time if seconds and minutes could not be conveniently measured or counted? The apparent motion of the sun across the sky, the phases of the moon, and the beat of a heart must have served some such purpose while day and night marked longer periods.

Time calculation still takes two distinct forms – the calendar, a geometrical way of organising intervals of time, and the clock, a physical device that counts the passage of time.

The ancient Egyptians may have divided the day into twelve smaller parts on their sundials, influenced no doubt by the 12 lunar cycles in a year. Their sundial used a gnomon to cast a shadow on a set of markings calibrated to the hour. The Romans used a clepsydra, a vessel with holes for the outflow of water. As the water emptied, it measured time, sort of like an hour glass with sand.

May the gods destroy that man who discovered hours and first set up a sundial here, to cut up my day!’

All of the above led me to the impending solstice which occurs when the Sun appears to reach its most northerly or southerly point as the Earth rotates and orbits the Sun. Bizarrely, I have been attempting to record daily sunrise and sunset where I live here in Perth – see the chart above – and it literally comes down to one or two minutes difference over the course of a week or so.

The Sun’s daily arc affects the length of daytime experienced and amount of daylight received during a given season. The two moments, when the angle of Earth’s rotational axis is towards the Sun, are the solstices, occurring annually, around June 21 and December 21, when the Sun’s motion comes to a stop. When it is the summer solstice in the Southern Hemisphere in Australia, it is the winter solstice in the Northern. The day of a solstice in either hemisphere has either the most sunlight of the year (summer solstice) or the least sunlight of the year (winter solstice). The summer solstice is when the Sun reaches its highest position in the sky and has the longest period of daylight. Conversely, the winter solstice is the day with the shortest period of daylight and longest night of the year, when the Sun is at its lowest daily position in the sky.

The word solstice is derived from the Latin sol (‘sun’) and sistere (‘to stand still’) because at the solstices, the Sun’s daily path seems to stop at a northern or southern limit before reversing direction. The Ancient Greeks used the term “ηλιοστάσιο” (heliostāsio), meaning stand of the Sun. The Sun’s westerly motion never ceases as Earth is continually in rotation.

The concept of the solstices was embedded in ancient Greek celestial navigation. When they realised that the Earth was spherical they devised the idea of an imaginary spherical surface rotating with the planets and stars fixed in it. The stars move across the inner surface of this imaginary celestial sphere along the circumferences of circles in parallel planes perpendicular to the Earth’s axis. The Sun and the planets do not move in these parallel paths but at an angle to the axis, bringing the Sun and planets across the paths of, and in among, the stars.

The exact solstice time is not easy to determine as I found out. The angle become smaller as the Sun gets closer to its maximum/minimum declination. The days before and after the solstice, are barely detectable with devices like sextant and impossible with more traditional tools like a gnomon. It is also hard to detect the changes on sunrise/sunset azimuth due to the atmospheric refraction changes.

According to Google, in 2019 the summer solstice will occur between December 20 and December 23 in the Southern Hemisphere and between June 20 and June 22 in the Northern.

How did Iron Age societies calculate such things as the start or separation of seasonal change? Did the druids base their pronouncements on observations of the sky and how was time calculated?

Ancient Indian philosophers believed that Time repeated ages over the lifespan of the universe, leading to beliefs in rebirth and reincarnation, as did the Celtic druids.

Ancient Greek philosophers wondered if time was linear or cyclical and if it was endless or finite while the Islamic and Judeo-Christian world-view regard time as linear and directional, beginning with God creating the world. The traditional Christian view sees the present order of things coming to an ‘end time’ although I don’t think that was ever mentioned in my primary school catechism which was a list of questions and answers that we were expected to memorise. I can only remember two of them now – ‘Who is God? God is our father in heaven’ and ‘Who made the world? God made the world.’

Celtic and other nomadic groups more than likely used the moon as a time-measuring device as early as 6,000 years ago with Lunar calendars of 12 or 13 lunar months corresponding to cycles of Moon phases. Seasons quickly come adrift in a lunar calendar and it became the prerogative of the druid class to calculate and add days or months to some years to balance things out.

Julius Caesar put the Roman world on a solar, or Julian, calendar in 45 BC indicating the seasons relative to the apparent position of the Sun but that, too, was faulty as the astronomical solstices and equinoxes moved by more than ten minutes per year. The solar day was the time interval between two successive journeys of the Sun across the local meridian – an imaginary line running from north to south – passing directly overhead. At this ‘solar noon’ the Sun reaches its highest point or zenith on its daily arc across the sky. It was not until 1582 that Pope Gregory XIII introduced a correction with the Gregorian solar calendar, now the most commonly used calendar around the world but which does not correspond to cycles of Moon phase.

The Celtic Coligny calendar, found in France in 1887, dates back to the first century A.D. and was used to define the beginning and length of the day, the week, the month, the seasons, quarter days, and festivals and was an attempt to reconcile the cycles of the moon and sun, as did the modern Gregorian calendar. However, the Coligny calendar considers the phases of the moon to be important, and each month always begins with the same moon phase.Coligny4

Among the Celts, the year was divided into a light half and a dark half. As the day was seen as beginning at sunset, so the year was seen as beginning with the arrival of the darkness at the feis of Samhain (around 1 November in the modern calendar).The light half of the year started at Bealtaine (around 1 May, modern calendar). Longer periods were reckoned in nights, as in the surviving English term fortnight meaning two weeks.

Those accuracy issues make it possible to determine the solstice day only with the use of more complex tools, which of course I don’t have, nor would I know how to use them, but my smart phone weather gizmo gives me – I presume – accurate readings for sunrise and sunset each day, as in the attached table. How did ancient societies do it? This is not a rhetorical question!

For vague comparison, the Southern Hemisphere (Winter) Solstice of 21 June 2019 had sunrise 07:15 and sunset at 17:18 giving a meagre daylight of 10 hours and three minutes.

Author: serkeen

I am Irish, currently living in West Australia. I have a degree in Old & Middle English, Lang & Lit and, despite having worked in Kuwait, Italy, Malaysia, USA, Brunei, Australia and Hong Kong over the last 40 years, I have a strong interest in Ireland’s ancient pre-history and the heroes of its Celtic past as recorded in the 12th and late 14th century collection of manuscripts, collectively known as The Ulster Cycle. I enjoy writing historical novels, firmly grounded in a well-researched background, providing a fresh and exciting look into times long gone. I have an empathy with the historical period and I draw upon my experiences of that area and the original documents. I hope, by providing enough historical “realia” to hook you into a hitherto unknown – or barely glimpsed - historical period.

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