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Scientific principles behind cultural festivals (solstice and equinox) - Or why we have seasons

Environment

Author:

Gudrun Bruckner, January 2020

Short summary:

Have you ever wondered why Christmas is celebrated on the 25th of December or why Halloween is dated on the 31st of October? Of course, there exist stories about Jesus or Jack Oldfield, but they are not proven with scientific evidence. In this article I will try to find scientific explanations for the dates of these cultural events. You will see that they often match with astronomic occurrences. It is not just a coincidence that winter solstice is happening a few days before Christmas or that Halloween falls on a Cross-Quarter Day exactly between equinox and solstice.

Throughout a year, humans celebrate several festivals in different cultures. For most of these festivals, roots can be traced back and an obvious relation to four special events in the astronomic year becomes visible. Since ages humans adjust their lives to the annual seasons, which are determined by these four major events, Quarter Days – vernal equinox, summer solstice, autumnal equinox and winter solstice.

In the western culture a solstice marks the first day of winter or summer. Others refer to them as the shortest and longest day of the year. To understand what makes these events outstanding, from an astronomical point of view, we need to look at the relation and the location of the sun and the earth.

Relation of sun and earth

Since Kepler’s discoveries we know that the earth’s orbit is not a circle but an ellipse. [1] Might this also be an explanation for the yearly seasons? Are we effected of the changing distance to the sun through the earth’s orbit?

During the year the distance between earth and sun fluctuates about 5 million kilometres. [2] The earth stands the closest to the sun in the beginning of January, when there is winter on the northern hemisphere. This event is called Perihelion. The opposite event, called Aphelion happens during the first days of July – the earth is the furthest away from the sun. [3] If the earth is closer to the sun the gravity is stronger. Through that, the earth experiences an acceleration and moves further away from the sun on the opposite side of its orbit, where the Aphelion appears. There a deceleration happens, and the earth moves closer to the sun again. The acceleration and deceleration are not the only reasons why the orbit is an ellipse instead of a circle but also why the summer on the northern hemisphere is a few days longer compared to the summer on the southern hemisphere. [4] However, the small differences in the distance to the sun does not visibly affect the sun’s energy reaching the earth and is therefore not an explanation for the changing of the seasons.

major events during the journey of the earth around the sun

Image 1: major events during the journey of the earth around the sun [5]

People, who live at different places around the globe, experience different seasons at the same time, but the relation of sun and earth at this point of time is the same for all of them. Therefore, the distance between the sun and the earth does only have a marginal influence on the seasons and we cannot explain the equinoxes and the solstices with the aberration of a circular orbit. Let’s investigate another interesting aspect of astronomy to find a proper explanation: the tilt of the earth’s axis

The axis of the earth is tilted in relation to the sun’s axis, which means they are not parallel to each other. When the angle of the tilt reaches a maximum, the earth is tilted away from the sun and it is winter in the northern hemisphere. The day that the angle reaches a minimum is called summer solstice. The sun reaches its highest point in the sky on the northern hemisphere throughout the year. 

In between, there are two events when the earth is neither tilted away nor tilted toward the sun. These are the vernal equinox in March — astronomically regarded as the first day of spring — and the autumnal equinox in September, which marks the end of summer and the beginning of autumn. Equinox comes from Latin and means "equal night”. At these special events, the hours of daylight and darkness are equal. [6] But where is the connection of these natural events with for example Christmas?

The History Behind Quarter & Cross-Quarter Days

The timing of present holidays was influenced by ancient cultures like the Celts, who lived in harmony with nature and adapted their traditions to the annual changing of the seasons. [7]
In general, summer starts with June 21st, which is one of the 4 Quarter Days. There exists a less famous parallel system, which was for example used by the Celts where June 21st is called Midsummer's Day, and the beginning of summer falls in early May. Followed by this autumn begins in August, winter in the beginning of November and Spring in February. These dates are known as Cross-Quarter Days, because they separate the quarters of the year again in eight similar parts. Ancient cultures aligned their farming rhythms with this wheel of the year. [8]
The Celts divided the year in a dark and a light season. A festival called Samhain was the beginning of the dark half, end of October/ beginning of November (nowadays we celebrate Halloween or All Saint's Day around this time). Beltane festival marked the beginning of the light season (beginning of May). Between these two major changes, in between summer and winter, fell two further Celtic festivals called Imbolc, beginning of February, and Lughnasadh, beginning of August 1. These quarters were divided by the solstices and equinoxes. [9] With the adoption of the Roman calendar, which is commonly used today, these Celtic celebrations conformed over the time with the liturgical year of Christian church. Even though the modern calendar has 12 months the separation of the year in eight equal parts is still visible. The Cross-Quarter Days and the Quarter Days were all some sort of holidays for the Celts or other pagan cultures who lived in dependency to nature. Therefore, it is not by chance that several relatively “modern” holidays and religious festivities like Christmas (winter solstice) or Easter (vernal equinox) appear at these times. Even in far eastern cultures we can find traces of ancient pagan festivals – Chinese New Year for example falls together with the Celtic Imbolc. [10]

This scientific background shows once more that humans were and still are connected to planet earth and its relation to the universe. Even though modern men sometimes think they can rule over the planet and control nature, it is nature which shaped humankind before it even believed in scientific principles. With sharing these insights, I want to remind you, when celebrating Christmas, Easter, Chinese New Year or Halloween, the very root of these holidays lies in astronomical events. Humans are and have ever since been driven by nature.

Further Reading