Solstices, Equinoxes, Seasons and Science of Time

My business is all about devices built to measure time. For most of us time is a framework for where we have been, where we are (or should be!), and where we will be - a scale by which we plot our path in our own little worlds. It is good, therefore, to reflect occasionally that time is a function not only of the world in which we live but, of our solar system itself. Our time and seasons are governed by our planet's rotation and its orbit around the sun.

One curiosity for me in moving from Great Britain to the United States was how the two countries view differently the arrivals of each season. Here, June 21st is often thought of as heralding the start of Summer, while in Britain it is known as "MidSummer" with "MidSummer's Day" being celebrated on June 24th. I guess it has partly to do with the more temperate climate in the British Isles; the temperature difference between the coldest day and the hottest day is much narrower in Britain. Add to that the very real potential for any Summer to be as wet as the preceding Spring, and you might start to grasp that June has as as much chance of being the best Summer month as do July or August.

An equinox in astronomy is that moment in time (not a whole day) when the center of the Sun can be observed to be directly above the Earth's equator, occurring around March 20 and September 23 each year.


Solstices occur twice a year, when the tilt of the Earth's axis is oriented directly towards or away from the Sun, causing the Sun to appear to reach its northernmost and southernmost extremes. The name is derived from the Latin sol (sun) and sistere (to stand still) because at the solstices, the Sun stands still in declination; that is, its apparent movement north or south comes to a standstill.

There is either an equinox (autumn and spring) or a solstice (summer and winter) on approximately the 21st day of the last month of every quarter of the calendar year. On a day which has an equinox, the center of the Sun will spend a nearly equal amount of time above and below the horizon at every location on Earth, and night and day will be of nearly the same length. The word equinox derives from the Latin words aequus (equal) and nox (night). In reality, the day is longer than the night at an equinox.

The cause of the seasons is that the Earth's axis of rotation is not perpendicular to its orbital plane around the Sun but currently makes an angle of about 23.44 degree. As a consequence, for half the year (from around 20 March to 22 September) the northern hemisphere tips to the Sun with the maximum around 21 June, while for the other half year the southern hemisphere has this distinction with the maximum around 21 December.

So, as you can see, time is a much broader thing than just what hour of the day it is. From ancient times (pun intended), Man has been consumed with understanding his world and his universe. Science is a process of observation, measurement, and explanation. Time has been a continual subject of that process ever since science was part of philosophy. In the next several articles, we will look at some of the amazing devices created to perform those measurements,such as astronomical clocks.