The Earth’s astronomical seasons, as seen from the northern hemisphere. - Illustration by Tauʻolunga, Wikimedia Commons

Summer is upon Vancouver at last. While Vancouverites revel in the sunshine, how many actually know the way the passage of the seasons is marked?

It is interesting how different countries mark the transition of seasons. For example, in Ukraine every season starts on the first day of March, June, September and December. Canada, on the other hand, uses the astronomical calendar that places the first day of spring on Mar 20, the beginning of summer on June 21, the commencement of autumn on Sept. 23 and the start of winter on Dec. 22.

The solstices and equinoxes that signify the beginning and the end of the seasons should technically occur in the middle of their respective months. Because of thermal lag, however, regions with a continental climate often use the astronomical calendar dates to mark the seasons. The length of the seasons is not uniform because of the elliptical orbit of the Earth and its varying orbital speed.

“The summer solstice is the longest day of the year and the winter solstice is the shortest,” says Stan Geller, an Ecology Master of Science candidate at UBC. “The equinox is the time when the day and the night are the same number of hours.”

In Sweden and Finland, meteorologists mark seasonal transitions with a method based on temperature. Spring starts when the daily average temperature permanently rises above 0 C, summer begins at a permanent temperature of above 10 C, summer ends when the temperature falls below 10 C and winter begins below 0 C.

“Permanently” is defined as the daily average temperature that remained above or below the limit for seven consecutive days. For the Swedes and the Finns, the seasons do not begin at fixed dates but must be determined through observation and are known only after the fact. Furthermore, seasons can (and do) begin on different dates in different parts of Sweden and Finland.

The ancient Celts, on the other hand, only celebrated the passage of summer and winter (the symbolic death and rebirth of the sun). The occasions were marked by the harvest festival of Samhain and the spring festival of Beltaine. Both events are celebrated among the Six Celtic Nations and their diasporas.

Chinese seasons are based on 24 periods known as “solar terms” in traditional East Asian lunisolar calendars (calendars that indicate moon phases and the solar time of year). Seasons begin at the midpoint of solstices and equinoxes. The Hindu calendar, by contrast, is unique in the sense that it recognizes – and marks – six seasons in a year, which are divided into two-month intervals in the twelve months of the calendar.

Regardless of how many seasons or months each calendar has, there are still 365 days – 366 in a leap year – to enjoy. Though there are many warm days ahead for Vancouverites who love to soak in the sun, the warm, heady afternoons of summer won’t last long before autumn makes its return, so make the best of it.