Understanding Seasons – Northern and Southern Hemisphere, Meteorological and Astronomical

Good gardeners learn to time their work according to the seasons, but when do the seasons begin and end?

There are different ways to divide up a year into seasons, changing when each actual season starts. Seasons can be meteorological or more traditionally astronomical, so which is which and how do you convert between the two?

Confusion about seasons doesn’t end there! Ever read a gardening book written on the other side of the world that talks about what month to do something in the garden, rather than what season, leaving you confused? Ideally it would be nice if gardening books were written to be more universal, but often they’re not so a way of converting months to seasons and translating northern hemisphere seasonal references in the southern hemisphere and vice versa in invaluable.

I’ve always wanted a quick reference guide for this purpose so I created a simple conversion table for gardeners which will make sense of overseas gardening books and local seasonal timing. Feel free to share!

 

Gardening Season Timing and Conversion Chart

season timing and conversion chart

Note: click on graphic above to enlarge and save image, or download the PDF version of the gardening season timing and conversion chart for printing

Using the chart is fairly straightforward to use.. If you’re reading a northern hemisphere gardening book, US or UK for example, and live in the southern hemisphere, say Australia, and the book refers to a task carried out in July, we can see that this refers to the middle of the meteorological summer or the beginning of the astronomical summer.

 

Meteorological or Astronomical Seasons – What’s the Difference?

Before anyone gets the wrong impression, the title says astronomical and not astrological!!! Some people mix these two words up, which brings an amusing little anecdote to mind. When I was working in the corporate world in a technical area many eons ago, I overheard a male colleague blurt out “I don’t believe in all that astronomy stuff” to which a female colleague sitting nearby wittily responded “So you don;t believe there’s a sun in the centre of our galaxy with planets revolving around it?” Quite embarrassed, the male colleague sheepishly replied “No, I mean the other one, you know what I mean…”

Astronomical Seasons

Traditionally, seasons began at the solstices and equinoxes. Solstices are the longest and shortest days of the year, so the midsummer solstice is the longest day of the year and the and midwinter solstice  is the shortest day of the year. The equinoxes are the days when the length of day and night are exactly equal, such as what happens in the spring and autumn equinoxes. These solstices and equinoxes occur around the third week of the month, and the days change from year to year, dependent upon the positions of the Earth in relation to the sun, as explained below.

So, in summary, Astronomical Seasons change (begin) at the equinoxes and solstices, the dates when they start are based on the position of Earth in relation to the sun.

If you want to understand how astronomical seasons work, it’s not that complicated. The key is the tilt of the Earth’s axis in relation to the sun.

If we look at a Northern Hemisphere example, when the Earth’s axis is tilted furthest towards the sun, the sun’s light shines more directly on the northern latitudes (northern hemisphere), producing the astronomical summer, which occurs approximately on June 20-22. (This is also the time of the winter solstice in the southern hemisphere).

640px-Earth-lighting-summer-solstice_EN
Illumination of Earth by Sun on the day of summer solstice on northern hemisphere
(Image source: public domain image by Przemyslaw Idzkiewicz)

Staying with our Northern Hemisphere example, when the Earth’s axis is tilted furthest away from the sun, the sun’s light shines more directly on the southern latitudes (southern hemisphere) and less on the northern latitudes (northern hemisphere), producing the astronomical winter, which occurs approximately on December 20-23. (This is also the time of the summer solstice in the southern hemisphere).

640px-Earth-lighting-winter-solstice_EN
Illumination of Earth by Sun on the day of winter solstice on northern hemisphere
(Image source: public domain image by Przemyslaw Idzkiewicz)

The equinoxes are fairly easy to understand, they occur when the Earth is tilting halfway between the summer and winter extremes and the sun’s light shines more directly on the equator, and the length of night and day are nearly equal, producing the astronomical spring approximately on  March 20 and astronomical autumn approximately on September 23 in the northern hemisphere.

Meteorological Seasons

There’s an even simpler way we can divide up the seasons – we can divide up the year into four 3-month periods which have similar temperatures, and we call these meteorological seasons. The meteorological seasons begin at the beginning of a particular month, and end three months later at the end of the month. Meteorologists (weather scientists) implemented this system which deals with temperatures over whole months (rather than astronomical part months) to allow them to more easily compare weather patterns from one season to another.

So, in summary, Meteorological Seasons change (begin) every 3 months, the dates when they start are based on groups of whole months that are similar in temperatures.

Using meteorological seasons for weather seasonal comparisons is easier because temperatures are more consistent across a season this way. Using astronomical seasons is more difficult because there is a seasonal lag, a delay between the time the astronomical season changes and the seasonal temperatures settling in.

We can see from our season conversion chart that meteorological seasons and astronomical seasons don’t neatly coincide or marry up, they’re out by approximately one month. In other words, the spring equinox may occur on March 20, which is closer to the start of April,  but real spring temperatures will arrive earlier at the start of March in the northern hemisphere. In the southern hemisphere, the spring equinox will occur on September 23, which is closer to the start of October,  but real spring temperatures will arrive earlier at the start of September.

Which Set of Seasons Do We Use?

Modern gardening calendars typically just use the meteorological seasons, so each season begins at the start of a certain month.

  • In the northern hemisphere, this corresponds to spring (March), summer (June), autumn (September), winter (December)
  • In the southern hemisphere, this corresponds to spring (September), summer (December), autumn (March), winter (June)

The astronomical seasons are the more traditional way of defining seasons, so if you’re planting by the moon (lunar calendar planting) or practising biodynamic gardening where the equinoxes and solstices are important, then this way of defining seasons may be more useful.

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One Response to Understanding Seasons – Northern and Southern Hemisphere, Meteorological and Astronomical

  1. loridorchak says:

    Thanks
    This was a timely article for me. I live in southern Chile and I have Steve Solomon’s book on PNW gardening which I really love and my climate is the same but the months and seasons always mess me up so I wrote myself a monthly gardening schedule based on his recommendations but using my southern hemisphere seasons.

    Like

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