Converting Months to Seasons – Northern and Southern Hemisphere, Meteorological and Astronomical

Good gardeners understand the importance of timing their work to the seasons of the year, but to be able to do that, we must know when the actual seasons start and end!

There are different ways to divide up a year into seasons, which changes when each season begins. 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 though. Have you 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? Ideally, it would be nice if gardening books were written to be more universal, but they’re often not, so a way of converting months to seasons and translating northern hemisphere seasonal references to ones that are meaningful in the southern hemisphere and vice versa would be invaluable for gardeners.

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

Gardening Season Timing and Conversion Chart

Want to convert seasons from say a northern hemisphere gardening book, US or UK for example, and live in the southern hemisphere, say Australia? It’s easy, just look up the Meteorological season for the month in the table below to find out which season and part of that season that month refers to.

Northern Hemisphere		Month	Southern Hemisphere
Astronomical	Meteorological		Meteorological	Astronomical
Winter	Winter	January	Summer	Summer
Winter	Winter	February	Summer	Summer
Winter	Spring	March	Autumn	Summer
Spring	Spring	April	Autumn	Autumn
Spring	Spring	May	Autumn	Autumn
Spring	Summer	June	Winter	Autumn
Summer	Summer	July	Winter	Winter
Summer	Summer	August	Winter	Winter
Summer	Autumn	September	Spring	Winter
Autumn	Autumn	October	Spring	Spring
Autumn	Autumn	November	Spring	Spring
Autumn	Winter	December	Summer	Spring

Note: Meteorological seasons are grouped by months (and used by gardeners), while astronomical seasons change at the equinoxes and the solstices.

I’ve also created the graphic of the table below, which is colour-coded to make it easier to read:

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

The Difference Between Meteorological or Astronomical Seasons

As we can see from the conversion table, meteorological and astronomical seasons don’t match up, as they differ by a month from each other. In the two sections below we explain in detail how each seasonal system is calculated, and what it’s based on.

What are 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).

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).

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.

What are 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 System 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.