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 do that, we must know when the seasons actually start and end!

There are different ways to divide a year into seasons, which changes when each season begins. Seasons can be meteorological or 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 tells you 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 often aren’t. A way to convert months to seasons and translate northern hemisphere seasonal references into ones 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 to 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 a northern hemisphere gardening book, such as a US or UK book, if you 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 the month falls into.

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), whereas astronomical seasons change at the equinoxes and solstices.

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

Note: click the graphic above to enlarge and save the 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 align, as they differ by a month. 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 mark the longest and shortest days of the year, so the midsummer solstice is the longest day of the year and the midwinter solstice is the shortest day of the year. The equinoxes are the days when the length of day and night are exactly equal, as in the spring and autumn equinoxes. These solstices and equinoxes occur around the third week of the month, and the exact dates change from year to year, depending on the positions of the Earth in relation to the Sun, as explained below.

So, in summary, Astronomical Seasons begin at the equinoxes and solstices. The dates when they start are based on Earth’s position relative 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 relative 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 tilted halfway between the summer and winter extremes, the Sun’s light shines more directly on the equator, and the length of day and night are nearly equal, producing 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 to divide the year into seasons – we divide it into four 3-month periods with similar temperatures, and we call these meteorological seasons. The meteorological seasons begin at the start of a particular month and end three months later at the end of that month. Meteorologists (weather scientists) implemented this system, which uses whole months (rather than astronomical parts of 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 seasonal weather comparisons is easier because temperatures are more consistent across a season. Using astronomical seasons is more difficult because there is a seasonal lag, a delay between the time the astronomical season changes and when seasonal temperatures settle 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 use the meteorological seasons, so each season begins at the start of a specific month.

• In the Northern Hemisphere, this corresponds to spring (March), summer (June), autumn (September), and winter (December)
  
• In the Southern Hemisphere, this corresponds to spring (September), summer (December), autumn (March), and winter (June).

The astronomical seasons are the 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, this way of defining seasons may be more useful.