Two ways to divide the year
Both systems split the year into spring, summer, autumn and winter. What differs is where the lines are drawn.
- Astronomical seasons begin at an equinox or solstice, the moments defined by the Sun’s position relative to Earth’s equator. They follow the planet’s orbit and tilt.
- Meteorological seasons begin on the first day of a month and each last exactly three calendar months. They follow the annual temperature cycle and the convenience of monthly records.
Dates in the Northern Hemisphere
| Season | Meteorological | Astronomical |
|---|---|---|
| Spring | March 1 to May 31 | March equinox (about March 20) to June solstice |
| Summer | June 1 to August 31 | June solstice (about June 21) to September equinox |
| Autumn | September 1 to November 30 | September equinox (about September 22 or 23) to December solstice |
| Winter | December 1 to February 28 or 29 | December solstice (about December 21 or 22) to March equinox |
Dates in the Southern Hemisphere
The seasons are reversed south of the equator, so the same dates mean the opposite season.
| Season | Meteorological | Astronomical |
|---|---|---|
| Spring | September 1 to November 30 | September equinox to December solstice |
| Summer | December 1 to February 28 or 29 | December solstice to March equinox |
| Autumn | March 1 to May 31 | March equinox to June solstice |
| Winter | June 1 to August 31 | June solstice to September equinox |
Australia and New Zealand commonly use the meteorological dates in everyday life, so summer there is generally said to start on December 1.
Why meteorologists use whole months
Climate scientists compare seasons across many years and many places. That is much easier when each season is made of complete calendar months. With astronomical seasons, the start date shifts by a day or so from year to year, the seasons have different lengths, and every monthly dataset would have to be split partway through.
The meteorological grouping also fits the temperature cycle better. In most mid-latitude places, the three coldest months are roughly December, January and February in the north, and the three warmest are June, July and August. The astronomical seasons start about three weeks later than that.
Why the astronomical seasons lag the weather
The June solstice gives the Northern Hemisphere its strongest sunshine, but the hottest weather usually comes weeks later. Land and especially oceans store heat, so temperatures keep rising for a while after the incoming sunlight peaks. The same delay happens in winter. This seasonal lag varies by place: coastal and island climates often lag more than continental interiors.
So in temperature terms, the solstices sit close to the start of the warmest and coldest periods rather than in the middle of them, which is one reason many people think of the solstice as the start of summer or winter.
Why astronomical seasons differ in length
Earth’s orbit is slightly elliptical. Earth is closest to the Sun (perihelion) in early January and farthest (aphelion) in early July, and it moves faster near perihelion. As a result:
- Northern summer (June solstice to September equinox) is the longest astronomical season, about 93 to 94 days.
- Northern winter (December solstice to March equinox) is the shortest, about 89 days.
Meteorological seasons avoid this by using calendar months, so they last 90, 91 or 92 days.
Which one should you use?
Neither is wrong. Use meteorological seasons for weather and climate comparisons, and astronomical seasons when you care about daylight and the Sun’s position. You can find the exact astronomical dates on the seasons page, daylight hours on the sunrise and sunset page, and more on the individual events in the guides to the March equinox and the June solstice.