Every year in early September, a familiar question resurfaces: has autumn already begun, or do we have to wait until late September? The answer depends entirely on whether you look at the calendar through the lens of atmospheric climate data or orbital mechanics.
The Underlying Distinction
Meteorological seasons are designed for climate science and statistical consistency. Meteorologists split the year into four fixed, three-month periods that align with the Gregorian calendar and annual temperature patterns.
Astronomical seasons, by contrast, are determined by the Earth’s orbit and its axial tilt relative to the Sun. Because the Earth’s orbit is elliptical and a solar year is roughly 365.25 days, the astronomical start dates shift slightly every year, typically falling between the 20th and 23rd of the month.
Seasonal Breakdown
​Autumn: Meteorological autumn always begins on September 1 and ends on November 30. Astronomical autumn begins at the autumnal equinox, which usually occurs around September 22 or 23, when day and night are of roughly equal length.
Winter: Meteorological winter runs from December 1 through the end of February. Astronomical winter starts at the winter solstice, usually around December 21 or 22, marking the shortest day and longest night of the year in the Northern Hemisphere.
Spring: Meteorological spring begins on March 1 and ends on May 31. Astronomical spring starts at the vernal equinox, around March 20 or 21, when the Sun crosses the celestial equator northward.
Summer: Meteorological summer spans June 1 to August 31. Astronomical summer starts at the summer solstice, around June 20 to 22, bringing the longest day of the year in the Northern Hemisphere.
Why Both Systems Exist
​Meteorological definitions simplify long-term record-keeping. If climate researchers had to adjust the boundaries of seasons each year based on fractional orbital timings, tracking global temperature anomalies and multi-decade trends would be unnecessarily messy. Grouping months into tidy, uniform quarters provides a stable baseline.
Astronomical seasons remain the natural markers of light and celestial geometry, reflecting the physical progression of our planet around the Sun rather than human record-keeping convenience.