Astronomical seasons are based on the position of Earth in relation to the Sun, and Earth's elliptical orbit makes them unequal, so they vary in length between 89 and 93 days. They are set by Earth's tilt and the Sun's alignment over the equator. The exact dates of the solstices and equinoxes vary from year to year. Meteorologists often use weather-service seasons that follow the calendar instead. Meteorological winter includes December, January and February, and the months listed for each meteorological season apply to the Northern Hemisphere. They use whole months because it becomes much easier to calculate seasonal statistics from the monthly statistics, and the result is more even: about 90 to 92 days, from 90 days for winter of a non-leap year to 92 days for spring and summer.
The unevenness of the astronomical seasons has a physical cause. Eccentricity measures how much the shape of Earth's orbit departs from a perfect circle, and it is the reason our seasons are slightly different lengths. Earth's closest point to the Sun is called perihelion, and it comes in early January; its farthest point is called aphelion, and it comes in July, as of November 2026. Perihelion does not cause the seasons, but it makes the seasons last different lengths: because Earth moves faster in early January, the seasons that occur during that time pass more quickly. Kepler's second law explains why Earth speeds up near the Sun, since the smaller the distance, the faster the motion, and Earth travels about 2,000 miles per hour faster at perihelion than at aphelion. Because it moves a little faster in northern winter, northern winter is about three days shorter than northern summer.
Even the solstices are not the extremes of daily life. A National Weather Service office in the Quad Cities area of Iowa and Illinois says its earliest sunset of the year comes around December 8 and its latest sunrise near January 5, so those are not the winter solstice. Its earliest summer sunrise is near June 14 and its latest sunset near June 28. The hottest days of summer also come after the summer solstice, not on it, because land and oceans need extra weeks to warm up. By August, the ocean water on the same beach will be much warmer.
The tropics keep their own timetable. Over land the tropical rain belt is distorted by monsoon circulations, in which the prevailing winds over land and nearby ocean reverse directions on a seasonal basis. In summer Asian land surfaces get hotter than the surrounding oceans. As days lengthen and heating increases, warm, moist air from the Indian Ocean flows onshore, bringing rain. The rising air near the equator is replaced by air from north and south of the equator, and seasonal shifts of the belt produce the wet and dry seasons of the tropics rather than the cold and warm seasons of higher latitudes.
The clock also runs over thousands of years. As of November 2026 Earth's tilt, called obliquity, is about 23.4 degrees, roughly halfway between the 22.1 and 24.5 degrees it has reached over the last million years, and it is very slowly decreasing in a cycle that spans about 41,000 years. The greater the tilt angle, the more extreme the seasons are. Axial precession makes seasonal contrasts more extreme in one hemisphere and less extreme in the other, and it affects seasonal timing relative to Earth's closest and farthest points around the Sun. Larger tilt angles favor periods of deglaciation, the melting and retreat of glaciers and ice sheets, and tilt, eccentricity and precession are three slow orbital cycles that help set Earth's long-term climate.
Living things read these clocks and leave a record of them. Phenology is the study of the timing and cyclical patterns of events in the natural world, such as the budding of leaves, the arrival of migratory birds and the flowering of plants. It helps us understand how living organisms respond to environmental cues such as day length, temperature and rainfall. Phenology records the timing of leaf-out, flowering, migration and spawning, and it helps assess the vulnerability of species, populations and ecological communities to ongoing climate change. It also tracks how global warming is altering the timing of key biological events, and it supports understanding of the timing of ecosystem processes, such as carbon cycling.
Words to know
- meteorological: by weather calendar
- eccentricity: orbit's stretch
- precession: slow axis wobble
- phenology: timing of nature
- perihelion: closest to Sun
- aphelion: farthest from Sun
