Seasons Calculator

Astronomical seasons are not defined by weather but by four exact instants: the moments when the apparent geocentric longitude of the sun along the ecliptic reaches 0, 90, 180 and 270 degrees. Those instants are the March equinox, June solstice, September equinox and December solstice. They exist because Earth's rotational axis is tilted about 23.44 degrees from the perpendicular to its orbital plane, a value called the obliquity of the ecliptic that is currently decreasing by roughly 0.013 degrees per century. The tilt, not the varying Earth-sun distance, drives the seasons, a point underlined by perihelion occurring in early January, during northern winter. The seasons are also unequal in length. Because Earth's orbit is an ellipse and the planet sweeps equal areas in equal times, it travels fastest near perihelion. In the current epoch northern spring lasts about 92.8 days, summer about 93.6 days, autumn about 89.8 days and winter about 89.0 days; in the southern hemisphere the labels swap and winter is the long season. The computation used here applies the mean-equinox polynomials from chapter 27 of Jean Meeus's Astronomical Algorithms, which give the Julian Ephemeris Day of each event to within about a day; the full method adds periodic terms for planetary perturbations and nutation to reach minute-level accuracy. It returns a moment in Terrestrial Time, so the local calendar date of a solstice can differ by one day between time zones. Meteorological seasons, which begin on 1 March, 1 June, 1 September and 1 December, are a separate convention used for climate statistics.

Tracking the solstices was among the earliest applications of systematic observation. The passage tomb at Newgrange in Ireland, built around 3200 BCE, admits winter solstice sunrise light through a roof box into its inner chamber, and the main axis of Stonehenge, developed between roughly 3000 and 2000 BCE, aligns with the summer solstice sunrise. Hipparchus of Nicaea, working in the second century BCE, measured the intervals between equinoxes and solstices with enough precision to establish that the seasons are unequal, and discovered the precession of the equinoxes around 128 BCE; Ptolemy transmitted his results in the Almagest about three centuries later. The physical explanation came only with Johannes Kepler, whose Astronomia Nova of 1609 established that planets move on ellipses and sweep equal areas in equal times, accounting for Earth's faster motion near perihelion. Calendar reform followed a parallel path: by the sixteenth century the Julian calendar had allowed the March equinox to drift to about 11 March, and the bull Inter gravissimas issued by Pope Gregory XIII in 1582 removed ten days and revised the leap year rule to hold the equinox near 21 March. In the twentieth century Milutin Milankovic worked out how slow changes in obliquity, eccentricity and precession modulate the seasonal distribution of sunlight. The polynomial approximations most software now uses were published by the Belgian astronomer Jean Meeus in Astronomical Algorithms in 1991, while the United States Naval Observatory publishes the definitive tabulated times.