Solstices and equinoxes: the astronomical seasons

Four points on the Sun’s annual path

Over a year, the Sun appears to travel along the **ecliptic**, the projection of Earth’s orbital plane onto the sky. Earth’s axial tilt places the ecliptic at about 23.4° to the celestial equator. Solar declination therefore ranges between about +23.4° and −23.4°.

Four positions define the seasonal markers:

• **The March equinox, apparent ecliptic longitude 0°:** the Sun crosses the celestial equator from south to north. Northern astronomical spring and southern astronomical autumn begin.
• **The June solstice, 90°:** the Sun reaches its greatest northerly declination. This gives the longest day in the Northern Hemisphere and the shortest in the Southern Hemisphere.
• The September equinox, 180°: the Sun crosses from north to south. Northern autumn and southern spring begin.
• The December solstice, 270°: the Sun reaches its greatest southerly declination, giving the shortest day in the north and the longest in the south.

Month-based names avoid calling June “summer” for readers in Australia or other southern locations. Precise event definitions use the Sun’s apparent geocentric position, including small effects such as nutation and aberration.

Instants rather than entire days

A solstice or equinox happens at one instant, shared worldwide. The local date and clock time depend on the time zone: an event occurring late in the evening in New York can already fall on the next calendar date in Japan. That does not make two different solstices.

“Solstice” comes from a term meaning the Sun stands still. Around the event, its noon altitude changes only slowly before reversing its seasonal trend. “Equinox” means equal night, although the standard sunrise-to-sunset interval is not exactly twelve hours because of refraction and the solar disk.

Why the dates move between years

The tropical year is about 365.2422 days, longer than a common calendar year. Without corrections, the seasonal instants would drift through the calendar. Leap days move the calendar relationship back in the other direction, while the Gregorian system omits three leap days in four hundred years to keep the long-term alignment close.

This creates a familiar pattern of small changes in date and time rather than a permanently fixed school-calendar date. The local time zone can shift the named date as well. Always read the event’s date in the time zone relevant to your observing location.

Why the seasons have unequal lengths

The intervals between seasonal instants are not all the same. In the Northern Hemisphere, astronomical summer is roughly 93.6 days, spring 92.8, autumn 89.8 and winter about 89 days. These are broad present-era values rather than exact durations for every year.

Earth moves faster near perihelion in early January and more slowly near aphelion in early July. The orbital geometry therefore gives unequal travel times between the seasonal markers. Northern summer is not warmer because Earth is nearer the Sun; it occurs when Earth is farther away. The higher Sun and longer days explain the seasonal heating pattern at northern middle latitudes.

The Earth–Sun distance changes from roughly 147 to 152 million kilometres across the orbit, changing incoming solar intensity by around seven percent. At middle latitudes, the seasonal change in solar angle and day length is more important. Meteorological seasons use convenient three-month calendar blocks for statistics, which are distinct from these astronomical instants.

Observing the seasonal turning points

From a fixed viewpoint, watch where the Sun disappears behind the horizon across several weeks, using safe indirect observations rather than staring at the disk. The sunset point shifts, slows near a solstice and then reverses. Nearby trees, buildings or canyon features can give memorable reference directions, although terrain changes the apparent event.

Historical monuments show that this annual pattern has long mattered to people. Stonehenge’s familiar solstitial alignments are a real historical example and remain in the English account; a historical site need not be replaced merely because it is outside the United States. A simple local horizon record can reveal the same broad turning pattern without claiming that every monument has the same purpose.

Over millennia, **precession** shifts the equinox relative to the stars, taking about twenty-six thousand years for a full circuit. This is why the traditional “first point of Aries” no longer lies in the constellation Aries.