Equinoxes and Solstices: Earth's Tilted Seasons

Four times each year, Earth reaches a special point in its orbit that marks the beginning of a new season. The two equinoxes (March and September) and two solstices (June and December) are defined by the relationship between Earth's tilted axis and its position around the Sun. Understanding these events explains why seasons exist, why day length changes, and why different constellations appear at different times of year. Track the Sun's position and the seasonal sky with StarGlobe.

Earth's Axial Tilt

Earth's rotational axis is tilted approximately 23.4 degrees from the perpendicular to its orbital plane. This tilt is the fundamental cause of seasons. As Earth orbits the Sun, the tilt remains pointing in the same direction in space (toward Polaris), but Earth's position changes. This means that for part of the year, the Northern Hemisphere is tilted toward the Sun, receiving more direct sunlight, and for the other part, it is tilted away.

A common misconception is that seasons are caused by Earth's varying distance from the Sun. In fact, Earth is slightly closer to the Sun in January (perihelion) than in July (aphelion). The roughly 3 percent difference in distance has a much smaller effect than the 23.4-degree axial tilt. Our article on Sun position and seasons explores this relationship further.

The March Equinox

Around March 20, the Sun crosses the celestial equator moving northward. On this date, the Sun rises due east and sets due west for all locations on Earth (except the poles). Day and night are approximately equal in length everywhere, which is why the event is called an equinox (from Latin for "equal night").

The March equinox marks the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. After this date, the Sun continues moving northward, and Northern Hemisphere days grow longer while Southern Hemisphere days shorten. Astronomically, the March equinox defines the zero point of right ascension in the celestial coordinate system.

The June Solstice

Around June 21, the Sun reaches its northernmost point on the celestial sphere, at a declination of +23.4 degrees. For the Northern Hemisphere, this is the longest day and shortest night of the year. The word solstice means "Sun stands still," referring to the apparent pause in the Sun's northward motion before it begins moving south again.

At the Tropic of Cancer (latitude 23.4 degrees north), the Sun passes directly overhead at noon on the June solstice. At the Arctic Circle (66.6 degrees north), the Sun does not set at all, creating the famous midnight sun. The reverse occurs in the Southern Hemisphere, where the June solstice is the shortest day. For stargazers, the short summer nights in the north mean less observing time but also access to unique constellations high in the sky. See our summer sky guide.

The September Equinox

Around September 22, the Sun again crosses the celestial equator, this time moving southward. Day and night are again approximately equal worldwide. The September equinox marks the start of autumn in the Northern Hemisphere and spring in the Southern Hemisphere. After this date, northern nights grow longer, providing more time for observing the rich autumn sky described in our autumn guide.

The December Solstice

Around December 21, the Sun reaches its southernmost point at a declination of -23.4 degrees. This is the shortest day in the Northern Hemisphere and the longest in the Southern Hemisphere. The Tropic of Capricorn (latitude 23.4 degrees south) experiences the Sun directly overhead at noon.

For Northern Hemisphere stargazers, the December solstice brings the longest nights of the year, maximizing observing time. The winter sky is rich with bright stars and constellations including Orion, Canis Major, Auriga, and Gemini. The Winter Hexagon dominates the sky, containing more first-magnitude stars than any other region.

Effects on the Night Sky

Equinoxes and solstices affect stargazing in several practical ways. The length of night determines how many hours you have for observing. Near the June solstice at mid-northern latitudes, astronomical darkness may last only five or six hours. Near the December solstice, you may have fourteen hours or more of darkness.

The Sun's position on the ecliptic determines which constellations are hidden behind solar glare. Constellations opposite the Sun are visible all night, while those near the Sun are invisible for weeks or months. This is why different constellations dominate different seasons and why sidereal time matters for sky observation.

Precession and the Moving Equinoxes

The equinox points are not fixed among the stars. Earth's axis wobbles like a spinning top in a motion called precession, completing one full cycle in about 25,800 years. This wobble slowly shifts the equinox points westward along the ecliptic at a rate of about one degree every 72 years.

Two thousand years ago, the March equinox point (the "First Point of Aries") was in the constellation Aries. Due to precession, it has shifted into Pisces and will eventually move into Aquarius. Despite this shift, the point is still traditionally called the First Point of Aries in astronomical coordinate systems. The zodiac constellations article discusses this discrepancy further.

Cultural Significance

Equinoxes and solstices have been celebrated by cultures worldwide for thousands of years. Stonehenge, Newgrange, Chichen Itza, and many other ancient structures are aligned with solstice or equinox sunrise or sunset directions. These alignments demonstrate the importance of astronomical observation in ancient societies and served practical calendrical functions.

Many modern holidays and festivals have roots in solstice and equinox traditions. Understanding the astronomy behind these events connects seasonal celebrations to the mechanics of Earth's orbit and tilt.

Observing the Effects

You can observe the effects of equinoxes and solstices throughout the year. Note where the Sun rises and sets on the horizon: at an equinox, it rises due east and sets due west. At the June solstice, it rises and sets at its northernmost points. At the December solstice, at its southernmost points. This annual shift of the Sun's rising and setting positions is one of the most fundamental observable astronomical phenomena.

Use StarGlobe to see how the visible sky changes from season to season. Compare the constellations visible at 9 PM in March versus June versus September versus December, and you will see the entire celestial sphere parade past over the course of a year, driven by Earth's orbit and tilt.