Solstices and Equinoxes Explained

Solstices and equinoxes are four important moments in Earth’s yearly orbit around the Sun. They mark the turning points of the astronomical seasons and help explain why daylight grows longer or shorter during the year.

The two solstices occur in June and December. At a solstice, one of Earth’s hemispheres is tilted most strongly toward the Sun while the other is tilted most strongly away. This creates the longest day of the year in one hemisphere and the shortest in the other.

The two equinoxes occur in March and September. At an equinox, the Sun’s center crosses Earth’s equatorial plane, and both hemispheres receive nearly equal amounts of daylight.

These events happen because Earth’s rotational axis is tilted by about 23.5 degrees relative to its orbit. They are not caused by Earth moving dramatically closer to or farther from the Sun. [1]

Quick Summary

• Earth’s tilted axis creates the seasons.
• A solstice occurs when the Sun reaches its farthest apparent position north or south of the equator.
• The June solstice brings astronomical summer north of the equator and winter south of it.
• The December solstice brings astronomical winter north of the equator and summer south of it.
• An equinox occurs when the Sun’s center crosses the celestial equator.
• The March equinox begins astronomical spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
• The September equinox begins astronomical autumn in the Northern Hemisphere and spring in the Southern Hemisphere.
• Day and night are nearly, but not exactly, equal on an equinox.
• The four-season model is not equally useful in every climate.
• Many tropical communities organize seasonal life around rainfall, rivers, crops, grazing, or stars rather than solstice-based seasons.
• Cultural celebrations associated with these dates are diverse and should not be combined into one universal solar religion.

What Causes Earth’s Seasons?

Earth rotates around an axis that is tilted rather than perfectly upright. As Earth orbits the Sun, this axis continues pointing in approximately the same direction in space.

For part of the year, the Northern Hemisphere tilts toward the Sun and receives:

• More direct sunlight
• A higher daily Sun path
• Longer daylight hours
• Shorter nights

At the same time, the Southern Hemisphere tilts away and receives less direct sunlight and shorter days. Six months later, the relationship reverses. [1]

Concise Definition

Solstices and equinoxes are astronomical events created by Earth’s tilted axis and orbit, marking the yearly extremes and crossing points of the Sun’s apparent north-south movement.

In Simple Terms

Imagine Earth as a tilted spinning ball moving around a lamp.

The lamp represents the Sun. Because the ball remains tilted as it moves, one half receives longer and more direct sunlight, while the other receives shorter and less direct sunlight. The solstices are the maximum points of that difference. The equinoxes are the crossing points between them.

Seasons Are Not Caused by Earth’s Distance From the Sun

A common misconception says summer occurs because Earth is closer to the Sun.

That explanation cannot be correct because the Northern and Southern Hemispheres experience opposite seasons at the same time. Earth is one distance from the Sun, yet one hemisphere may be in summer while the other is in winter.

The decisive factors are:

• The angle of sunlight
• The length of daylight
• Earth’s axial tilt
• The Sun’s path through the sky

What Is a Solstice?

The word solstice comes from Latin terms commonly translated as “Sun standing still.”

Around each solstice, the Sun’s sunrise and sunset positions along the horizon change very slowly before reversing direction. To a careful observer, the Sun appears to pause at its farthest northern or southern point.

Astronomically, a solstice is the moment when the Sun reaches its greatest declination north or south of Earth’s equator. NOAA describes solar declination as reaching approximately plus 23.44 degrees at the Northern Hemisphere summer solstice and minus 23.44 degrees at its winter solstice. [4]

What Is an Equinox?

The word equinox comes from Latin words meaning “equal night.”

Astronomically, an equinox occurs when the center of the Sun crosses the plane of Earth’s equator. At that moment, the Sun is directly above the equator, and daylight is distributed nearly equally between the hemispheres. [3]

There are two equinoxes each year:

• The March equinox
• The September equinox

The Four Astronomical Turning Points

• Event
• Approximate date
• Northern Hemisphere
• Southern Hemisphere
• March equinox
• March 19–21
• Spring begins
• Autumn begins
• June solstice
• June 20–22
• Summer begins
• Winter begins
• September equinox
• September 21–24
• Autumn begins
• Spring begins
• December solstice
• December 20–23
• Winter begins
• Summer begins

Exact dates and times vary slightly because the calendar year does not perfectly match Earth’s orbital period and because leap-year corrections shift civil dates.

1. The June Solstice

At the June solstice, the Northern Hemisphere reaches its maximum tilt toward the Sun.

For locations north of the equator, this generally produces:

• The longest daylight period of the year
• The shortest night
• The Sun’s highest annual noon position
• The beginning of astronomical summer

South of the equator, the same event produces the shortest day, longest night, lowest annual Sun path, and beginning of astronomical winter. [1]

Is June 21 Always the Solstice?

No.

The June solstice may occur on June 20, 21, or occasionally June 22 depending on the year and time zone. The astronomical event happens at one instant worldwide, but local clocks and calendar dates differ.

What Happens at the Tropic of Cancer?

Near the June solstice, the midday Sun can appear directly overhead at locations near the Tropic of Cancer, approximately 23.5 degrees north latitude.

This is the northernmost latitude where the Sun can be directly overhead at noon.

2. The December Solstice

At the December solstice, the Southern Hemisphere reaches its maximum tilt toward the Sun.

For the Southern Hemisphere, this generally produces:

• The longest daylight period
• The shortest night
• The Sun’s highest annual path
• The beginning of astronomical summer

For the Northern Hemisphere, it produces the shortest day, longest night, lowest annual Sun path, and beginning of astronomical winter.

What Happens at the Tropic of Capricorn?

Near the December solstice, the midday Sun can appear directly overhead at locations near the Tropic of Capricorn, approximately 23.5 degrees south latitude.