Astronomy has been important to people for thousands of years. The ancient construction known as Stonehenge in England may have been designed, among other purposes, to pay special honor to the solstices and equinoxes. These are the times and locations during Earth's journey around the sun that we humans have long used to mark our seasons.
Here is a guess at how Stonehenge might have looked about 2400 BC. At dawn on the Summer Solstice, the rays of the sun would have shone straight through what is called the "slaughter stones" to exactly strike the "altar stone" in the center.
But what is the solstice exactly?
It has to do with some imaginary lines on our planet. These lines are important, because they help people navigate and measure time.
The equator is an imaginary line drawn right around Earth’s middle, like a belt. It divides Earth into the Northern Hemisphere and the Southern Hemisphere.
Another imaginary line drawn straight through Earth connecting the North Pole to the South Pole is Earth’s axis of rotation. This line is tilted 23.5° from Earth’s orbital path around the sun. This tilt is the cause of Earth’s seasons.
Earth's axis of rotation is tilted 23.5°. That makes the latitudes of +23.5° and −23.5° special. But how? Other useful, but imaginary, lines around Earth that are parallel to the Equator are called lines of latitude. They are numbered from 0° to 90°. The one at 0° is the equator itself. The higher the number, the farther north (if it’s a + number) or south (if it’s a − number).
You may have noticed two special lines of latitude on a globe of the world: One in the Northern Hemisphere called the Tropic of Cancer at +23.5° latitude and one in the Southern Hemisphere called the Tropic of Capricorn at − 23.5° latitude.
At these latitudes, the sun is directly overhead around noon on the solstices. In the Northern hemisphere, the Summer Solstice occurs when the sun is directly above the Tropic of Cancer, usually June 21. In the Southern Hemisphere, the Winter Solstice occurs when the sun is directly above the Tropic of Capricorn, usually December 21. The solstice days are the days with the most (for Summer) or fewest (for Winter) hours of sunlight during the whole year.
The sun is directly overhead at "high-noon" on Summer Solstice at the latitude called the Tropic of Cancer. Credit: Przemyslaw Idzkiewicz, via Wikipedia Commons.
The sun is directly overhead at "high-noon" on Winter Solstice at the latitude called the Tropic of Capricorn. Credit: Przemyslaw Idzkiewicz, via Wikipedia Commons.
The sun is directly overhead at "high-noon" on the equator twice per year, at the two equinoxes. Spring (or Vernal) Equinox is usually March 20, and Fall (or Autumnal) equinox is usually September 22. Except at the equator, the equinoxes are the only dates with equal daylight and dark. At the equator, all days of the year have the same number of hours of light and dark.
Between the two tropics zones, which includes the equator, the sun is directly overhead twice per year. Outside the tropic zones, whether to the south or north, the sun is never directly overhead.
Two other significant lines of latitude are the Arctic Circle (around the North Pole) and the Antarctic Circle (around the South Pole). These circles are as far from the poles as the Tropic of Cancer and the Tropic of Capricorn are from the equator. On the Arctic Circle, the sun does not set at all on the Summer Solstice. On that one day, the sun traces a complete circle just above the horizon as the Earth rotates. On the Antarctic Circle, the sun does not set at all on the Winter Solstice.
As you go closer to the poles, you have more and more days when the sun does not set (or rise), until, at the poles, the sun remains above or below the horizon for six months at a time.
A webcam at the North Pole captured this picture on June 21, 2010.
A webcam at the South Pole captured this picture on June 18, 2010, at high noon.
More than "imaginary" to GOES . . .
The equator is a very important "imaginary line" for the GOES and GOES-R series weather satellites. They orbit exactly above the equator, at a very great distance (22,300 miles), which allows them to make just one orbit per day.
They "hover" over one point on Earth's equator. That way, they have a full view of almost one-half of Earth and can keep a continual watch on developing weather. Find out more about satellite orbits.
Tropic of Cancer? Tropic of Capricorn? Who came up with those names?
These names were thought up about 2,000 years ago. At that time, the Sun was in the direction of the constellation Cancer at the Summer Solstice in June. However, this is no longer true. Earth’s axis wobbles a bit, slowly changing the direction in which it points.
Over 26,000 years, the axis traces out a small cone shape. At this time, the Sun is in Taurus or Gemini (depending on where you draw the boundary between them) at the Summer Solstice. The word "tropic" itself comes from the Greek τροπή (tropi), meaning turn, referring to the fact that the sun appears to "turn back" at the solstices.
In this chart of the zodiac, the Sun is in the constellation Cancer.
When the Tropic of Capricorn was named, the Sun was entering the constellation Capricorn at the Winter Solstice in December. In modern times the Sun appears in the constellation Sagittarius during this time.
SkyTellers Seasons activities for young children
See also: Middle school Seasons activities and resources
About Our Seasons
What causes our seasons?
We have seasons because Earth's axis – the imaginary line that goes through the Earth and around which the Earth spins — is tilted. It's tilted about 23.5 degrees relative to our plane of orbit (the ecliptic) around the Sun. As we orbit our Sun, our axis always points to the same fixed location in space. Our northern axis points almost directly toward Polaris, the North Star.
This picture shows Earth from its side as it orbits our Sun. The axis is tilted and points to the North Star no matter where Earth is in its orbit. Because of this, the distribution of the Sun's rays changes. In June, in the northern hemisphere summer, the Sun's rays reach the north pole and beyond, enveloping the Arctic circle. In December, in the northern hemisphere winter, the north pole is tilted away from the incoming sunshine.
The “fixed” tilt means that, during our orbit around our Sun each year, different parts of Earth receive sunlight for different lengths of time. It also means that the angle at which sunlight strikes different parts of Earth's surface changes through the year. Sunlight striking the surface at an angle is “spread” across a wider area compared to sunlight striking perpendicular to Earth's surface. Areas that receive more scattered sunlight receive less energy from our Sun. All of these factors combine to give Earth its annual cycle of seasons!
For part of our orbit the northern half of Earth is tilted toward the Sun. This is summer in the northern hemisphere; there are longer periods of daylight, the Sun is higher in the sky, and the Sun's rays strike the surface more directly, giving us warmer temperatures. The north pole is in constant daylight!
When the northern half of Earth is tilted toward the Sun, the southern hemisphere is tilted away. People in the southern hemisphere experience the shorter day lengths and colder temperatures of winter.
During winter in the northern hemisphere, our northern axis continues to point to the North Star, but, because we have moved in our orbit around the Sun, our northern hemisphere now points away from our Sun. The north pole is completely dark and other places in the northern hemisphere experience the shorter day lengths and colder temperatures of winter as the Sun traces a lower arc across the southern sky and the Sun's rays strike the surface at a lower angle. When it is winter in the northern half of Earth, the southern hemisphere, tilted toward our Sun, has summer.
During fall and spring, some locations on Earth experience similar, milder, conditions. Earth has moved to a position in its orbit where its axis is more or less perpendicular to the incoming rays of the Sun. The durations of daylight and darkness are more equally distributed across all latitudes of the globe.
What doesn't cause the seasons?
The seasons are not caused by how far Earth is from our Sun. Earth's orbit around our Sun has a slightly elliptical path (very slight!), and the Sun is not exactly in the center of the ellipse. This means that, during the year, Earth is sometimes farther from our Sun, and sometimes closer — but the difference is small (not so for some other planets!). Earth is closest to our Sun in January (perihelion) and the farthest away in July (Earth is 147.5 million kilometers from the Sun when it reaches aphelion). If distance were the most important factor, the entire Earth would have summer in January when we are closest to our Sun and winter in July when we are farthest away!
What are solstices and equinoxes?
Solstices occur when Earth's axis is pointed directly toward our Sun. This happens twice a year during Earth's orbit. Near June 21 the north pole is tilted 23.5 degrees toward our Sun and the northern hemisphere experiences summer solstice, the longest day of the northern hemisphere year. On that same day, the southern hemisphere is tilted 23.5 degrees away from our Sun and the southern regions of Earth experience the shortest day of the year — the winter solstice.
The second solstice occurs on December 21 or 22 when the north pole is tilting 23.5 degrees away from our Sun and the south pole is inclined toward it. This is the shortest day of the year in the northern hemisphere — the northern hemisphere winter solstice.
Twice each year, during the equinoxes (“equal nights”), Earth's axis is not pointed toward our Sun, but is perpendicular to the incoming rays. During the equinoxes every location on our Earth (except the extreme poles) experiences 12 hours of daylight and 12 hours of darkness. The vernal or spring equinox occurs in the northern hemisphere on March 21 or 22 (the fall equinox of the southern hemisphere). September 22 or 23 marks the northern hemisphere autumnal or fall equinox.
National Maritime Museum
As Earth orbits our Sun, the position of its axis relative to the Sun changes. This results in a change in the observed height of our Sun above the horizon. For any given location on Earth, our Sun is observed to trace a higher path above the horizon in the summer, and a lower path in the winter. During spring and fall, it traces an intermediate path. This means that our Sun takes a greater amount of time tocross the sky in the summer and a shorter amount of time in the winter. This effect is greater as you move toward the poles; people living near the equator experience only small changes in daylight during the year. The change is more extreme toward the poles.
During the northern hemisphere summer solstice, Earth is tilted such that the Sun's rays strike perpendicular to the surface at the Tropic of Cancer (23.5 degrees north latitude, corresponding to the tilt of Earth's axis). At (solar) noon, our Sun is directly overhead in this location (and at a decreasing height above the horizon north and south of the Tropic of Cancer). At locations north, our Sun will be at its highest position above the horizon and will take the greatest amount of time to cross the sky. All northern locations have more than 12 hours of daylight. All locations south experience less than 12 hours of daylight. Locations above the Arctic Circle (north of 66.5 degrees latitude; 90 degrees minus the tilt of Earth's axis) receive 24 hours of sunlight. Locations below the Antarctic Circle (66.5 degrees south latitude) experience 24 hours of darkness.
During the northern hemisphere winter solstice, the Sun's incoming rays are perpendicular to the Tropic of Capricorn at 23.5 degrees south latitude. The Sun's path is the lowest above the horizon in locations north of the equator, and these regions experience the shortest day of the year. Between the winter and summer solstices, daylight increases as Earth continues its orbit around our Sun.
During the equinoxes, sunlight strikes perpendicular to the surface at Earth's equator. All locations on Earth, regardless of latitude, experience 12 hours of daylight and 12 hours of darkness. The spring equinox marks the change from 24 hours of darkness to 24 hours of daylight at Earth's poles . In these extreme locations, our Sun moves above the horizon at the spring equinox and does not go below the horizon until the fall equinox.
Do other planets have seasons?
Yes! Other planets in our solar system experience seasons for the same reason Earth does; their axis of rotation is tilted. However, some planets — like Mars and Pluto — have elliptical orbits that result in more extreme variations in distance from the Sun as they revolve around it. This, combined with the axial tilt, causes greater seasonal variation.
Uranus has an extreme tilt of 82 degrees. It takes Uranus almost 84 Earth years to complete its nearly circular path around the Sun. The tilt means that the pole of each hemisphere is exposed almost directly to the Sun's rays during the summer solstice, and the opposite hemisphere is in constant darkness. Given Uranus' long period of orbit, this translates into a 20-year winter or summer!
| Length of year (days) | Spin axis tilt (degrees) | Spring begins | Summer begins | Autumn begins | Winter begins |
| 88 | <1 | n/a | n/a | n/a | n/a |
| 224.7 | 2.6 or 1.77 | n/a | n/a | n/a | n/a |
| 365.25 | 23.4 | Mar 20, 2018 | Jun 21, 2018 | Sept 22, 2018 | Dec 21, 2018 |
| 687 | 25.2 | Mar 23, 2019 | Oct 8, 2019 | Apr 8, 2020 | Sep 2, 2020 |
| 4331 | 3.1 | n/a | n/a | n/a | n/a |
| 10,747 | 26.7 | 2009 | 2017 | 2025 | 2032 |
| 30,589 | 97.8 | 2050 | 2072 | 2007 | 2030 |
| 59,800 | 28.3 | 2046 | 2087 | 2128 | 2005 |
*Summer solstice refers to the time the north pole of a planet is tilted toward the Sun.
Based on data from 1990.