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NGSS Crosscutting Concepts:NGSS Disciplinary Core Ideas:It may seem strange that the northern hemisphere is closer to the sun during the winter and farther away during summer! However, the seasons are determined primarily by the tilt of the earth, not the distance between the earth and the sun. The earth is tilted on its axis approximately 23.5˚ relative to its orbital plane around the sun. When a hemisphere is tilted toward the sun, that hemisphere receives more direct sunlight and experiences summer. At the same time, the other hemisphere is tilted away from the sun and receives less direct sunlight, experiencing winter. The northern hemisphere is more directly exposed to the sun’s rays during June and July (summer in the northern hemisphere, SF Fig. 6.11 A). The southern hemisphere receives less direct sunlight during these months, resulting in winter. Exposure to direct sunlight alternates as the earth revolves in its orbit around the sun. The southern hemisphere is most directly exposed to the sun’s rays in December and January (SF Fig. 6.11 B). During this time the northern hemisphere experiences winter as it is receiving indirect sunlight. Even though the tilt of the earth is responsible for the seasons, the elliptical orbit of the earth moderates the northern hemisphere's winters and summers. This means the temperatures of the northern hemisphere’s winters and summers are not as extreme as could be. In the southern hemisphere, the opposite effect occurs, with more extreme winters and summers. Special Feature Type:Table of Contents:
 Summer happens in the hemisphere tilted towards the Sun, and winter happens in the hemisphere tilted away from the Sun. Click on image for full size Windows to the Universe original image
Climate Varies With Latitude What Is Climate? The Earth travels around the Sun one full time per year. During the year, the seasons change depending on the amount of sunlight reaching parts of the Earth. The seasons are caused because the Earth is tilted 23.5 degrees on its axis. Summer happens to the hemisphere tilted towards the Sun, and winter happens to the hemisphere tilted away from the Sun. The hemisphere that is tilted towards the Sun is warmer because sunlight travels more directly to the Earth’s surface so less gets scattered in the atmosphere. That means that when it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere. The hemisphere experiencing summer, tilted towards the Sun, has longer days and shorter nights than the hemisphere tilted away from the Sun.
In general, summer and winter temperatures get lower the further you travel from the equator. At the equator, there are no seasons because each day the Sun strikes at about the same angle. Every day of the year the equator receives about 12 hours of sunlight. The poles remain cool because they are never tilted in the direct path of the sunlight. Light must travel through so much atmosphere that much of it is scattered before reaching the Earth surface. During midwinter, when a pole is tilted away from the Sun, there is no daylight at all at the pole. The Sun never rises. However, during the summer, a pole receives sunlight all the time and there is no night! Many people believe that Earth is closer to the Sun in the summer and that is why it is hotter. And, likewise, they think Earth is farthest from the Sun in the winter. Although this idea makes sense, it is incorrect. It is true that Earth's orbit is not a perfect circle. It is a bit lop-sided. During part of the year, Earth is closer to the Sun than at other times. However, in the Northern Hemisphere, we are having winter when Earth is closest to the Sun and summer when it is farthest away! Compared with how far away the Sun is, this change in Earth's distance throughout the year does not make much difference to our weather. There is a different reason for Earth's seasons. Earth's axis is an imaginary pole going right through the center of Earth from "top" to "bottom." Earth spins around this pole, making one complete turn each day. That is why we have day and night, and why every part of Earth's surface gets some of each. Earth has seasons because its axis doesn't stand up straight. But what caused Earth to tilt?Long, long ago, when Earth was young, it is thought that something big hit Earth and knocked it off-kilter. So instead of rotating with its axis straight up and down, it leans over a bit. By the way, that big thing that hit Earth is called Theia. It also blasted a big hole in the surface. That big hit sent a huge amount of dust and rubble into orbit. Most scientists think that that rubble, in time, became our Moon. As Earth orbits the Sun, its tilted axis always points in the same direction. So, throughout the year, different parts of Earth get the Sun’s direct rays. Sometimes it is the North Pole tilting toward the Sun (around June) and sometimes it is the South Pole tilting toward the Sun (around December). It is summer in June in the Northern Hemisphere because the Sun's rays hit that part of Earth more directly than at any other time of the year. It is winter in December in the Northern Hemisphere, because that is when it is the South Pole's turn to be tilted toward the Sun. Earth's lopsided orbitEarth's perihelion (point closest to Sun) = 91,400,000 miles from Sun Earth's aphelion (point farthest from Sun) = 94,500,000 miles from Sun While that is a difference of over 3 million miles, relative to the entire distance, it isn’t much. And, believe it or not, aphelion (when Earth is farthest from the Sun) occurs in July, and perihelion (when we are closest) occurs in January. For those of us who live in the Northern Hemisphere where it's summer in July and winter in January, that seems backwards, doesn't it? That just goes to prove that Earth's distance from the Sun is not the cause of the seasons. Related Resources for Educators Seasons (Educator Guide to go with Seasons Spotlite video)
SkyTellers Seasons activities for young children About Our SeasonsWhat causes our seasons?
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? What are solstices and equinoxes? 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? 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!
*Summer solstice refers to the time the north pole of a planet is tilted toward the Sun. |
When the Northern Hemisphere has summer the Southern Hemisphere is tilted away from the Sun?
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