Earthworks & Systems, Climate Change

The Tilting of the Earth: Shaping Our Seasons and Climates

By Eric McLamb

On January 3, 2008, at 2:00 PM (Universal Time), Earth will reach its closest distance to sun. At this time, the northern hemisphere experiences winter while the southern hemisphere experiences summer. The Earth will travel to its farthest point from the sun on July 4, 2008, at 8:00 AM (UT), when it will be summer in the Northern Hemisphere and winter in the Southern Hemisphere.

Earth travels around the Sun at about 18.4 miles per second while rotating on its axis at about 1,070 miles per hours. Pictured above is Earth during summer for the Northern Hemisphere where the North Pole (center of the Arctic Ice Cap) receives sunlight 24 hours a day. (Image: University of Toledo)

Hurtling through space at an average speed of 18.4 miles per second, Earth is constantly changing its position with the sun. Not only is the Earth orbiting around the sun , but it also rotating on its axis at 1,070 miles per hour (speed at the equator). It is, however, a common misconception that the Earth's distance from the sun determines how warm or cold the planet gets.

While it is true that Earth does have a perihelion, or point at which it is closest to the sun, and an aphelion, its farthest point from the sun, the difference between these distances is too minimal as to have any significant impact on the Earth's seasons and climate. The average distance of the Earth from the sun is about 93 million miles (which is also referred to as one astronomical unit or AU). At its closest point, the Earth is about 91.1 million miles from the sun; conversely, the sun is about 94.8 million miles away when it is at aphelion. With these numbers it's easy to figure out that the Earth's orbit around the sun is not so much elliptical (oval) as it is circular, and that the Earth's distance from the sun remains relatively constant throughout its annual orbit.

So what does the Earth's orbit around the sun have to do with our planet's constantly changing temperatures and changing seasons? The answer is... everything! Earlier we said that the Earth is constantly changing its position with the sun. That's because the Earth is tilted in relation to the sun. That is what creates the differences in the seasons and the annual warming and cooling cycles of the Earth's Northern and Southern Hemispheres.

The Earth is tilted 23.5 degrees on its axis, a straight line through the planet from the North Pole to the South Pole. The Earth spins around, or rotates, on this axis as it orbits the sun. The key here is that as the Earth orbits the sun, different regions on Earth are tilted both towards and away from the sun depending on the region's respective hemisphere. This causes the sun's light and energy to hit the different regions of the Earth at different angles throughout the course of one orbit, or one full year.

The Seasons

When the North Pole is tilted most toward the sun, the Northern Hemisphere experiences summer. This occurs when the Earth is farthest away from the sun, and begins on June 21-22. Astronomers refer to the arrival of this event as the Summer Solstice. The sun's energy is more concentrated on the Northern Hemisphere where its rays hit the Earth more directly and are thus more intense. At the same time, however, the Southern Hemisphere is tilted away from the sun causing the sun's rays to hit the region more at an angle and with less intensity. This brings about winter in the Southern Hemisphere, the arrival of which is called the Winter Solstice.

This diagram indicates the orbital position of the Earth as it reaches winter, spring, summer and fall. Note the tilt in relation to the Sun. (NOAA Image)

As the Earth continues along its path around the sun, its angle constantly shifts the North Pole away from the sun and the South Pole toward the sun. On December 21-22 -- called the Winter Solstice in the Northern Hemisphere, the Earth's North Pole is tilted farthest from the sun and the South Pole is pointed the closest to the sun (or Summer Solstice in the Southern Hemisphere). This is also the time when Earth reaches its closest orbital distance to the sun, and winter arrives in the north and summer in the south.

The Equinoxes

Now, just to close the loop on the effects Earth's solar orbit has on its seasons and climates, here's what happens in-between the winter and summer solstices. Spring and Fall (or Autumn) occur midway on the Earth's journey from winter to summer and from summer to winter. These times occur when the sun appears to be directly over the Earth's equator, and the length of day and night are equal over most of the planet. On March 20 or 21 of each year, the Earth reaches the vernal equinox which marks the arrival of Spring in the north and Fall in the south. The autumnal equinox occurs on September 22-23 and marks the arrival of Fall in the north and Spring in the south.

Putting It All Together

So, now, what do we know? Let's review. We now know that Earth rotates on its axis as it travels around the sun in an almost circular orbit. We also know that, because the Earth is tilted on its axis, its seasons change as it orbits the sun. When it is Summer in the Northern Hemisphere, it is Winter in the Southern Hemisphere, and this has nothing to do with how close or far the Earth gets to or away from the Sun in its orbit. It's all because the Earth is tilted on its axis.

(Now, just to make sure we don't mislead you, the Earth's overall orbit or distance from the sun would make a difference if it were located say where Mercury is or where Pluto is. As Dr. Jack Hall from ecology.com's Dr. Jack's Natural World says: "It's the three bears syndrome. We're not too close to the sun, and we're not too far away. We're j-u-u-u-u-st right!)

Venus' surface (pictured) maintains an average temperature of about 882°F due to the extremely high concentrations of carbon dioxide (CO2) in its atmosphere. CO2 is a greenhouse gas that also exists in Earth's atmosphere but at lower concentration levels that help maintain life-sustaining temperatures. (NASA photo)

Good! But this does not explain why our planet maintains its relatively warm, life-sustaining temperatures and climates that can -- for the most part -- sustain life, does it? It also does not explain why life on Earth doesn't burn up. For Earth to maintain its average temperature of 61°F (16.1°C), it requires a very delicate balance within its atmosphere, oceans and solid Earth. Consider the planet Mercury, which is the planet closest to the Sun at a distance of only 36 million miles. But its temperatures range from minus 280°F to plus 800°F, depending on whether or not its surface is facing the Sun. Venus, on the other hand -- which is the second closest planet to the Sun at a distance of about 67 million miles, maintains an average surface temperature of about 882°F regardless of which part of the planet is facing the Sun. Neither planet is able to sustain life as we know it on Earth, nor is any other planet in our solar system.

The Sahara Desert, pictured, records the hottest temperatures on Earth. Located in the northern part of Africa, the Sahara's highest temperatures average around 130°F. (MIT Photo)

Earth's... coldest temperature averages about minus 60°F (-45°F to -97°F) and its hottest temperature averages about 130°F-plus. While these extremes make most life impossible to naturally exist or thrive, they occur only in remote areas of the planet, such as the Antarctic (coldest average temperatures) or the Sahara Desert (hottest). Still, these temperatures are relatively warm (or cool) compared to other planets.

Earth's "Checks & Balances"

Earth has a built-in, naturally-occurring "force field" around it that creates and helps maintain viable living conditions for its plant and animal inhabitants. The atmosphere contains greenhouse gases like carbon dioxide to keep the Earth warm, and an ozone layer to protect the Earth from harmful and incinerating (burning) radiation.

The Earth also has other checks and balances that help it absorb and retain the sun's heat and energy (such as the oceans) as well as reflect its energy back into space (such as the polar ice caps). Wind and ocean currents also help distribute this heat around the globe, all within the Earth's protective atmosphere. The point is that Earth is unique in its ability to create and maintain sustainable living conditions because all of its systems and influences are connected to each other, from its atmosphere, oceans and land, to its seasons, its living inhabitants and the sun.

Venus' extremely hot temperature exists because of the very high concentrations of carbon dioxide (a greenhouse gas) in its atmosphere. In fact, Venus has the thickest atmosphere of all the planets. Heat gets in, but doesn't leave. Mercury, on the other hand, has no atmosphere and relies totally on direct sunlight for heat. But the side of Mercury facing away from the sun, which experiences extreme cold, loses all this heat because there is nothing there to hold it in. Earth's systems are j-u-u-u-u-st right!

-- Eric McLamb

Did you know....?

• The hottest temperature ever recorded on Earth is 136°F in El Azizia, Libya, in Northern Africa on the northern fringe of the Sahara Desert. A close second is Death Valley in California's Mojave Desert which registered 134°F on July 10, 1913. (Source: The Physics Factbook)

• The coldest temperature ever recorded on the planet's surface did not occur at the South Pole but at Vostok, Antarctica, on July 31, 1983. It was -128.6°F!!! (Source: The Physics Factbook)

• It's the Sahara Desert's dryness, not heat, that makes it a desert. The frozen continent of Antarctica, the coldest place on Earth, is so dry that some scientists consider it a desert, too.

• Our solar system orbits around the center of the Milky Way, our home galaxy. It takes approximately 225 million years for the sun to make one revolution or trip around the Milky Way. (Source: US Department of Energy)

• The speed of the Earth is fastest when it is closest to the Sun, in January, and slowest when it is farthest away from the sun, in July. In other words, in January it will be moving faster than average, and in July it will be moving slower than average.

• Earth's solar system is located on the outer edge on the Milky Way, about 28,000 light years from the center of the galaxy. Most of the Milky Way's 200 billion other stars and their planets are clustered in the center. This leads many scientists to believe that if the solar system were located more toward the center of our giant galaxy, life on Earth would be impossible because of the overwhelming heat and energy emitting from those stars.

• Although most planets spin on their axis that are tilted only a few degrees, the third largest planet, Uranus, spins on an axis that is virtually perpendicular to the sun. Still, it is hotter at its equator than at its poles, the reasons for which are unknown.

Also, check out these sites:

• Equinox and Solstice
• An Overview of the Solar System
• Zoom Astronomy: The Seasons and Axis Tilt
• Misconceptions About Sun-Earth Science