As we eagerly look toward longer days and warmer temperatures here in the Northern Hemisphere, let’s learn more about what other areas of our Earth are experiencing by taking a look at our Earth’s seasons around the globe.
The seasons on the Earth are largely due to the Earth's tilted axis as it revolves around the sun; this is because one of the most important factors that determines the seasons is the intensity of the sun's rays over a given area. This changes as the Earth revolves around the sun.
The Earth is tilted 23.5 degrees on its axis, which affects the distribution of the sun’s energy across the surface of the planet. As the Earth orbits the sun every 365 ¼ days, the axis is always pointing in the same direction into space, with the North Pole tilted toward Polaris, the North Star. Note that the distinctions in season are caused by the tilt of the earth—not how close or how far the Earth is from the sun. In fact, the Earth is farthest from the sun (aphelion) on July 3, which is full summer in the Northern Hemisphere.
Broadly speaking, the Northern and Southern Hemispheres of the Earth have opposite seasons. As the Earth moves around the sun, the tilt of the earth’s axis shifts; during the months of June, July, and August, the Northern Hemisphere is angled toward the sun and therefore receives the most direct radiation and energy. In December, January, and February, the Earth’s revolution around the sun angles the Northern Hemisphere away from the sun, and it receives less energy than the Southern Hemisphere, which now experiences most of the sun’s energy.
The summer and winter solstices, the longest and shortest days of the year, occur when Earth's axis is either closest or farthest from the sun. The summer solstice in the Northern Hemisphere occurs around June 21, the same day as the winter solstice in the Southern Hemisphere. The south's summer solstice occurs around December 21, which is the winter solstice for the north. In both hemispheres, the summer solstice marks the first day of astronomical summer, while the winter solstice is considered the first day of astronomical winter.
During the vernal and autumnal equinoxes, the sun’s rays strike the planet exactly perpendicular to the Earth’s surface at the equator. At the equinoxes, all areas of the Earth experience nearly equal periods of day and night. The spring, or vernal, equinox for the Northern Hemisphere takes place around March 20, the same day as the autumnal equinox in the Southern Hemisphere. The vernal equinox in the Southern Hemisphere occurs around September 20. The vernal equinox marks the first day of astronomical spring for a hemisphere, while the autumnal equinox ushers in the first day of fall.
To make matters more complicated, in the United States, we are taught that there are four seasons (spring, summer, fall, and winter) that begin and end with the specific astronomical points in time—the solstices and the equinoxes. But culture and geography do not make it that simple.
Areas near the equator tend to only have two seasons: rain/monsoon or dry. As they are near the equator the amount of light they have during the year does not change much, nor does the average temperature. However, areas at the poles tend to fall into a summer/winter cycle, which include a “midnight sun” in summer and “polar night” in winter.
In other areas of the world, although four recognized seasons are common, some regions recognize two, three, or six. In fact, many ecologists note that in temperate regions there are six identifiable seasons based on plant growth and hibernation cycles.
Historically speaking, the Egyptians had three seasons based on the Nile—flood, growth, and low water. The Celts in early Britain marked the beginning of summer at Beltane and its end at Samhain (Hallowe’en). The Romans recognized four seasons and the Greeks recognized three or four depending on what source you read. The Hindu calendar identifies six.
In this activity, which can be done at home, students use a light source and a globe to model sunlight striking Earth. They monitor the temperature in the globe’s Northern Hemisphere and Southern Hemisphere and use their observations to understand how the tilt of Earth’s axis causes the seasons.
Materials: light source (e.g., lamp with an incandescent bulb), globe, meter stick, thermometers (2), masking tape
Steps:
Place a lamp and a globe 25 inches apart. (Remind students not to touch the light bulb when the lamp is turned on.) Make sure the equator line on the globe is at the same height above the table as the light bulb. If needed, adjust the height of the globe using books. Also make sure the Northern Hemisphere is tilted away from the lamp.
Use the tape to attach thermometers on the globe along the 15° north latitude line and 15° south latitude line.
Record the temperature on each thermometer.
Then turn on the lamp, wait 5 minutes, and record the temperature on each thermometer again. Determine the change in temperature in the globe’s Northern Hemisphere and Southern Hemisphere.
Check out this sun, earth, and moon orbital model from Eisco Labs -- the 3D model allows students to study the motion of the earth and moon around the sun, and demonstrates the science behind phenomena such as seasons, phases of the moon, eclipses, and tides.
