Earth-Sun Relations
Although Earth is considered a closed system with regards to physical matter, it is an open system in terms of energy - specifically, solar energy.
The sun provides over 99% of the total energy used on Earth. As described in the Biosphere module, sunlight is the foundation of Earth's biosphere: sunlight is converted by producers (plants) into matter, thus sustaining all life on earth. Solar energy also drives most of Earth's climate patterns, winds, and currents. Even fossil fuels were created by the metamorphosis of woody plants - producers which originally chemically transformed solar energy into plant tissue billions of years ago.
Insolation
Insolation is the solar radiation intercepted by Earth - so, basically another word for solar energy. Insolation hits Earth at consistent parallel rays. Each ray contains the same amount of solar energy. However, the solar energy from these rays is unevenly distributed across Earth's surface.
Direct Rays
Direct rays distribute the most insolation over the smallest area of Earth's surface. They also have the shortest distance to travel from the sun.
Oblique Rays
Oblique rays have to travel a farther distance, and have to distribute the same amount of insolation over a larger surface. This means that these parts of the globe get less insolation than those hit by direct rays.
Tangent Rays
Tangent rays are those hitting the very top and bottom of Earth, and thus receive very little insolation. These rays could be described as 'glancing blows'. Thus, whatever part of Earth is at the top/bottom of the globe at a given time will receive the least amount of insolation.
Axial Tilt
The distribution of solar energy on Earth's surface is also affected by Earth's axial tilt. Earth tilts 23.5 degrees on its axis. The axial tilt remains the same throughout the year.
This means that the same location on Earth's surface will receive different amounts of insolation at different times. So for instance, Earth's equator (0 degrees latitude) is not always receiving the most insolation! Likewise, Earth's Geographic North Pole (90 degrees N of the equator) and Geographic South Pole (90 degrees S of the equator) do not always receive the least incoming insolation. In fact, Earth's axial tilt means that locations north of the Arctic Circle (66 degrees N of the equator) or south of the Antarctic Circle (66 degrees S of the equator) will experience 24 hours of continuous sunlight when the pole is tilted towards the sun, and 24 hours of continuous darkness when the pole is tilted away from the sun.
Subsolar Point
The point of direct insolation is known as the subsolar point, or the location on Earth's surface that is receiving the most solar energy at any given time. In other words, the subsolar point is where Earth is receiving direct rays of insolation.
Because Earth is tilted, the subsolar point moves up and down as the Earth rotates. It moves from the Tropic of Cancer (23.5 degrees north of the equator) to the Tropic of Capricorn (23.5 degrees south of the equator).
- You can see where the subsolar point is right now using online trackers, like this one Links to an external site.!
- Within the zone between the Tropics of Cancer and Capricorn, there is a phenomenon called Lāhainā Noon, where shadows completely disappear! This happens when the sun is directly over head and the sun's rays hit Earth's surface at exactly 90 degrees - in other words, when that location is sitting at the subsolar point. For a fun video discussing this phenomenon, check out this link Links to an external site.. (To see subtitles on the videos, click the small closed caption or "CC" button at the bottom of the video.)
As noted above, each ray of insolation has the same wattage, i.e. the same amount of solar energy. The farther you get from the subsolar point, the more each ray of solar energy has to spread out - these are oblique rays of insolation. The amount of space each ray has to cover increases, the further north/south you move from the subsolar point.
Seasons
Earth's axial tilt is the reason Earth experiences seasons, or divisions of the year distinguished by changes in weather and the number of daylight hours.
As Earth revolves around the sun (which takes 365.25 days, or one year), it spins on its axis. This means that some parts of Earth are tilted more towards the sun at certain times of year, receiving more light and heat (summer). Other parts of Earth are tilted away from the sun at other times, receiving less light and heat (winter).
To see an animation depicting the relationship between Earth's axial tilt, incoming solar radiation, and seasonality, check out this video Links to an external site. from the California Academy of Sciences. (Note: to see subtitles on the videos, click the small closed caption or "CC" button at the bottom of the video.)