16.5: Earth-Sun Relationships
- Page ID
- 32270
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Earth's Rotation
The Earth rotates once on its axis about every 24 hours. If you were to look at Earth from the North Pole, it would be spinning counterclockwise. As the Earth rotates, observers on Earth see the Sun moving across the sky from east to west with the beginning of each new day. We often say that the Sun is “rising” or “setting,” but it is the Earth’s rotation that gives us the perception of the Sun rising or setting over the horizon. When we look at the Moon or the stars at night, they also seem to rise in the east and set in the west. Earth’s rotation is also responsible for this. As Earth turns, the Moon and stars change position in our sky.
Another effect of Earth’s rotation is that we have a cycle of daylight and darkness approximately every 24 hours. This is called a day. As Earth rotates, the side of Earth facing the Sun experiences daylight, and the opposite side (facing away from the Sun) experiences darkness or nighttime. The circle of illumination is the line separating the part of Earth receiving sunlight and the part of Earth in darkness. Since the Earth completes one rotation in about 24 hours, this is the time it takes to complete one day-night cycle.
Earth's Tilt
It is a common misconception that summer is warm, and winter is cold because the Sun is closer to Earth in the summer and farther away from it during the winter. The seasons are actually caused by the 23.5° tilt of Earth’s axis of rotation and Earth’s yearly revolution around the Sun. This axis of rotation is tilted 23.5° relative to its plane of orbit around the Sun. The axis of rotation is pointed toward Polaris, the North Star. As the Earth orbits the Sun, the tilt of Earth’s axis stays lined up with the North Star. The tilt of the axis of rotation results in one part of the Earth being more directly exposed to rays from the Sun than the other part. The sunlight is most concentrated at the point where the Sun's rays are perpendicular to the surface of the Earth, forming a high Sun angle. As the Sun angle becomes smaller, the effect is that the same amount of Sun energy is spread over a larger area, which also means that each area gets less energy. This uneven distribution of sunlight on the tilted Earth produces the seasons. The part tilted away from the Sun experiences a cool season, while the part tilted toward the Sun experiences a warm season. Seasons change as the Earth continues its yearly revolution around the Sun, causing the hemisphere tilted away from or towards the Sun to change accordingly. When it is winter in the Northern Hemisphere, it is summer in the Southern Hemisphere, and vice versa.
As the Earth rotates, different places on Earth experience sunset and sunrise at a different time. As you move towards the poles, summer and winter days have different lengths of daylight hours in a day. For example, in the Northern Hemisphere, summer begins on or around June 21. At this point, the Earth’s North Pole is tilted toward the Sun. Therefore, areas north of the equator experience longer days and shorter nights because the northern half of the Earth is tilted toward the Sun. Since the southern half of the Earth is tilted away from the Sun, the opposite effect occurs, longer nights and shorter days. In the Northern Hemisphere, winter begins on or around December 21. At this point, the Earth’s South Pole is tilted toward the Sun, so there are shorter days and longer nights for areas north of the equator.
Northern Hemisphere Summer
The North Pole is tilted towards the Sun and the Sun’s rays strike the Northern Hemisphere more directly in summer. At the summer solstice, around June 20 or 21, the Sun’s rays hit the Earth most directly along the Tropic of Cancer (23.5° N). The Arctic Circle (66.5°N) is in constant sunlight and experiences 24 hours of daylight. By the same token, the Antarctic Circle (66.5°S) is in constant darkness and experiences 24 hours of night. When it is summer solstice in the Northern Hemisphere, it is winter solstice in the Southern Hemisphere.
Northern Hemisphere Winter
The winter solstice for the Northern Hemisphere happens around December 21 or 22. The tilt of Earth’s axis points away from the Sun. The Sun's rays hit the Earth most directly along the Tropic of Capricorn (23.5° S). Light from the Sun is spread out over a larger area, so that area isn’t heated as much. With fewer daylight hours in winter, there is also less time for the Sun to warm the area. The Arctic Circle experiences 24 hours of darkness while the Antarctic Circle experiences 24 hours of daylight. When it is winter solstice in the Northern Hemisphere, it is summer solstice in the Southern Hemisphere.
Northern Hemisphere Equinox
Halfway between the two solstices, the Sun’s rays shine most directly at the equator, called an equinox. The circle of illumination passes through the Earth's axis of rotation at the poles. The daylight and nighttime hours are exactly equal on an equinox. The autumnal equinox happens on September 22 or 23 and the vernal or spring equinox happens around March 20 or 21 in the Northern Hemisphere.
References
- Dynamic Earth: Introduction to Physical Geography. Authored by: R. Adam Dastrup. Located at: http://www.opengeography.org/physical-geography.html. Project: Open Geography Education. License: CC BY-SA: Attribution-ShareAlike
- Earth's rotation and the circle of illumination. Authored by: Waverly Ray. Located at: https://geo.libretexts.org/Bookshelves/Geography_(Physical)/Physical_Geography_Lab_Manual_(Ray_et_al.)/01%3A_Labs/1.03%3A_Lab_3_-_Earth-Sun_Relationships. License: CC BY-NC-SA 4.0
- Tilt of Earth's axis of rotation. Authored by: LuckySoul, N. Ikeda. Located at: stock.adobe.com. License: CC BY-NC-SA 4.0
- Earth on June 21. Authored by: Andrew Fraknoi, David Morrison, Sidney Wolff. Title: Astronomy 2e. Publisher: OpenStax. Located at: https://openstax.org/books/astronomy-2e/pages/4-2-the-seasons. License: CC BY 4.0
- Earth on December 21. Authored by: Andrew Fraknoi, David Morrison, Sidney Wolff. Title: Astronomy 2e. Publisher: OpenStax. Located at: https://openstax.org/books/astronomy-2e/pages/4-2-the-seasons. License: CC BY 4.0
Public Domain content
- The Science of the Seasons. Provided by: NOAA. Located at: https://scienceinprek.si.edu/resource/image/science-seasons. License: Public Domain