The motion of Earth is a dynamic and intricate dance orchestrated by a combination of forces that drive its daily and yearly cycles. This celestial ballet, governed by fundamental principles of physics and astronomy, shapes the very fabric of our planet’s existence. The Earth’s rotation on its axis, responsible for the alternation of day and night, and its orbit around the sun, marking the passage of seasons, are pivotal to the forces at play. In this intricate interplay of gravitational pulls, axial tilts, and orbital paths, Earth’s geography becomes a canvas upon which these cosmic forces paint the mesmerizing portraits of our daily and yearly experiences. Understanding the forces driving these cycles is fundamental to unraveling the mysteries of our planet’s dynamic existence and the diverse landscapes it hosts. This exploration delves into the captivating realm where terrestrial geography meets celestial mechanics, unraveling the tapestry of Earth’s motion and the forces that propel it through the vastness of space and time.
Motions of the Earth
Rotation of earth
Earth rotates, one half of the planet faces the Sun, experiencing daytime, while the other half faces away from the Sun, experiencing nighttime.
The rotation of the Earth around its axis takes approximately 24 hours to complete, which defines one Earth day.
This is what causes the cycle of day and night on our planet.
The circle of illumination is the boundary between the illuminated and dark parts of the Earth, and it constantly moves as the Earth rotates.
The angle of 66.5° between the Earth’s axis and its orbital plane is also known as the axial tilt or obliquity, and it causes seasonal variations on our planet. It takes 365¼ days (one year) to revolve around the sun.
Six hours saved every year are added to make one day (24 hours) over four years. This surplus day is added to February. Thus, every fourth year, February is 29 days instead of 28 days. Such a year with 366 days is called a leap year.
Why are days always longer than nights at the equator?
If there were no atmosphere, the Sun’s rays would travel in a straight line and there would be no refraction, which means that the Sun’s position in the sky would be exactly where it would be if it were not refracted by the Earth’s atmosphere. In this case, the length of day and night would be close to equal at the equator during the equinoxes, when the Sun’s rays are perpendicular to the Earth’s axis.
The Earth’s atmosphere causes the Sun’s rays to refract, or bend, which makes the Sun appear to be above the horizon even when it is actually below it.
This effect is strongest during the morning and evening hours when the Sun’s rays are at a slant. As a result of this refraction, the days at the equator appear to be longer than the nights, even during the equinoxes when the Sun’s rays are perpendicular to the Earth’s axis.
The exact amount of time that the days are longer than the nights depends on factors such as latitude, time of year, and atmospheric conditions, but in general, the effect is most pronounced at the equator.
Revolution
The second motion of the Earth around the Sun in its orbit is called revolution, and it takes approximately 365¼ days (one year) to complete one revolution. To account for the extra ¼ day, every fourth year, an additional day (February 29) is added to the calendar, making that year a leap year with 366 days instead of the usual 365 days.
This adjustment to the calendar system is necessary because the Earth’s orbit around the Sun is not precisely 365 days long. It takes approximately 365.2422 days for the Earth to complete one orbit around the Sun, which is why an extra day is added to the calendar every four years to account for the accumulated difference.
The practice of adding an extra day to the calendar every four years is known as the Gregorian calendar system, named after Pope Gregory XIII who introduced the system in 1582. This calendar system is now widely used around the world and is the internationally accepted civil calendar.
Solstice
| Summer Solstice | Winter Solstice |
| The summer solstice occurs when the North Pole is tilted closest to the Sun | The winter solstice occurs when the North Pole is tilted farthest from the sun |
| It occurs on 21st June | It occurs on 22nd December |
| The summer solstice brings the longest day in the Northern Hemisphere, as it is tilted towards the sun. | The winter solstice brings the longest night in the Northern Hemisphere as it is tilted away from the sun |
| The Southern Hemisphere has the shortest night | The Southern Hemisphere has the longest days |
| Sun rays directly fall over the Tropic of Cancer | Sun rays directly fall over the Tropic of Capricorn |
| The places beyond the Arctic Circle experience continuous daylight for about six months | The places beyond the Antarctic Circle experience continuous daylight for about six months |
| As a large portion of the Northern Hemisphere receives sunlight and heat during the summer solstice, it summers in the Northern Hemisphere; whereas winters in the Southern Hemisphere | As a large portion of the Southern Hemisphere receives sunlight and heat during the winter solstice, it is summer in the Southern Hemisphere, whereas winter in the Northern Hemisphere |
| Although the summer solstice is the longest day of the year for the Northern Hemisphere, the dates of the earliest sunrise and latest sunset vary by a few days. This is because the Earth orbits the Sun in an ellipse, and its orbital speed varies slightly during the year. | Although the winter solstice itself lasts only a moment, the term sometimes refers to the day on which it occurs. |
| Although the Sun appears at its highest altitude from the viewpoint of an observer in outer space or a terrestrial observer outside tropical latitudes, the highest altitude occurs on a different day for certain locations in the tropics. | Traditionally, in many temperate regions, the winter solstice is seen as the middle of winter, but today in some countries and calendars, it is seen as the beginning of winter. |
Equinox
The word equinox comes from the Latin words “aequus” meaning equal and “nox” meaning night. Equinoxes refer to the two times of the year when the Earth’s axis is tilted neither toward nor away from the sun, resulting in a nearly equal amount of daylight and darkness at all latitudes.
The equinoxes occur around March 21 and September 23 every year, and on these days, the Sun is exactly above the Equator, which makes day and night of equal length. However, due to the atmospheric refraction of sunlight and the way day length is defined, most places on Earth receive slightly more than 12 hours of daylight on the equinoxes.
In addition, the equinoxes are also a prime time for the Northern Lights, also known as the Aurora Borealis. This is because the geomagnetic activity that causes the Northern Lights is twice as likely to occur during the spring and fall equinoxes than in the summer or winter.
Thus, you find that there are days and nights and changes in the seasons because of the rotation and revolution of the earth respectively.
Rotation = Days and Nights.
Revolution = Seasons.
Prime meridian
The Prime Meridian is an imaginary line that passes through Greenwich, British Royal Observatory and is considered as the base longitude, or 0 degrees longitude.
The line divides the Earth into the Eastern Hemisphere and the Western Hemisphere, and all locations on Earth are measured in terms of their distance to the east or west of the Prime Meridian.
The concept of the Prime Meridian and the 24-hour time zones based on it is crucial for establishing a global standard time. The Prime Meridian is considered the base of world time, and it serves as a reference point for determining the longitude of other places on the Earth.
This enables everyone around the world to refer to the same time, regardless of their location.
The 180 degrees east and 180 degrees west of the Prime Meridian form a complete circle around the Earth, known as the International Date Line. This line marks the change of one calendar day to the next, and when traveling across the line, the date changes by one day.
Antarctic Circle
The Antarctic Circle is an imaginary line of latitude that circles the Earth at approximately 66.33 degrees south of the Equator. It is one of the five major circles of latitude, the others being the Equator, Tropic of Cancer, Tropic of Capricorn, and the Arctic Circle.
The region located to the south of the Antarctic Circle is known as the Antarctic. This area is characterized by extremely cold temperatures, vast expanses of ice and snow, and unique flora and fauna adapted to harsh conditions.
The zone immediately to the north of the Antarctic Circle is called the Southern Temperate Zone, which includes regions of South America, southern Africa, Australia, and New Zealand.
Due to the tilt of the Earth’s axis, the position of the Antarctic Circle is not fixed and varies slightly over time. However, it is currently located approximately 66.33 degrees south of the Equator. South of the Antarctic Circle, the Sun is above the horizon for 24 continuous hours at least once per year during the southern summer solstice in December, and below the horizon for 24 continuous hours at least once per year during the southern winter solstice in June. The same is true for the equivalent polar circle in the Northern Hemisphere, the Arctic Circle.
Daylight saving in some temperate regions
Daylight saving time (DST) or summer time is the practice of advancing clocks during summer months by one hour.
In DST, evening time is increased by sacrificing the morning hours.
[Normal days = Start office at 10 AM and close at 5 PM
In DST = Advance clock by one hour (can be more) = Start office at 9 AM and Close at 4 PM]
Daylight Saving Time (DST) is a practice of advancing the clock by one hour during summer months, typically from March to November, in regions with summer time.
One of the main problems with DST is that it can disrupt people’s sleep patterns and lead to fatigue, particularly when the clocks are set forward in the spring.
It can also confuse travelers, who may not be aware of the time changes in the regions they are traveling to or from. Additionally, it can complicate timekeeping and billing, as some systems may not account for the change in time.
There have also been reports of medical devices and heavy equipment malfunctioning due to DST changes.
Equator
The equator is located at zero degrees latitude.
It is the longest circle of latitude on the Earth and is approximately 40,075 km (24,901 mi) in circumference.
The equator is an imaginary line located at 0 degrees latitude, halfway between the North and South poles.
The equator passes through 13 countries in total, including those you mentioned. The other countries are Colombia, Sao Tome and Principe, Gabon, Somalia, Kiribati, Ecuador (again because the line passes through the country twice), and the international waters of the Atlantic, Pacific, and Indian Oceans.
On the equator, the sun is directly overhead at noon on the two equinoxes – near March 21 and September 21. This means that on these two days, the length of day and night is nearly equal all around the world.
On the equator, the sun rises due east and sets due west every day of the year, which causes the length of day and night to be almost equal (about 12 hours) throughout the year. This is because the equator is perpendicular to the Earth’s axis of rotation, so it receives roughly the same amount of sunlight throughout the year.
Tropic of Cancer and Tropic of Capricorn
The Tropic of Cancer and the Tropic of Capricorn are two circles of latitude that are located 23.5 degrees north and south of the Equator, respectively.
The equator is the circle where the sun is directly overhead at noon on the equinoxes (around March 20 and September 22), while the Tropic of Cancer and the Tropic of Capricorn are the circles where the sun is directly overhead at noon on the solstices (around June 21 and December 21).
These latitudes are important because they mark the boundaries of the tropics, where the climate is generally warmer and wetter than in other parts of the world.
As for the naming of the Tropic of Cancer, you’re right that it was named about 2,000 years ago based on the position of the sun in the sky during the June solstice when the sun appears to be in the direction of the constellation of Cancer.
The name “tropic” comes from the Greek word “Tropos,” meaning turn, because the sun appears to turn back at these latitudes during the solstices.
The Tropic of Cancer is located approximately 23.5° north of the equator and passes through Mexico, the Bahamas, Egypt, Saudi Arabia, India, and southern China. Meanwhile, the Tropic of Capricorn is located at approximately 23.5° south of the equator and passes through countries such as Australia, Chile, southern Brazil, and northern South Africa.
Brazil is indeed the only country that passes through both the equator and the tropics, making it an interesting location for astronomical and geographical studies.
Effects of Rotation
The rotation of the Earth on its axis is one of the fundamental motions of our planet and has important environmental and societal consequences. Here are some additional details to expand on your points:
The diurnal cycle of light and darkness created by the Earth’s rotation drives various natural processes, such as photosynthesis in plants and the sleep-wake cycle in animals. It also affects the distribution of heat and moisture across the planet, which in turn influences weather patterns and the distribution of biomes.
The day-night cycle created by the Earth’s rotation also leads to a corresponding cycle of temperature and humidity changes. For example, daytime heating of the land and ocean surfaces leads to convection and the formation of clouds, which can affect rainfall patterns and regional climates.
The rotation of the Earth requires the creation of standardized time zones to maintain consistency in timekeeping across the world. There are 24 time zones, each separated by one hour of the Earth’s rotation. This allows for efficient coordination of activities and communication across long distances.
Rotation causes the tides
The rotation of the Earth is also responsible for causing tides, which are the regular rise and fall of sea levels around the world. Here are some additional details to expand on your points:
Tides are caused by the gravitational forces of both the Moon and the Sun acting on the Earth’s oceans. The gravitational force of the Moon is stronger because it is closer to the Earth, but the Sun’s gravity also plays a role.
The alignment of the Earth, Sun, and Moon influences the height of the tides. When the Sun, Moon, and Earth are in a straight line (either during a new moon or a full moon), their gravitational forces combine to create higher high tides (spring tides) and lower low tides (neap tides).
The geography of coastlines and ocean basins also affects the magnitude and timing of tides. In narrow bays or estuaries, for example, the shape of the coastline can cause amplification of the tidal range, resulting in extremely high or low tides.
Tides have many important ecological and economic impacts, such as influencing the distribution and behavior of marine organisms, shaping coastal habitats, and affecting navigation and commerce. They also provide a source of renewable energy through tidal power generation.
The Coriolis Force
The rotation of the Earth also causes a deflection of ocean and air currents, which has important consequences for climate and weather patterns. Here are some additional details to expand on your points:
The Coriolis effect, which is caused by the Earth’s rotation, causes the deflection of winds and ocean currents. As you mentioned, the rotation of the Earth is much faster than the movement of winds and currents, resulting in a deflection to the right in the Northern Hemisphere and the left in the Southern Hemisphere.
This deflection ultimately results in the creation of rotating patterns of high and low pressure in the atmosphere, known as anticyclones and cyclones, respectively. These pressure systems help to drive the global circulation of air and moisture, influencing regional weather patterns and climate.
In the oceans, the deflection of currents due to the Coriolis effect creates large rotating pools of water called gyres. There are five major gyres in the world’s oceans, each centered around a subtropical high-pressure system. These gyres influence ocean circulation, heat transport, and nutrient cycling, as well as affect marine ecosystems and weather patterns in coastal regions.
The deflection of winds and currents due to the Earth’s rotation also affects the distribution and movement of pollutants, such as air pollution and plastic debris in the oceans.
Frequently Asked Questions (FAQs)
1. FAQ: What causes the Earth’s daily rotation?
Answer: The Earth’s daily rotation is primarily caused by its initial angular momentum acquired during its formation. As the solar system formed from a rotating disk of gas and dust, the gravitational collapse of this material led to the conservation of angular momentum. As the collapsing material came together to form the Earth, its rotation rate increased due to the conservation of angular momentum. The Earth continues to rotate on its axis, completing one full rotation approximately every 24 hours, causing day and night cycles.
2. FAQ: What forces drive the Earth’s yearly orbit around the Sun?
Answer: The Earth’s yearly orbit around the Sun, known as revolution, is primarily driven by gravitational forces. The gravitational attraction between the Earth and the Sun keeps the Earth in orbit around the Sun. This force is balanced by the Earth’s inertia, which tends to keep it moving in a straight line. The combination of gravitational attraction and the Earth’s inertia results in a stable orbit. The elliptical shape of the Earth’s orbit and the tilt of its axis contribute to the changing seasons as different parts of the Earth receive varying amounts of sunlight during different times of the year.
3. FAQ: How does the tilt of the Earth’s axis impact its seasons?
Answer: The tilt of the Earth’s axis plays a crucial role in the changing seasons. The Earth’s axis is tilted at an angle of approximately 23.5 degrees relative to its orbit around the Sun. This tilt causes different parts of the Earth to receive varying amounts of sunlight at different times of the year. When a hemisphere is tilted toward the Sun, it experiences summer because sunlight is more concentrated, and days are longer. Conversely, when a hemisphere is tilted away from the Sun, it experiences winter due to less concentrated sunlight and shorter days. The tilt, combined with the Earth’s orbit, results in the annual cycle of seasons.
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