View Full Version : How Does The Earth Rotate?

2016-Jan-30, 08:30 PM
Beginning in the 16th century, humanity's understanding of our world and the cosmos was revolutionized as we became aware of a simple fact. The Earth moved! Not only did it move about the Sun, but it also rotates. And while it would take several centuries before this became universally accepted (aka. a few people had to endure persecution and house arrest) the fact that the Earth rotates about our Sun and on its axis soon became accepted fact.
In fact, the rotation of our planet on its axis and around the Sun are the cause of just about every stellar phenomenon we humans have come to take for granted. It is the reason the Sun rises in the East and sets in the West, the reason why the Moon goes through phases, and the reason the stars appear to rotate around the Earth once every day. So let's address this whole "Eppur Si Mouve (http://www.universetoday.com/48756/galileo-facts/)" business, shall we?
As already noted, Earth experiences two kinds of rotation. On the one hand, there is the rotation of Earth on its axis, which is known as sidereal rotation. This is what allows for the diurnal cycle and makes it appear as if the heavens are revolving around us. On the other hand, the Earth orbits about the Sun, which is known as its orbital period. This revolution (among other things) is responsible for the seasons, the length of the year, and variations in our diurnal cycle. Let's break these rotational habits down by the numbers...
Sidereal Rotation:Earth rotates once on its axis every 23 hours, 56 minutes and 4.1 seconds. Also known as a sidereal day, this period of rotation is measured relative to the stars. Meanwhile, Earth's solar day (i.e. the amount of time it takes for the Sun to reappear in the same place in the sky) is an even 24 hours. Naturally, we use this latter value when it comes time to measure calendar days.
Earth's sidereal rotation is responsible for the pattern of sunrises and sunsets that we are so familiar with. Using celestial objects as a reference point (i.e. the Moon (http://www.universetoday.com/19424/the-moon/), the stars, etc) the Earth rotates at a rate of 15°/h (or 15'/min) in a western direction. If viewed from space above the North Pole, Earth would appear to be rotating counter-clockwise. Hence why the Sun rises in the East and sets in the West.
The speed of the rotation of Earth has had various effects over time, including the Earth's shape (an oblate spheroid with flattening at the poles), Earth's climate, the depth and currents of its oceans, as well as tectonic forces. This should come as no surprise, considering that it's rotational velociy is 1,674.4 km/h.
However, the planet is slowing slightly with the passage of time, due to the tidal effects the Moon has on Earth's rotation. Atomic clocks show that a modern day is longer by about 1.7*milliseconds than a century ago, slowly increasing the rate at which UTC is adjusted by leap seconds. The Earth's rotation also goes from the west towards east, which is why the Sun rises in the east and sets in the west.
Orbital Period:Earth orbits the Sun at an average distance (aka. semi-major axis) of 149,598,023*km or 92,955,902*mi (about 1 AU), and completes a single rotation every*365.2564 mean solar days. This is what is known as a sidereal year, or Earth's orbital period. This creates the appearance of the Sun moving eastward through the sky at a rate of about 1° per day.
At this rate, it takes the Sun the equivalent of 24 hours - i.e. one solar day - to complete a full rotation about the Earth's axis and return to the meridian (a point on the globe that runs from north to south through the poles). Viewed from the vantage point above the north poles of both the Sun and Earth, Earth orbits in a counterclockwise direction about the Sun.
This Earth's rotation around the Sun, or the precession of the Sun through the equinoxes, is the reason a year lasts approximately 365.2 days. It is also for this reason that every four years, an extra day is required (a February 29th during every Leap Year). Also, Earth's rotation about the Sun is sujbect to a slight eccentricity of (0.0167°), which means that it is periodically closer or farther from the Sun at certain times of the year.
Earth's perihelion (147,098,074 km) occurs around January 3rd, and the aphelion around July 4th (152,097,701 km). The changing Earth-Sun distance results in an increase of about 6.9% in solar energy reaching the Earth at perihelion as related to aphelion. The southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, so the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year.
It is also the reason for the Moon's apparent phases, and the occasional lunar (http://www.space.com/15689-lunar-eclipses.html) and solar eclipse (http://www.space.com/15584-solar-eclipses.html). A lunar eclipse occurs where the Moon passes into the shadow of the Earth (umbra) relative to the Sun, which causes it to darken and take on a reddish appearance (aka. a “Blood Moon” or “Sanguine Moon”.)
A solar eclipse occurs during a new Moon, when the Moon is between the Sun and Earth. Since they are the same apparent size in the sky, the moon can either partially block the Sun (annular eclipse) or fully block it (total eclipse). In the case of a total eclipse, the Moon completely covers the disc of the Sun and the solar corona becomes visible to the naked eye.
Were it not for Earth's axial tilt (which is inclined 24.3° to the ecliptic), there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses. It is also because of it's axial tilt that the amount of sunlight reaching the Earth's surface varies over the course of the year. This is what causes seasonal changes, changes in the diurnal cycle, and changes in the Sun's position in the sky relative to equator. When one hemisphere is tilted towards the Sun, it experiences summer, characterized by warmer temperatures and longer days. Every six months, this situation is reversed.
History of Study:In ancient times, astronomers naturally believed that the Earth was a fixed body in the cosmos, and that the Sun, the Moon, the planets and stars all rotating around it. By classical antiquity, this became formalized into cosmological systems by philosophers and astronomers like Aristotle and Ptolemy - which later came to be known as the Ptolemaic Model (or Geocentric Model (http://www.universetoday.com/32607/geocentric-model/)) of the universe.
However, there were those during Antiquity that questioned this convention. One point of contention was the fact that the Earth was not only fixed in place, but that it did not rotate. For instance, Aristarchus of Samos (ca. 310 – 230 BCE) published writings on the subject that were cited by his contemporaries (such as Archimedes). According to Archimedes, Aristarchus espoused that the Earth revolved around the Sun and that the universe was many times greater than previously thought.
And then there was Seleucis of Seleucia (ca. 190 – 150 BCE), a Hellenistic astronomer who lived in the Near-Eastern Seleucid empire. Seleucus was a proponent of the heliocentric system of Aristarchus, and may have even proven it to be true by accurately computing planetary positions and the revolution of the Earth around the Earth-Moon ‘center of mass’.

The geocentric model of the universe would also be challenged by medieval Islamic and Indian scholars. For instance, In 499 CE, Indian astronomer Aaryabhata published his magnum opus Aryabhatiya (http://www.wilbourhall.org/pdfs/aryabhatiyaenglish.pdf), in which he proposed a model where the Earth was spinning on its axis and the periods of the planets were given with respect to the Sun.
The 10th-century Iranian astronomer Abu Sa’id al-Sijzi contradicted the Ptolemaic model by asserting that the Earth revolved on its axis, thus explaining the apparent diurnal cycle and the rotation of the stars relative to Earth. At about the same time, Abu Rayhan Biruni* 973 – 1048) discussed the possibility of Earth rotating about its own axis and around the Sun – though he considered this a philosophical issue and not a mathematical one.
At the Maragha and the Ulugh Beg (aka. Samarkand) Observatory, the Earth’s rotation was discussed by several generations of astronomers between the 13th and 15th centuries, and many of the arguments and evidence put forward resembled those used by Copernicus. It was also at this time that Nilakantha Somayaji published the Aryabhatiyabhasya (a commentary on the Aryabhatiya)in which he advocated a partially heliocentric planetary model. This was followed in 1500 by the Tantrasangraha, (https://archive.org/details/TantraSangrahaOfNilakantha) in which Somayaji incorporated the Earth’s rotation on its axis.
In the 14th century, aspects of heliocentricism and a moving Earth began to emerge in Europe. For example, French philosopher Bishop Nicole Oresme (ca. 1320-1325 to 1382 CE) discussed the possibility that the Earth rotated on its axis. However, it was Polish astronomer Nicolaus Copernicus who had the greatest impact on modern astronomy when, in 1514, he published his ideas about a heliocentric universe in a short treatise titled Commentariolus (http://copernicus.torun.pl/en/archives/astronomical/1/?view=transkrypcja&lang=en) (“Little Commentary”).
Like others before him, Copernicus built on the work of Greek astronomer Atistarchus, as well as paying homage to the Maragha school and several notable philosophers from the Islamic world (see below). Intrinsic to his model was the fact that the Earth, and all the other planets, rolved around the Sun, but also that the Earth revolved on its axis and was orbited by the Moon.
In time, and thanks to scientists sch as Galileo and Sir Isaac Newton (http://www.universetoday.com/91785/isaac-newton/), the motion and revolution of our planet would become an accepted scientific convention. With the advent of the Space Age, the deployment of satellites and atomic clocks, we have not only confirmed that it is in constant motion, but have been able to measure the its orbit and rotation with incredibly accuracy.
We have written many interesting articles about the motions of the Earth here at Universe Today. Here's How Fast Does The Earth Rotate? (http://www.universetoday.com/26623/how-fast-does-the-earth-rotate/), Earth's Orbit Around The Sun (http://www.universetoday.com/61202/earths-orbit-around-the-sun/), How Fast Does The Earth Rotate? (http://www.universetoday.com/26623/how-fast-does-the-earth-rotate/),*Why Does The Earth Spin? (http://www.universetoday.com/14491/why-does-the-earth-rotate/), What Would Happen If The Earth Stopped Spinning? (http://www.universetoday.com/66570/what-would-happen-if-the-earth-stopped-spinning/), and What Is The Difference Between the Heliocentric and Geocentric Models Of The Solar System? (https://www.google.com/url?q=http://www.universetoday.com/36487/difference-between-geocentric-and-heliocentric/&sa=U&ved=0ahUKEwi22pixg9DKAhUD92MKHcISBVAQFggNMAM&client=internal-uds-cse&usg=AFQjCNFnP0Xw0dn7c4jUrsVhVSqAUnkHJA)
Astronomy Cast also has a relevant episode on the subject - Episode 171: Solar System Movements and Positions (http://www.astronomycast.com/2010/02/ep-171-solar-system-movements-and-positions/)
For more information, be sure to check out NASA: Why Do We Study the Sun? (http://www.nasa.gov/vision/universe/solarsystem/sun_for_kids_main.html) NASA Eclipse (http://eclipse.gsfc.nasa.gov/5MCLE/5MCLE-Text10.pdf), and all about Earth (http://solarsystem.nasa.gov/planets/profile.cfm?Object=Earth).
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