From time immemorial, humanity has wondered about the shape of the Earth. Over the centuries countless studies exploring the Earth and its shape have been conducted, and they still continue today. With advancements in technology, the methods and measuring devices have constantly changed. In earlier periods the Earth was believed to be flat; nevertheless from around the fifth century bc there were varying opinions suggesting that the Earth was actually round and calculations were conducted to measure its radius. Particularly from the seventh century ce onwards, the number of studies regarding what the Earth really looked like have increased tremendously.1
In later years, with the advent of Islam and its open encouragement of Muslims to explore the universe and make advances in science, Muslim scholars made huge progress in astronomical research. Historians who have studied these advancements in astronomical science agree that the era between the eighth and fourteenth centuries can aptly be designated as a period of Islamic astronomy. In the years following the sixteenth century, significant research was undertaken in relation to the shape of the Earth and measurements of its radius in both the Islamic world and the West.
In the eighteenth century astronomic research and the advancement of technology proved not only that the Earth was round, but that it had a distinctive shape. According to calculations, the Earth was bulging around the equator and flattened at the poles. The maps produced by satellite systems show that the Earth is not completely round or smooth, but rather it has protrusions, creating an uneven surface that resembles a face with spots.2
"Geoid" is the term scientists prefer to use when referring to the physical depiction of the Earth's surface. The shape of the Earth is not a perfect ellipsoid, thus, scientists use this representative surface that is thought to be most approximate to sea level, in order to identify departures from the ellipsoid shape. Due to the events of nature and human-related factors the geoid constantly changes, and this is why a precise mathematical account of the geoid has not yet been possible.
The Earth is known to be a geologically active planet. Just as everything else in the universe, from atoms to galaxies, has not been left to their own fate, the Earth is also constantly being transformed, thus making our magnificent ecosystem possible. The continuous geological process of changes in the Earth's crust is related to a variety of factors: the varying density of the rock layers which form the Earth's crust, the activity of the tectonic plates, as well as the movement of the continents, shifts in the center of gravity, tidal activity, hydro-spherical and atmospheric phenomena, and human intervention in some regions.
Researching the variations in the gravity of the Earth with satellite systems is a relatively new method of recording the changes in geoid elevations. The distance between the center of the Earth and its surface is constant (the tallest mountains will rise or decrease 1-2cm per year at most); if we take into consideration that the Earth's physical body does not vary much and disregard the other forces, then we can say that the main reason for these infinitesimal changes in gravity on the surface of the Earth is due to differences in mass. While there is a decrease in weight in specific regions from melting glaciers, in other areas there is an increase in weight due to melting water flowing into reservoirs; both these phenomena play a significant role in the variations of the Earth's gravity.
Even a minor variation in mass can be detected by measuring gravity. The change of mass location on the Earth's surface results in gravity variations in the same region; in brief, today the commonly used gravity measurements are the most important source for detecting and identifying variations in geoid elevations, as well as determining the actual reasons for these changes. Gravity measurements are conducted via satellite systems; these indicate changes in mass location by detecting an increase or decrease in weight. The most modern technological satellite systems that can detect gravity change and allow us to follow the variations in masses on the Earth's surface are the satellite used by the European Space Agency, called GOCE, and NASA's satellite, called GRACE. GOCE has been designed to perform accurate studies of the Earth's gravity field as it progresses into orbit. As the satellite passes over the regions where gravity is intense or weak, it measures the variations in gravity with signals that have been conveyed by a device called a gradiometer. GRACE is a pair of identical satellites that are flying in the same orbit, 136 miles apart; they orbit the Earth at a distance of 300 miles. These satellites can measure distances with microwave signals, and can detect changes of less than 1% the thickness of human hair; thus the twin satellites are able to accurately measure the distance to the surface of the Earth. The measurements provided by this satellite system make it possible for changes in gravity to be calculated. The GRACE satellite data is 1,000 times more accurate than other gravity field detection systems.
The enormous waves that occurred on the sea surface as a result of the Sumatra Island earthquake, which measured 9 on the Richter scale, caused a level ridge, measuring about six meters in height, to form on the shore. According to data produced by GOCE, such changes in the mass of the Earth's surface caused a variation of 18 mm to occur on the geoid; this is recognized as a relatively high degree of change.
Changes in the polar glaciers also cause variations in the geoid; data provided from satellite GRACE shows that the layers of ice in Greenland and the Antarctica are melting at a higher rate than previously expected. The melting icebergs are causing a rise in sea levels of up to 0.41 mm every year and the weight of water produced from the melting icecaps is causing changes to the shape of the Earth's surface.3
One of the interesting facts attained by GRACE is the changes in the Earth's gravity field that have been caused by Three Gorges in China, the largest reservoir ever built. The lake region of the reservoir that is being built measures around 372 miles long, 70 miles wide and approximately 574 feet deep; when the casing of the reservoir is completed it will house an amazing 39.3 billion m3 (9.4 cu mi) of water. The area that this reservoir will cover once the project is completed is so great that it will make an estimated 1.5 million people homeless. It has been observed that the enormous accumulation of water in the completed sections of the reservoir has increased the gravity level in that region, which in turn has caused changes in the geoid structure.4
Scientists have confirmed that the Earth's shape is becoming rounder as a result of the construction of projects like the Three Gorges reservoir. It is also estimated that the weight decrease due to the melting icecaps has played a major role in these changes. In some regions of Scandinavia and Canada the ground is rising 1 cm every year due to the melting glaciers. The water produced from the melting glaciers is forcing the currents in the Atlantic Ocean towards the equator, while the decrease of mass at the poles and the increase of weight in the equator region have caused significant changes in the shape of the Earth.
Many scientists claim that changes in the Earth's surface have been caused by changes in the climate. Unfortunately, according to a report published by the UN Intergovernmental Panel on Climate Change (IPCC), humans are responsible for 90% of global warming. As a result of these vast variations, the geoid shape of the Earth is becoming rounder and its radius is increasing annually by 0.4–0.8 mm. The reasons for these changes are being closely monitored by scientists. According to scientists, the variation of the geoid that has been caused by changes in mass location is having an effect on the Earth's dynamics, with the transfer of mass demonstrated by the changes in gravity causing a reduction in the speed of the Earth's rotation around its axis; this is expected to result in variations in the daily time-zone.
Abdullah Sancak is pursuing an academic career in engineering in Turkey.
1. For more information on this topic see James R. Smith, Introduction to Geodesy, The history and Concepts of Modern Geodesy, John Wiley and Sons, Inc., 1997.
2. There are interesting images that show how the earth looks in the following link: University of Texas Center for Space Research and NASA (7 Haziran 2005) http://www.csr.utexas.edu/grace/gallery/gravity/
3. G. Ramillien, A. Lombard, A. Cazenave, E. R. Ivins, M. Llubes, F. Remy, R. Biancale, Interannual variations of the mass balance of the Antarctica and Greenland ice sheets from GRACE, Global and Platenary Change 53 (2006) 198-208.
4. San Shaoan, Institute of seismology, CEA, Wuhan, China, Gravity change before and after the first water impoundment in Three Gorges Project. http://www.sgg.whu.edu.cn/icct/html/icct_ppt/S1/sun.s.A___fourth.pdf