My Science School

You need to know two things: how long it takes to make a full rotation, and Earth’s circumference. The time it takes Earth to rotate so the sun appears in the same position in the sky, known as a solar day, is 24 hours. However, the time it takes Earth to complete one full rotation on its axis with respect to distant stars is actually 23 hours 56 minutes 4.091 seconds, known as a sidereal day.

With this information, to work out how fast Earth is spinning we need only our planet’s circumference. At the equator, its circumference is roughly 40,075 kilometres, so dividing this by the length of day means that, at the equator, Earth spins at about 1670 kilometres per hour.

However, this speed of rotation isn’t consistent across the planet. As you move north or south, the circumference of Earth gets smaller, so the speed of spin reduces until it reaches its slowest at both poles. And all of this is nothing compared with the 107,000 kilometres per hour at which Earth orbits the sun.

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At the start, it was just a fiery ball of molten (liquid) rock. As it cooled, lumps formed on the surface of the molten rock. The surface gradually hardened into a crust. Volcanoes kept on pouring steam and gases onto the surface, which led to the atmosphere being formed. As Earth cooled further, clouds of steam became water, creating vast oceans. The crust eventually cooled to form the continents.

Three recent studies improve our understanding of environmental conditions on early Earth—important not just for reconstructing the history of our own planet, but for assessing the habitability of planetary bodies in general.

The first of these studies was led by John Tarduno from the University of Rochester and reported in Proceedings of the National Academy of Sciences. The authors present evidence of a strong magnetic field around Earth, from about 4.1 billion to 4 billion years ago. Their conclusion is based on magnetite inclusions in certain minerals (zircons), and thus appears to be very reliable. A strong magnetic field would have been critical for life to originate on Earth, because it would have protected the surface from the solar wind. Stars like our Sun are known to expel large amounts of harmful radiation when they are still young, and without a magnetic field it is doubtful life on Earth’s surface would have been able to survive the barrage.

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Yes, Earth spins around a line between the poles called its axis. The axis is tilted over at 23.5° in relation to the Sun. Earth rotates once every 23 hours, 56 minutes and 4.09 seconds. The Sun thus appears to come back to the same place in the sky once every 24 hours.

Earth’s spin, tilt, and orbit affect the amount of solar energy received by any particular region of the globe, depending on latitude, time of day, and time of year. Small changes in the angle of Earth’s tilt and the shape of its orbit around the Sun cause changes in climate over a span of 10,000 to 100,000 years, and are not causing climate change today.

Daily changes in light and temperature are caused by the rotation of the Earth, and seasonal changes are caused by the tilt of the Earth. As the Earth orbits the Sun, the Earth is pulled by the gravitational forces of the Sun, Moon, and large planets in the solar system, primarily Jupiter and Saturn. Over long periods of time, the gravitational pull of other members of our solar system slowly change Earth’s spin, tilt, and orbit. Over approximately 100,000 – 400,000 years, gravitational forces slowly change Earth’s orbit between more circular and elliptical shapes. Over 19,000 – 24,000 years, the direction of Earth’s tilt shifts (spins). Additionally, how much Earth’s axis is tilted towards or away from the Sun changes through time, over approximately 41,000 year cycles. Small changes in Earth’s spin, tilt, and orbit over these long periods of time can change the amount of sunlight received (and therefore absorbed and re-radiated) by different parts of the Earth. Over 10s to 100s of thousands of years, these small changes in the position of the Earth in relationship to the Sun can change the amount of solar radiation, also known as insolation, received by different parts of the Earth. In turn, changes in insolation over these long periods of time can change regional climates and the length and intensity of the seasons. The Earth’s spin, tilt, and orbit continue to change today, but do not explain the current rapid climate change.

credit: UNDERSTANDING GLOBAL CHANGE

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