Our PLANET EARTH

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Size, Mass, and Orbit:
With a mean radius of 6371 km and a mass of 5.97×1024 kg, Earth is the fifth largest and fifth most-massive planet in the Solar System. In essence, it is the largest terrestrial planet, but is smaller and less massive than any of the gas/ice giants of the Outer Solar System. And with a mean density of 5.514 g/cm³, it is the densest planet in the Solar System.


In terms of its orbit, Earth has a very minor eccentricity (approx. 0.0167) and ranges in its distance from the Sun from 147,095,000 km (0.983 AU) at perihelion to 151,930,000 km (1.015 AU) at aphelion. This works out to an average distance (aka. semi-major axis) of 149,598,261 km, which is the basis of a single Astronomical Unit (AU).

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The Earth has an orbital period of 365.25 days, which is the equivalent of 1.000017 Julian years. This means that every four years (in what is known as a Leap Year), the Earth calendar must include an extra day. Though technically a full day is considered to be 24 hours long, our planet takes precisely 23h 56m and 4 s to complete a single sidereal rotation (0.997 Earth days).


Viewed from the celestial north pole, the motion of Earth and its axial rotation appear counterclockwise. From the vantage point above the north poles of both the Sun and Earth, Earth orbits the Sun in a counterclockwise direction.


Earth’s axis is tilted 23.439281° away from the perpendicular of its orbital plane, which is responsible for producing seasonal variations on the planet’s surface with a period of one tropical year (365.24 solar days). In addition to producing variations in terms of temperature, this also results in variations in the amount of sunlight a hemisphere receives during the course of a year.


Basically, when the North Pole is pointing towards the Sun, the northern hemisphere experiences summer and the southern hemisphere experiences winter.  During the summer, the day lasts longer and the Sun climbs higher in the sky; while in winter, the climate becomes generally cooler, the days are shorter and the Sun appears lower in the sky.


Above the Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year – up to six months at the North Pole itself, which is known as a “polar night”. In the southern hemisphere, the situation is exactly reversed, with the South Pole experiencing a “midnight sun” – i.e. a day of 24 hours.

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Earth’s Structure and Composition:
The shape of Earth approximates that of an oblate spheroid, a sphere flattened along the axis from pole to pole such that there is a bulge around the equator. This bulge results from the rotation of Earth, and causes the diameter at the equator to be 43 kilometres (27 mi) larger than the pole-to-pole diameter.


Earth’s interior structure, like that of other terrestrial planets, is differentiated between a metallic core and mantle composed of rock and silicate materials. However, unlike other terrestrial planets, it has a distinct inner core of solid material and a liquid outer core. This inner core has an estimated radius of 1,220 km, while the outer core extends beyond it to a radius of about 3,400 km.


Extending outwards from the core are the mantle and the crust. Earth’s mantle extends to a depth of 2,890 km, making it the thickest layer of Earth. This layer is composed of silicate rocks that are rich in iron and magnesium relative to the overlying crust. Although solid, the high temperatures within the mantle cause the silicate material to be sufficiently ductile that it can flow on very long timescales.


The upper layer of the mantle is divided into the lithospheric mantle (aka. the lithosphere) and the asthenosphere. The former consists of the crust and the cold, rigid, top part of the upper mantle (which the tectonic plates are composed of) while the asthenosphere is the relatively low-viscosity layer on which the lithosphere rides.

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The average surface temperature on Earth is approximately 14°C; but as already noted, this varies. For instance, the hottest temperature ever recorded on Earth was 70.7°C (159°F), which was taken in the Lut Desert of Iran. These measurements were part of a global temperature survey conducted by scientists at NASA’s Earth Observatory during the summers of 2003 to 2009. For five of the seven years surveyed (2004, 2005, 2006, 2007, and 2009) the Lut Desert was the hottest spot on Earth.


However, it was not the hottest spot for every single year in the survey. In 2003, the satellites recorded a temperature of 69.3°C (156.7°F) – the second highest in the seven-year analysis – in the shrublands of Queensland, Australia. And in 2008, a yearly maximum temperature of 66.8°C (152.2°F) recorded on the Flaming Mountain, located near the Turpan Basin in western China.


Meanwhile, the coldest temperature ever recorded on Earth was measured at the Soviet Vostok Station on the Antarctic Plateau. Using ground-based measurements, temperatures reached an historic low of -89.2°C (-129°F) on July 21st, 1983. Analysis of satellite data indicated a probable temperature of around -93.2 °C (-135.8 °F; 180.0 K) on August 10th, 2010, also in Antarctica. However, this reading was not confirmed by ground measurements, and thus the previous record stands.


All of these measurements were based on temperature readings that were performed in accordance with the World Meteorological Organization standard. By these regulations, air temperature is measured out of direct sunlight – because the materials in and around the thermometer can absorb radiation and affect the sensing of heat – and thermometers are to be situated 1.2 to 2 meters off the ground.