Earth

Earth is the third planet from the Sun and the only known celestial body to support life. Formed approximately 4.54 billion years ago, it occupies a unique position in the Solar System, with conditions ideal for the sustenance of liquid water, a diverse atmosphere, and a magnetic field that protects it from harmful solar radiation.

Position and Classification in the Solar System

Earth is the fifth-largest planet in the Solar System, following Jupiter, Saturn, Uranus, and Neptune, and preceding Venus, Mars, and Mercury in size. It is the largest of the terrestrial planets—a group that includes Mercury, Venus, Earth, and Mars—characterised by their rocky composition and solid surfaces.
A terrestrial planet is primarily made up of silicate rocks and metals, unlike the gas giants (Jupiter, Saturn, Uranus, and Neptune), which are predominantly composed of hydrogen, helium, and water in various states and lack a definite solid surface.
Earth orbits the Sun at an average distance of 149.6 million kilometres, completing one revolution every year. Unlike other planetary names derived from mythology, Earth’s name originates from Old English and Germanic roots meaning ground or soil, making it the only planet not named after a Greek or Roman deity.

Physical Characteristics

Earth is an oblate spheroid, meaning it is slightly flattened at the poles and bulges at the equator due to its rotation. The equatorial diameter measures approximately 12,756 kilometres, while the polar diameter is about 12,714 kilometres. The equatorial circumference is 40,077 kilometres, and the polar circumference is 40,009 kilometres.
The total surface area of Earth is roughly 510 million square kilometres, of which 29% is land and 71% is covered by water. The planet’s mass is estimated at 5.9736 × 10²⁴ kilograms, and its mean density is 5.517 g/cm³, the highest among all planets in the Solar System.
The highest land point on Earth is Mount Everest (8,848 metres), and the lowest exposed land point is the shore of the Dead Sea (-397 metres). The mean ocean depth is about 3,730 metres, while the Mariana Trench, the deepest oceanic trench, reaches depths of around 10,924 metres. Located in the western North Pacific Ocean near Guam, it represents the deepest natural point in Earth’s crust.

Motions of the Earth

Earth exhibits five principal motions:

1. Motion with the Milky Way galaxy as it travels through space.
2. Motion following the Sun, which itself orbits the Milky Way’s galactic centre.
3. Precession, a slow wobble of the Earth’s axis similar to a spinning top.
4. Revolution around the Sun.
5. Rotation on its own axis.

Rotation

Earth completes one full rotation on its polar axis in 23 hours, 56 minutes, and 4.09 seconds, moving from west to east. This period is known as a solar day. The rotational velocity at the equator is about 1,667 kilometres per hour, decreasing progressively toward the poles, where it becomes zero.
The rotation of the Earth is gradually slowing due to tidal interactions with the Moon. As a result, the length of a day increases by approximately two milliseconds per century. The French physicist Jean Bernard Léon Foucault was the first to publicly demonstrate Earth’s rotation using his famous pendulum experiment in 1851.

Revolution

Earth revolves around the Sun in an elliptical orbit, taking approximately 365.25636 days to complete one sidereal year. The orbital speed averages 29.8 kilometres per second, and the orbit’s eccentricity (degree of deviation from a perfect circle) is 0.0167.
Because of this slight eccentricity, the Earth’s distance from the Sun varies annually:

• Aphelion (maximum distance): about 152.10 million kilometres in early July.
• Perihelion (minimum distance): about 147.10 million kilometres in early January.

The tropical year, which defines the solar calendar, lasts 365 days, 48 minutes, and 45.68 seconds, and to adjust for the fraction of a day lost annually, a leap year with 366 days is added every fourth year.
Earth maintains an axial tilt of 23.5 degrees relative to the perpendicular of the plane of the ecliptic, creating a 66.5-degree angle between its axis and the plane of orbit. This inclination is responsible for the occurrence of seasons as the planet revolves around the Sun.

Gravitational Features

Earth’s escape velocity—the minimum speed required to overcome its gravitational pull—is 11.2 kilometres per second. This theoretical value disregards atmospheric resistance and represents the balance point where an object’s kinetic energy equals the gravitational potential energy of Earth.

Shape and Geographical Extremes

Earth’s shape deviates slightly from a perfect sphere, showing an 18-metre rise at the North Pole and a 26-metre depression at the South Pole. These variations are due to uneven mass distribution and centrifugal forces caused by rotation.
The mean surface temperature of Earth is about 14°C, maintained by its atmosphere through the greenhouse effect. This moderate temperature range, combined with the presence of water, allows for the existence of diverse ecosystems.

Satellites and Near-Earth Companions

Earth has one natural satellite — the Moon, which plays a crucial role in stabilising the planet’s axial tilt and producing ocean tides through its gravitational influence. Additionally, Earth shares complex orbital relationships with certain asteroids, notably 3753 Cruithne and 2002 AA29.

• 3753 Cruithne is an asteroid in a 1: 1 orbital resonance with Earth, tracing a horseshoe-shaped orbit around the Sun. It has been informally termed “Earth’s second moon”, though it does not orbit the planet directly.
• 2002 AA29, discovered in January 2002, follows a similar horseshoe orbit, coming near Earth roughly every 95 years. In about 600 years, it is predicted to temporarily become a quasi-satellite of Earth.

Historical and Exploratory Notes

The first recorded circumnavigation of the Earth was achieved by Ferdinand Magellan, a Portuguese explorer, during his expedition (1519–1522). This voyage provided empirical proof of Earth’s roundness and helped establish the concept of global geography.