Mercury is the smallest and innermost planet in the Solar System.
The Solar System is the gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly.
A planet is an astronomical object orbiting a star or stellar remnant that
Planet Mercury | Space School by Science Channel
Its orbital period is less than any other planet in the Solar System.
In physics, an orbit is the gravitationally curved path of an object about a point in space, for example the orbit of a planet about a star or a natural satellite around a planet.
The orbital period is the time taken for a given object to make one complete orbit around another object.
What Is Mercury? by MonkeySee
Seen from Earth, it appears to move around its orbit in about 116 days.
Earth is the third planet from the Sun, the densest planet in the Solar System, the largest of the Solar System's four terrestrial planets, and the only astronomical object known to harbor life.
D is the fourth letter of the modern English alphabet and the ISO basic Latin alphabet.
Partly because it has almost no atmosphere to retain heat, Mercury's surface temperature varies diurnally more than any other planet in the Solar System, ranging from 100 K at night to 700 K during the day in some equatorial regions.
The Equator usually refers to an imaginary line on the Earth's surface equidistant from the North Pole and South Pole, dividing the Earth into the Northern Hemisphere and Southern Hemisphere.
An atmosphere is a layer of gases surrounding a planet or other material body, that is held in place by the gravity of that body.
The poles are constantly below 180 K. Mercury's axis has the smallest tilt of any of the Solar System's planets, and its orbital eccentricity is the largest of all known planets in the Solar System.
The orbital eccentricity of an astronomical object is a parameter that determines the amount by which its orbit around another body deviates from a perfect circle.
At aphelion, Mercury is about 1.5 times as far from the Sun as it is at perihelion.
The perihelion is the point in the orbit of a planet, minor planet, or comet, where it is nearest to its orbital focus, generally a star.
Mercury's surface is heavily cratered and similar in appearance to the Moon, indicating that it has been geologically inactive for billions of years.
Mercury is tidally or gravitationally locked with the Sun in a 3:2 resonance, and rotates in a way that is unique in the Solar System.
As seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun.
The fixed stars are celestial objects that do not seem to move in relation to the other stars of the night sky.
As seen from the Sun, in a frame of reference that rotates with the orbital motion, it appears to rotate only once every two Mercurian years.
In physics, a frame of reference consists of an abstract coordinate system and the set of physical reference points that uniquely fix the coordinate system and standardize measurements.
An observer on Mercury would therefore see only one day every two years.
Because Mercury orbits the Sun within Earth's orbit, it can appear in Earth's sky in the morning or the evening, but not in the middle of the night.
Also, like Venus and the Moon, it displays a complete range of phases as it moves around its orbit relative to Earth.
Although Mercury can appear as a bright object when viewed from Earth, its proximity to the Sun makes it more difficult to see than Venus.
Two spacecraft have visited Mercury: Mariner 10 flew by in 1974 and 1975; and MESSENGER, launched in 2004, orbited Mercury over 4,000 times in four years, before exhausting its fuel and crashing into the planet's surface on April 30, 2015.
Mariner 10 was an American robotic space probe launched by NASA on November 3, 1973, to fly by the planets Mercury and Venus.
MESSENGER was a NASA robotic spacecraft that orbited the planet Mercury between 2011 and 2015.
A spacecraft is a vehicle, or machine designed to fly in outer space.