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 Mercury Contacts, infos and linksGlossary

The second of arc
Unit used for small angles. A degree is subdivided into 60 minutes of arc, themselves divided into 60 seconds of arc or arc-second.

Cassini (state and laws)
Cassini is a French-Italian scientific of the 17th century who discovered the laws of libration of the Moon. These laws have been developed empirically by Cassini, hence their name "the laws of Cassini". If a planet is in the state of Cassini, as the Moon and perhaps as Mercury, it meets these three conditions: a report of integers between the period of rotation and revolution (3 / 2 for Mercure), the obliquity should remain constant over time and finally, three axis must be coplanar: the axis of rotation, the perpendicular to the orbit and the normal to the plane of Laplace. he Cassini state is a state of équilibrium. A celestial body may have several possible Cassini states, each characterized by different obliquity. One of the objectives of the missions to Mercury and the Observatory is to determine if Mercury is in a Cassini state.

Doppler effect
This effect appears when the source which emits a wave and the receiver that picks are in motion relative to one another. The frequency received by the receiver is different from the frequency emitted by the source. This frequency shift is called the Doppler effect.

The orbital eccentricity
Characteristic of an elliptical orbit, it is between 0 and 1. The bigger it is, the more the orbit will have a crushed form. An eccentricity of zero corresponds to a circle. The eccentricity of Mercury is approximately 0.2, it is relatively high compared to other planets, the Earth has an eccentricity of 0.017.

The exosphere
Last layer of the atmosphere. It is believed that collisions between neutral particles are rare.

The ionosphere
Upper atmosphere containing a significant amount of charged particles.

The libration
Regular swings, periodicals, of a celestial body around a mean rotational movement. The Moon, for example, presents the same face to Earth. However, the libration movement reveals additional 9% of its surface. Mercury also has libration.

The magnetosphere
Region around a planet in which charged particles are controlled by the magnetic field.

The moment of inertia
The moment of inertia of a system rotating about an axis p quantifies the resistance of this system to a change in its speed of rotation around this axis p.

The obliquity
Angle between the axis of rotation of a planet and its orbital plane.

The perihelion
Point of the orbit of a celestial body that is closest to the Sun.

The synodic period
The time it takes for a planet to return to the same configuration planet-Earth-Sun.

The plane of Laplace
Reference plane defined as the normal plane to the orbital pole precession of Mercury.

The precession
Change of direction of the axis of rotation of Mercury by the action of gravitational forces due to other stars.

The resonance
There are two types of resonance:
  • The spin-orbit resonance of a celestial body: when there is a report of integers between its rotation period and the period of revolution.
  • Resonance between multiple celestial bodies: when there is a report of integers between their periods of rotation.

Measuring device that indicates the composition and structure of matter by measuring the distribution of radiation from the wavelength or frequency.

The solar wind
Flows from the sun consisting mainly of ions and electrons.

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