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 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.

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.

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.

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

Upper atmosphere containing a significant amount of charged particles.

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.

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

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.

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

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

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

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

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

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.

Flows from the sun consisting mainly of ions and electrons.