Characteristics of Planets

For the Sun we speak of a Aphelion (from the Greek "apo" away and "helios" sun), the farthest point between the object and the Sun and a perihelion ("peri" around and "helios" sun) , the nearest point.
But more generally speaking of apsis which designate the two extreme points of the orbit of a celestial object. The point at the minimum distance from the center of the orbit is called periapsis.
The point at the maximum distance from the center of the orbit is called Apoapside.
The main axis of the ellipse that connects the periapsis and apoapside an orbit is called line of apsis.
The names of these points, the closest and farthest from the central object, are specific of the central object name (Greek root of the name of the celestial object).

nota: A light-year is exactly 9 460 730 472 580 800 meters.

The planets glide majestically on one orbit around the sun, leaving no trace seen of gravitational constraints that lead. Yet an orbit is the path followed by a planet to meet the constraints of gravitational effects multiple heavenly bodies and in particular the sun. In the solar system, all objects, planets, asteroids and comets move in the same direction around the Sun. But no orbit is perfectly circular or perfectly coplanar i.e. on the same plane around the equator of the central object. If the orbits of planets have very low inclinations to the plane of the ecliptic, much less massive bodies as Pluto, Eris, asteroids or comets have orbits highly inclined to the plane. Orbits have a perihelion and aphelion therefore eccentricity and an inclination, an ascending node, a vernal point and an argument of perihelion. The orbits of the planets are all roughly in the same plane.
The orbital plane is called the ecliptic.

nota: The eccentricity defines the shape of an elliptical orbit, it varies between 0 and 1. 0 for circular orbits.
A high eccentricity decreases the minor axis (perihelion) and increases the larger axis (aphelion), but does not change the major axis.

Among the units of measure astronomical distances, the astronomical unit (AU or UA symbol) is the smallest units, it corresponds approximately to the length of the semi-major axis of Earth's orbit (the average distance of the Earth sun).

nota: The three units of measurement useful in astronomy to express the distances:
- light year (a.l.) A light year is a unit of distance used in astronomy. A light-year is equal to the distance that light travels in a vacuum in the space of one year (31,557,600 seconds), about 10,000 billion kilometers. is the 63242.17881 au, is exactly equal to 9 460 730 472 580 800 m.
- parsec (pc The parsec is the distance at which one astronomical unit subtends an angle of one arcsecond.) is equal to 206 AU or 270.6904 3.2616 years-light or 30 857 656 073 828 900 m.
- astronomical unit (au (symbol: ua ou au) Créée en 1958, c’est l'unité de distance utilisée pour mesurer les distances des objets du système solaire, cette distance est égale à la distance de la Terre au Soleil. La valeur de l'unité astronomique représente exactement 149 597 870 700 m, lors de son assemblée générale tenue à Pékin, du 20 au 31 août 2012, l'Union astronomique internationale (UAI) a adopté une nouvelle définition de l'unité astronomique, unité de longueur utilisée par les astronomes du monde entier pour exprimer les dimensions du Système solaire et de l’Univers. On retiendra environ 150 millions de kilomètres. Une année-lumière vaut approximativement 63 242 ua. Mercure: 0,38 ua, Vénus: 0,72 ua, Terre: 1,00 ua, Mars: 1,52 ua, Ceinture d’astéroïdes: 2 à 3,5 ua, Jupiter: 5,21 ua, Saturne: 9,54 ua, Uranus: 19,18 ua, Neptune: 30,11 ua, Ceinture de Kuiper: 30 à 55 ua, Nuage d’Oort: 50 000 ua.) is from August 30, 2012, exactly 149 597 870 700 meters.

Table of equivalences on units of distances.

The orbital velocity of a planet is the speed at which it orbits the Sun, the most massive object.
Instantaneous orbital velocity can be determined by Kepler's second law, the law of areas (1609).
A planet puts the same time to cover the same surface, along its elliptical orbit.
The speed of a planet becomes greater when the planet approaches the Sun.
Is maximum in the vicinity of the shortest radius (perihelion), and lowest in the vicinity of the largest radius (aphelion).
The average orbital speed is determined either by knowing the orbital period and semi-major axis of its orbit, either from the masses of the two bodies and semi-major axis.

In the solar system, the Sun has captured 99.86% of the total mass of dust and gas from the original nebula. Jupiter, the largest planet in the system, has captured 71% of the remaining mass.
The other planets are shared residue gravitational this development, i.e. 0.038% of the total mass.

The density is a physical quantity which characterizes the mass of an object per unit volume. It is denoted by the Greek letter ρ (rho).
The density is determined by the ratio:
ρ = m / V
m = mass of the homogeneous substance occupying a volume,
V = volume.
The unit of measurement of the density in the international system is the kilogram per cubic meter (kg/m3). Density at the center of the Sun is huge, it is more than 000 kg/m3 to 150, 150 times more than water. The center of the Sun the temperature reaches about 15 million degrees and the pressure is about 22,000 billion Pascal's (Pa) or 217 million times the atmospheric pressure on Earth. The pressure of the atmosphere varies around 101 325 Pa or 1013.25 hPa.

nota: table above we can see that the density of Saturn is less than that of water (1000 kg/m3).

The orbital period is the time taken by a planet to complete its trajectory, or revolution around another star. The planets and solar system objects it is the Sun.
The time required to accomplish this shift can be estimated by a return to the same position relative to a fixed star, or at the same position relative to the equinoctial point.
In this case, this period is called sidereal period of revolution.

nota: The Earth rotates around the Sun in 365.2564 days solar or sidereal year at an average speed of 29,783 km/s.

The escape velocity of a planet called the second cosmic speed is the speed that allows a projectile to escape once the gravitational pull of the planet.
Do not confuse with the first cosmic speed is the speed orbiting.
The second cosmic speed is the minimum speed that should theoretically reach a body to move away indefinitely from a star despite the gravitational pull of the latter.
Escape velocity is calculated from the following formula: v = √ 2GM / R
v = escape velocity
G (universal gravitational constant)
G = 6.6742 × 10-11 m3 · kg-1 · s-2
M is mass of the planet
R is radius of the planet.

nota: The escape velocity increases with increasing mass of the planet or when its radius decreases.

The apparent magnitude is a measure of the irradiance of a celestial object observed from Earth. This measures the power of the light flux per unit area delivered to the object.
The magnitude is a log Conversely, it increases by one when the irradiance is reduced by ≈ 2.51.
The star Vega (α Lyr) is a reference, with an apparent magnitude of zero is the brightest star in the constellation Lyra and the fifth brightest star in the sky, the second in the northern hemisphere after Arcturus.

nota: albedo (whiteness in Latin) is the ratio of solar energy reflected by a surface.

The inclination is the angle between the axis of rotation of the Earth and its orbital plane, it remains confined between 21.8 ° and 24.4 °. Currently, it is 23 ° 26.5 'axis but recovers about 50 "per year or 1 degree every 71.6 years. Moreover axis oscillates around a cone whose complete cycle (360 °) lasts 25,765 years. This angle (23 ° 27 ') is the succession of the seasons. Indeed, in the summer the sun is higher in the northern hemisphere than in the southern hemisphere. The sun is high in the sky of the northern part of the globe, in the southern part. Sunlight arrive on Earth with more intensity. The sun rises earlier sets later, and the days are longer. In the southern part it is winter. The Sun also appears lower on the horizon and the days are shorter, the sun rises later and sets earlier. At the equator the length of day and night does not change (although the Sun's position in the sky varies). The poles, day and night lasts for six months each.
The obliquity thus characterizes the inclination of the Earth's axis relative to the ecliptic and varies between 21.8 ° and 24.4 °. But the Earth is slightly flattened at the poles, the gravitational forces exerted by the Sun and the Moon rotate on itself not as a perfectly spherical ball but like a top. This small variation from 21.8 ° to 24.4 ° is due to the presence of the moon acts as a stabilizer on the equatorial bulge of the Earth. Nevertheless, small variations in the obliquity have broad implications for the insolation at the latitude of 65 °, which is considered the most reliable criterion of melting ice sheets. The combination of these two effects leads to an oscillation of the Earth's obliquity, very limited, about 1.3 ° about a mean value close to 23.5 °.

The combined period of these oscillations is about 41,000 years.
The obliquity of great importance in high latitudes because it is the origin of the seasons, if the obliquity were zero, there would be no seasons and hence little variation in temperature.
This is one of the parameters or Milanković Milanković cycles corresponding to three astronomical phenomena affecting the Earth's eccentricity, obliquity and precession.
These parameters are used within the astronomical theory of paleoclimate and are partly responsible for natural climate change whose main result, glacial and interglacial periods.

Image: The inclination of the Earth's axis is 23 ° 26 'axis but recovers about 50 " per year or 1 degree every 71.6 years.

The rotation period refers to the time taken by a star (star, planet, asteroid) for a ride on itself.
The rotation of the Earth is 86,400 seconds.
The Earth rotates on itself around an axis, the velocity at the equator is 1674.364 km/h, this axis is oriented toward the north celestial pole.
For a long time, the rotation of the Earth was considered the most accurate measurement of passing time, but its speed varies with time.
The speed of rotation of the Earth is not regular, small crises or stuttering time occur quite frequently.
Seconds disappear or rather minutes 61 seconds appear. Since the 1960s, 34 seconds are missing because of the imperceptible slow but steady rotation of our planet around its axis. All movements of the Earth are irregular and vary continuously in time, many local and cosmic events change the speed of rotation of the Earth. The speed of rotation at the equator is 1 674.364 km/h. The number of revolutions of the Earth itself is about a year 365.2425 or 365.2425 days sidereal (rotation relative to the celestial reference system).

The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface, trapping heat by greenhouse gases and reducing the temperature difference between day and night.
The atmosphere is divided into several layers of varying importance. Their limits were set according to discontinuities in the temperature variations, depending on the altitude as the temperature decreases with altitude. Uranus and Neptune are enveloped in clouds of gas icy, Mars and Mercury have an extremely tenuous atmosphere, Jupiter and Saturn are only the atmosphere without solid surface, are gas giants.
One planet has an atmosphere like Earth in its founding, our nearest neighbor Venus. But atmospheric pressure of Venus is huge, 90 times higher than on Earth.
This pressure is accompanied by very high temperatures, 480 ° C on average. This temperature is enough to melt lead on Earth.