Summary of Chapter

The Solar System consists of the Sun, nine planets with their satellites and rings, many asteroids (about 109 greater than 1 km across), the Edgeworth-Kuiper belt (more than 105 objects larger than 100 km), 1012-1013 small icy-rocky bodies in the Oort cloud, and an interplanetary medium of tenuous gas and small solid bodies ranging in size down to less than 10-6m.

Meteoroids are small rocky bodies, and those that fall to Earth are called meteorites, These have provided much information about the origin, evolution, and composition of the Solar System and the ages of events within it.

The planets orbit the Sun in one direction - the prograde direction - in approximately circular, coplanar orbits with the Sun near the centre. The orbital planes of the asteroids have a wider range of inclinations and eccentricities. The rotation of the Sun is prograde, as is that of most of the planets. If the inclination of the rotation axis to the orbital plane is more than a few degrees then the surface of the planet will experience seasonal changes. Rotation axes are subject to precession.

Some comet orbits reach to within a few AU of the Sun, but the great majority spend most or all of their time at far greater distances, where they are dormant icy-rocky bodies. There are two reservoirs. The Edgeworth-Kuiper belt lies immediately beyond the planetary domain, and contains bodies (EKOs) in orbits that are predominantly prograde and that are concentrated towards the ecliptic plane. The Oort cloud is more far flung and consists of bodies in a spherical distribution around the Sun, reaching out to the edge of interstellar space, about 105AU from the Sun.

The Sun is by far the largest, the most massive, and the most luminous body in the Solar System. It is fluid throughout, and consists largely of hydrogen and helium. Its luminosity is sustained by the nuclear fusion of hydrogen deep in its interior where temperatures reach 1.4 x 107K.

The four planets closest to the Sun - Mercury, Venus, the Earth, and Mars - are the terrestrial planets. They are comparable with the Earth in size, and consist of iron-rich cores overlain by rocky materials. The Earth is the largest of these bodies. The asteroids are rocky bodies concentrated between Mars and Jupiter.

The giant planets - Jupiter, Saturn, Uranus, and Neptune - are considerably larger and more massive than the terrestrial planets, Jupiter by some margin being the most massive planet of all. The giants consist largely of hydrogen, helium, and icy-rocky materials, and (like the Sun) are fluid throughout. The giants have richly varied families of satellites, and all giants have rings, those of Saturn being by far the most substantial. Beyond Neptune we come to the outermost planet, Pluto, smaller in size than the terrestrial planets, and more icy in its composition. The comets are also icy-rocky bodies. Beyond Pluto there is at least one body somewhat larger than Pluto - Eris. Pluto and Eris are regarded as large members of the Edgeworth-Kuiper belt.

The orbits of the planets are described to a very good approximation by Kepler's laws of planetary motion.

First law Each planet moves around the Sun in an ellipse, with the Sun at one focus of the ellipse.

Second law As the planet moves around its orbit, the straight line from the Sun to the planet sweeps out equal areas in equal intervals of time.

Third law If P is the sidereal period of a planet, and a is the semimajor axis of the orbit, then

Elliptical orbits are characterised by five orbital elements: the semimajor axis a, the eccentricity e, the inclination i, the longitude of the ascending node O, and the longitude of perihelion (O + «). To calculate the position of a body in its orbit we need a sixth element - a single position at any known time.

Newton's laws of motion and law of gravity account for Kepler's laws, and go further by accounting for the motion of comets and of other bodies, and for slight departures from Kepler's laws that have various causes. Major effects on orbits are caused by mean motion resonances and secular resonances.

The precession of the perihelion of Mercury shows that at the highest level of precision Einstein's theory of general relativity is superior to Newton's laws.

Our view from the Earth of the apparent motion of a planet depends on whether it is in a smaller or larger orbit than our own. Solar and lunar eclipses result when the Moon, Sun, and Earth line up. Figure 1.27 shows the umbral tracks of forthcoming total solar eclipses. Tables 1.1-1.6 list basic data on the Solar System.

Table 1.1 Orbital elements in 2006 and some physical properties of the Sun, the planets, and Ceres

Object

+1 0

Post a comment