Solar system

• Space physics

  Study of the physical properties of the universe beyond the Earth's atmosphere
• Motion of the Earth

  1. Travels in a nearly circular orbit around the sun each one year

  2. Keeps spinning on its axis about the line that represents its north and south poles each 24 hours
• Day and night

  • The half of the Earth facing the sun has day, the other half has night
• Rising and setting of the Sun

  1. The Earth's rotation on its axis causes the Sun to have a clear daily journey from east to west

  2. Sun rises exactly in the East and sets exactly in the west only at the equinoxes
• Equinoxes


  The time or date (twice each year) at which the sun crosses the celestial equator, when day and night are of approximately equal length (around 22 September and 20 March)
• Sun's position in the northern hemisphere

  1. During summer, it rises north of east and sets north of west

  2. During winter, it rises south of east and sets south of west
• Sun's position in the southern hemisphere

  1. During summer, it rises south of east and sets south of west

  2. During winter, it rises north of east and sets north of west
• Hemisphere


  A half of the Earth, usually as divided into northern and southern halves by the equator, or into western and eastern halves by an imaginary line passing through the poles
• Sun's position at noon

  The Sun is highest above the horizon at noon and directly due south in the northern hemisphere
• Sun's height in the northern hemisphere

  Greatest and the daylight hours longest about 21 June, after that the Sun's height gradually decreases
• Seasons


  • Depend on the Earth's motion round the sun each approximately 365 days
  • Depend on the tilt (23.5%) of the Earth's axis to the plane of its path around the Sun
• Seasons in the northern hemisphere

  1. Tilted towards the Sun so the hours of daylight are greater than those of darkness

  2. Receives more solar radiation and so the weather is warmer

  3. It is Summer and Spring time
• Seasons in the southern hemisphere

  Tilted away from the Sun and has shorter days than nights so it is autumn and winter there
• Equinoxes

  At the equinoxes (20 March and 23 September) night and day are equal in both hemispheres
• Motion of the Moon

  Due to the rotation of the Earth on its axis the moon has a daily trip rising at the east and setting at the west
• Phases of the Moon

  • Exactly half of the Moon is always illuminated by the Sun
  • New Moon: Moon is between the Sun and Earth, unlit side facing Earth
  • First Quarter: Half of the Moon's surface can be seen
  • Full Moon: Moon is on the opposite side of the Earth from the Sun, appears as a complete circle
  • Last Quarter: Only the old crescent can be seen
• Components of the Solar System

  • One star named the Sun
  • 7 planets
  • Dwarf plants and asteroids which orbit the sun
  • Moons that orbit plants
  • Smaller Solar System bodies such as comets and natural satellites
• Inner planets

  • Mercury, Venus, Earth and Mars, are all small, of similar size, solid and rocky, with a layered structure, and have a high density
• Outer planets

  • Jupiter, Saturn, Uranus and Neptune, are much larger and colder and consist mainly of gases; their density is low
• Mars

  • Colder than Earth, with a thin atmosphere mostly of carbon dioxide, has seasons longer than Earth, no liquid water on surface but polar ice-caps
• Jupiter


  • Largest planet, a gaseous planet with a Great Red Spot - a massive swirling storm
• Saturn

  • Has spectacular rings made up of ice particles, has a turbulent atmosphere of hydrogen, helium, ammonia and methane gas
• Uranus and Neptune

  • Large, cold and windy planets with small rocky cores surrounded by an icy mass of water, methane and ammonia, have rings and many moons, atmospheres of methane and helium
• Pluto


  • Smaller than the Moon, has an atmosphere of frozen methane and nitrogen, has 5 moons including the largest Charon
• Asteroids

  • Pieces of rock of various sizes which mostly orbit the Sun between Mars and Jupiter, have a density similar to the inner planets, the largest is Ceres which is classed as a dwarf planet
• Comets

  • Consist of dust embedded in ice made from water and methane, have a low density, orbit the Sun in highly elliptical orbits, develop a bright head and long tail when approaching the Sun
• The Sun

  • A star that produces its own light, the planets and moon are seen from Earth by reflected solar light
• Orbits

  • The planets, dwarf planets and comets orbit the Sun in an ellipse, with the Sun at one focus of the ellipse
• Origin of the Solar System

  1. Formed around 4.5 billion years ago from gravitational attraction pulling together swirling clouds of hydrogen gas and dust (nebulae)

  2. The planets formed from the remnants of the disc cloud of matter left over from the nebula that formed the Sun

  3. The inner, rocky planets formed where it was too hot for lighter molecules, the outer, gaseous planets formed where it was cool enough for lighter molecules to exist in solid form
• Gravitational field strength

  The force of gravity between two objects depends on their masses and the distance between them
• Inner planets


  • Made of materials with high melting temperatures such as metals (e.g. iron)
• Only 1% of the original nebula is composed of heavy elements, so the inner, rocky planets could not grow much and stayed as a small size, solid and rocky
• Outer planets


  • Large, gaseous and cold
• Gravitational field strength of a planet

  Depends on its mass, and is nearly uniform across its surface. The strength of the gravitational field decreases as the distance from the planet increases.
• Calculating travel times

  Use the equation: Average speed = total distance/total time
• The outer regions of the Solar System are over 5000 million kilometers, so it takes a long time to travel to such distant places.
• The planets orbiting the Sun in near circular paths

  The force of gravity between the Sun and the planet provides the necessary centripetal force
• The strength of the Sun's gravitational field decreases with distance


  The further a planet is away from the Sun, the weaker the centripetal force, resulting in a lower orbital speed and longer orbital duration
• A comet with a large elliptical orbit

  Its speed increases as it approaches the Sun and decreases as it moves further away
• The Moon is kept in a circular orbit around the Earth


  By the force of gravity between it and the Earth