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Origin is taught in the note for The Earth. Read it below and look for the section on this topic.

The Earth

14 min read

Origin of the Earth

The Earth formed about 4.6 billion years ago. The Big Bang theory says that the universe began with an enormous explosion about 13.8 billion years ago. The nebula (solar nebula) theory says that the Sun and planets formed from a spinning cloud of gas and dust. The cloud collapsed under gravity, the Sun formed at the centre, and the remaining material clumped together to form the planets, including the Earth.

The solar systemThe Sun at the centre with eight planets orbiting it in order of distance: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The four inner planets are small and rocky; the four outer planets are large gas or ice giants. The orbits and sizes are not drawn to scale.MercuryVenusEarthMarsJupiterSaturnUranusNeptunenot to scale

Shape and size

The Earth is almost a sphere, slightly flattened at the poles (an oblate spheroid, called a geoid). Its equatorial diameter is about 12 756 km and its polar diameter 12 714 km. The circumference at the Equator is about 40 075 km. Evidence for the shape: ships appear over the horizon hull-first, the Earth’s shadow on the Moon is curved in an eclipse, travellers can go round the world, and satellites photograph a round Earth.

Movements: rotation and revolution

The Earth rotates on its axis from west to east once in about 24 hours, causing day and night and the apparent movement of the Sun. It also revolves round the Sun once in about 365¼ days, along an elliptical path. The axis is tilted 23.5° from the vertical and always points in the same direction. This causes the seasons and the different lengths of day and night.

Why we have seasonsThe Earth circles the Sun once a year with its axis tilted by about 23.5 degrees and always pointing the same way. When the northern hemisphere leans towards the Sun it has summer, longer days and more direct light. Six months later it leans away and has winter. Near the equator, as in Tanzania, the change is small.SunJune: northern summerSeptember: equinoxDecember: northern winterMarch: equinox
Phases of the MoonThe Moon orbits the Earth once about every month. Sunlight from the right always lights half of the Moon. As the Moon moves round the Earth we see more or less of the lit half, from new moon through first quarter to full moon and back again.EarthNew moonWaxing crescentFirst quarterWaxing gibbousFull moonWaning gibbousLast quarterWaning crescentsunlight
DateEventWhere the noon Sun is overhead
21 MarchEquinoxEquator
21 JuneJune solsticeTropic of Cancer (23.5° N)
23 SeptemberEquinoxEquator
22 DecemberDecember solsticeTropic of Capricorn (23.5° S)

Parallels and meridians; time

Lines of latitude (parallels) run east–west and measure the angle north or south of the Equator (0°). Lines of longitude (meridians) run from pole to pole; the Prime Meridian at Greenwich is 0°. The Earth rotates 360° in 24 hours, which is 15° each hour or 4 minutes for each degree. Places east of Greenwich are ahead of GMT. Tanzania, at about 35° E, uses East Africa Time, which is GMT + 3.

Each degree of longitude east adds 4 minutes: the line shows the time ahead of Greenwich in hours.
Try itTime difference and longitude

Choose a longitude east of Greenwich.

Check yourself The Earth rotates once in about
Check yourself The seasons are caused by
Check yourself When it is noon at Greenwich, the time at 45° E is

Key terms

Rotation
The spinning of the Earth on its axis.
Revolution
The movement of the Earth round the Sun.
Latitude
Angle north or south of the Equator.
Longitude
Angle east or west of the Prime Meridian.

Summary

  • The Earth formed from a cloud of gas and dust about 4.6 billion years ago.
  • It is nearly spherical, with a circumference of about 40 000 km.
  • Rotation gives day and night; revolution with the tilted axis gives the seasons.
  • The time changes by 4 minutes for every degree of longitude.
Shape and size