Our Solar System
The earth is one of eight planets (with Mercury, Venus, Mars, Jupiter, Saturn, Uranus & Neptune) orbiting the sun; some planets are rocky and solid, others are giant balls of gas. The sun is a million times the size of the earth and the earth is orbiting the sun at over 95,000 km/hour.

Our galaxy
Our solar system is only a minute part of a galaxy known as the Milky Way which is so large that distances are measured in light years. Light travels at 300,000 km/sec and one light year is 9.5 million million km. Our moon is 1 light second away. The nearest star is over 4 light years away. The galaxy is 100,000 light years across and contains around 100,000 million stars. The known cosmos contains 120 galaxies, one of which identified by the Hubble telescope is 13 billion light years away.

Stars
Stars undergo a cycle from birth to death and they are different colours depending on their age. Massive stars may explode and contract down to form a black hole which is an area of extreme gravity. The earth would need to be squeezed down to the size of a marble before it became a black hole. At the centre of our galaxy is a black hole around which the sun is rotating at 800,000 km/hour.

The origin of the universe
The universe originated with the Big Bang 13.8 billion years ago. To get a perspective on the timing of later events it is useful to compare this with one of our calendar years:

• 1 Jan: The Big Bang
• Mar: Formation of the Milky Way galaxy
• Aug: Emergence of the sun and planets
• Sept: First simple life forms
• Nov: Multicellular life forms
• 19 Dec: The first plants
• 21 Dec: The first insects
• 24 Dec: Dinosaurs appeared
• 29 Dec: Dinosaurs were wiped out
• 31 Dec: Recognisable human forbears
• 10 secs to midnight: Modern humans emerged
• 1 sec to midnight: Development of our society, sciences and culture

Based on a lecture given in June 2013 to the Royal Society of Medicine Retired Fellows Society by Carolin Crawford (Gresham Professor of Astronomy) an account of which by Ken Citron was published in the RFS Newsletter in December 2013