The+evolution+of+the+Universe

The earlier section (Solar and Cosmology) includes material on the history of cosmology and how our understanding of the Universe evolved over time. This section looks at what our current understanding of the Universe is, what evidence there is to support that understanding and what that means for how the Universe itself evolved.

Overview
The mean distance from the Earth to the Sun is called an '**Astronomical Unit**'. It is a very hand unit of measurement for objects in the solar system. It is an experimentally measured unit, and is approximately 149 597 870.7 kilometres. So **1au = 1.5x10 8 km or 0.15 Tm** (terametres).

For larger distances the **light year** is used. This is the distance travelled by light in a vacuum in one year so, using 3x10 8 ms -1 **1ly = 9.5x10 15 m or 9.5 Pm** (pecametres)).

For even greater distances, a **parsec** can be used. A parsec is defined as the distance an object must be away for the parallax (p) to be one second of arc, where the base line for the parallax measurement is 1AU:

Therefore a distance measured in parsecs is 1/tan p, where p is measured in seconds of arc (1/3600 of a degree). However, for small angles (the closest star has a parallax angle of less than 1 arcsecond) tax P approximates to P, so frequently 1/p is used.


 * 1pc = 3.26 ly** = 31 trillion km

Example distances measured in astronomical units (au), light years (ly) and parsecs (pc):


 * < **Distance from Earth to:** ||= **au** ||= **ly** ||= **pc** ||
 * < Sun ||= 1 ||= 1.5x10 -5 ||= small ||
 * < 1ly ||= 63 000 ||= 1 ||= 0.3 ||
 * < 1pc ||= 206 000 ||= 3.26 ||= 1 ||
 * < Proxima Centauri (nearest star) ||= 252 000 ||= 4 ||= 1.23 ||
 * < Centre of galaxy ||= 6.3x10 9 ||= 100 000 ||= 30 700 ||

Measuring astronomical distances
This section considers how astronomical distances outside our solar system have been measured, broadly in historical order.
 * Parallax
 * Cephid variables
 * Hubble's Law and red shift
 * Supernovae
 * Structure of the Milk Way Galaxy