Light travels very quickly: 300,000 kilometers per second. The distance it travels in one year is called a light-year.
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A view of the Milky Way towards
the Galactic
Center, as seen from a non-light polluted area (the Black Rock Desert, Nevada). |
Objects in the Universe can be so far apart that the distances are measured in light-years, instead of kilometres. For example, the Milky Way (the galaxy containing our Sun) is about 100,000 light-years in diameter.
The table below shows the diameters of the Earth, Sun and Milky Way. Notice how huge the Milky Way is compared with the Earth and Sun.
Very large distances, such as those to a star, cannot be measured directly. This may cause some uncertainty about these measurements.
| Diameter in kilometres | Number of times larger than the Earth | |
| Earth | 12,756 | 1 |
| Sun | 1,380,000 | 108 |
| Milky Way | 1 million million million | 74,000,000,000,000 |
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science
]