Length

Let's start our considerations of the base units with an overview of length scales. For this, we have to resort to logarithmic scales and to working with powers of 10. The drawing here is in such a logarithmic scale. This means that equal distances in the drawing mean identical factors. Here the distance between each two horizontal lines represents a factor of ten.

• The diameter of a proton inside the nucleus of an atom is about 10^-15 m.
• The diameter of an atom is a factor of 100000 (= 10⁵) times bigger than that of the proton, about 10^-10 m. This is the smallest length scale that we can resolve with our best electron microscopes.
• Typical dimensions of microbes, bacteria, and cells in the human body are about 1 µm = 10^-6 m. We can still use optical microscopes to study this length scale.
• Another factor of 1 million brings us to the height of the human body, typically between 1.6 and 2.0 m.
• A typical large city has a diameter of about 10 km, another factor of 10⁴ bigger than the human body.
• The radius of the Earth is 6400 km, the distance between Earth and the Moon is approximately 384000 km.
• The distance between our planet and the Sun is 148 million km, and our solar system has a diameter of about 10¹³ m.
• A light year is the distance that a light beam can travel in vacuum in one year. The velocity of light is approximately 300000 km/s. Therefore we get: 1 light year = 9.5$\cdot$ 10¹⁵ m
• The nearest star is 4 light years away from our solar system.
• The Andromeda nebula, the nearest galaxy to our own Milky Way, is about 2 million light years away.
• The radius of the entire Universe is between 10 and 20 billion light years, or about 10²⁶ m.

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