• Question: how do you know how far away the earth is from the sun because it is always getting closer?

    Asked by 624spea43 to Francesca, Laura, Matthew, Andrew, Rebecca on 18 Nov 2014.
    • Photo: Andrew McKinley

      Andrew McKinley answered on 18 Nov 2014:

      I have not heard that it is always getting closer… I do know that the moon is getting further from earth though!

      It is actually a really interesting problem because the distance can’t be measured directly. In the early days of researching the solar system (17th, 18th and 19th centuries), as new planets were found we could not determine absolutely how far away they were – we could make observations and accurately say that “Mars is 1.6 times as far from the sun as we are”, or “Jupiter is five times further from the sun than we are”, but everything was always a multiple of the earth-sun distance. Until we knew that distance, we could not determine – in miles or kilometers – what distances planets were from the sun.

      What we *did* know however was something called the “angular size of the sun” – that is the amount of sky the disc of the sun takes up. By observing a “transit” of an inner planet across this disc – that is timing how long it takes for the dot of Venus or Mercury to travel across the disc of the sun, calculations can be done to determine the earth-sun distance, and hence unlock the structure of our solar system.

    • Photo: Laura Schofield

      Laura Schofield answered on 19 Nov 2014:

      The Sun is on average 149,597,870,700 metres away from the surface of the Earth (it actually varies between 146 billion metres and 152 billion metres because the Earth isn’t a perfect sphere!) In the early days (1600s) people used Pythagoras (see, your Maths teacher tells you it has its uses!) to calculate the distance by plotting triangles between Mars or Venus and the Earth and Sun. It took a very long time to do because they had to watch the planets move over time and work out how far away from the Earth they are.
      Nowadays, we use satellites that we know the location of! If we know the location of the satellite, we know the distance between Earth and the satellite, plus the angles between all 3 bodies and use Pythagoras equations to calculate the distance accurately!

    • Photo: Matthew Camilleri

      Matthew Camilleri answered on 19 Nov 2014:

      The distance between the Earth and the sun is not always constant, as the shape of orbit between the Earth and the sun is an ellipse rather then a circle. At its closest point, the Earth gets to 147 million km, and at its most distant point, it’s 152 million km.
      As you can see the distances are so huge that if there is a variation of a cm or 2 every year there is no way how we can realise that, but it seems that the earth is not getting closer to the sun as everything in our solar system is in balance. The only way to lose this balance would be if a meteorite hits earth and puts us out of orbit.

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