• # Question: How do scientist know how many days it took for each planet to travel round the Earth? Where did they get that infomation from?

• Keywords:
Asked by 10kniwilc to Daniela, Hannah, Ian, Jono, Mark on 25 Jun 2013.
• Mark Hodson answered on 25 Jun 2013:

Hi 10kniwilc,

if you can spot the planets with a telescope you can time them. These days people use calculations based on how far they think the planets are from the sun and their mass

• Hannah Brotherton answered on 25 Jun 2013:

Hi 10kniwilc,
This is all to do with physics ðŸ˜€
Using maths a man called Johannes Kepler noticed that the planets do not orbit the sun in perfect circles (this was in the 1500’s so he was really clever for his time). Also, understanding that a bigger object attracts things closer to it more strongly than things further away, he then went to find out how far the planets were away from the sun – he couldn’t do this accurately because his ruler wasn’t big enough – ha!!, he didn’t have the technology we have today :P. So he guessed, he knew mars was closer to the sun than Saturn etc. But he couldn’t work out how far earth was to the sun.

But a man called Gian Domenico Cassini in 1672, used just his eyes and his fingers to try and work out the distance from the sun – we have a couple of inches between each eye, so if we shut one and open the other and look at an object with our thumb covering it – like the sun we will see the sun shift side to side – using this and MORE MATHs, he figured out the distance of the sun from the earth (just by using a thumb and his eyes). He didn’t quite get it right…..

But because of all the technology we have today, we are able to work out the sun is 95 millions miles away from earth.

So we know the distance and the sizes (kinda). now we can calculate the speed and finally how many days they travel around the sun.

All because of two men looking at the planets with just their eyes (and thumbs) ðŸ˜›

So here are the days for each planet:

Planet orbit day length
Mercury 88 days 59 days
Venus 224.7 days 243 days
Earth 365.256366 days 24 hours
Mars 686.971 days 24.3 hours
Jupiter 4,331.572 days 9.8 hours
Saturn 10,832.327 days 10.2 hours
Uranus 30,799.095 days 17.14 hours
Neptune 60,190 days 16 hours

• Ian Wilson answered on 25 Jun 2013:

Hi 10kniwilc,

You could get a rough idea by looking through a telescope at a planet and counting the number of days until the same planet is back in the same position in our sky. This is very inaccurate though and would have meant sitting watching your telescope every day hoping to see something!

Accurate measurements were made using a complicated set of mathematical equations created by a crazily clever man called Kepler. ‘Kepler’s laws of planetary motion’ underpin all information we have on the way the planets move around the sun. I have no idea what any of it means though lol!

Sorry I couldn’t be of more help,

Ian