# Using gravity to measure time (and other measurements at home)

## How can you use gravity to measure time?  We are so used to our digital watches, clocks and mobile phones that it is easy to forget that it was not so long ago that most clocks relied on a pendulum to measure time, and the swing of the pendulum depends on gravity.

At the National Physical Laboratory (NPL) we have set some measurement challenges for you to do at home.  One of these challenges, called 'Give me a second', is to create a pendulum with a period of one second.  As explained in the video, you can use simple objects like string and a Lego brick to make your pendulum, and then you can submit your results on the website.

Other experiments include using toilet rolls to measure the speed of sound and measuring tap water temperature using sugar.  There are eight measurement challenges so far, with more to be released over the autumn.

### Thinking time

Tuesday 22 September is the autumn equinox: 12 hours of day and 12 hours of night.

Have you ever thought about why there are 24 hours in a day, and why 60 minutes in an hour, and 60 seconds in a minute? The ancient Egyptians divided the day into 10 hours and added a twilight hour at each end, giving 12 daylight hours and 12 night-time hours.  The minutes and seconds come from the Babylonians, who measured using numbers using base 60 (instead of base 10 that we use today).  So do we still measure a second as 1/24 of 1/60 of 1/60 of a day?  No, not any more.  It may seems surprising but the tidal gravitational drag from the Moon is causing the spin of the Earth to slow down, and so the days are getting very slightly longer all the time.  We won’t notice the changes in our lifetime but since many things we use today rely on very accurate timing, we have to use a new way to define time.  The second is now defined using a caesium atomic clock.  You can find out more about this and the other Standard International Units from the NPL website.