How It Works
Time

How is time measured?

Time

Our measurement of time is determined by the orbit of the Earth around the Sun, and the Moon around the Earth. In the western world we use what is known as the Gregorian calendar, as described here. However there are currently almost 50 different calendars in use around the globe.

Gregorian calendar

1. Observer

The Sun is directly above the observer. Let’s say the observer measures the time to be noon.

2. The sidereal day

23 hours and 56 minutes has passed and the Earth has made one full rotation, however Earth has also moved along its orbit, meaning the Sun no longer appears directly overhead. This is known as a sidereal day.

3. The solar day

Four minutes later and the Sun is in roughly the same spot as it was in the first instance (noon) corresponding to the measurement of time that we are most familiar with – the 24-hour solar day.

4. Earth’s orbit

It takes approximately 365.25 days for Earth to complete its orbit around the Sun, which is why we need to add an extra day to our calendar every four years (leap years). The rotation of the Earth is actually slowing down – albeit very slowly – due to gravitational forces between the Moon and our planet. Geological records show that around 620 million years ago there were actually 400 days in a year.

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Plus, take a look at:

Top 5 Facts: The physics of time 

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