How is time measured?
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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Top 5 Facts: The physics of time