What exactly do we mean by “time”? It’s a better question than you might think, as most will assume that of all things in life, time is one of the constants that can be counted on. But this isn’t so after all.
Time, as may seem obvious, is based on the motion of the planet. Earth rotates once on its axis every 24 hours and orbits the Sun once ever year. Or at least it seems to. So let me use this moment in time, the time of year we call the Winter Solstice, to reflect on the unusual nature of time.
To begin, when we say that one day on Earth is 24 hours, on what do we base the start of a rotation? If we use the Sun and start recording “time” as it passes over a familiar object such as the tree across the street from where I’m sitting, it will take slightly more than 24 hours to reach the same spot tomorrow. This is because it is necessary to consider that the Earth is moving, so to re-align with the starting point, we would need to rotate slightly more than 360°, taking slightly more than 24 hours. This is called a Synodic Day.
Alternatively, we could use the stars as a starting point. The stars, being so much farther away than the Sun, don’t appear to change their position in the sky and so timing a precise 360° rotation would take slightly less than 24 hours (23 hours 56 minutes and 4 seconds). This is called a Sidereal Day.
And what about a year? What should we use as the starting point for Earth to record one full revolution? It would make sense to start at the beginning of one of the seasons. For instance, on December 21st this year, Earth’s tilt will be pointing exactly away from the Sun. This will happen at 15:58 GMT and marks the beginning of winter.
However, the marker that is in fact used is the beginning of spring, which we call the Spring Equinox. This is the spot in Earth’s orbit where the Sun’s rays are exactly perpendicular to the tilt of Earth’s axis. If we measure the time from equinox to equinox, the exact time frame is slightly more than 365 days (365.24 days). We account for this discrepancy by adding in an extra day every four years, on February 29th. (They are added into years that are divisible by 4, so the next one will be in 2024).
There are two further factors to consider. One is that the Earth’s rotation is slowing due to the tidal action of the Moon, meaning we are headed towards a 25 hour day. The other is something that has perplexed me for quite some time which is this word “year”, which we use to describe the age of the universe (13.8 billion years). This is a time frame based on Earth, a planet that has been orbiting for only 4.6 billion years. which of course means that for approximately 9 billion years, the universe was being timed by a clock that didn’t exist!
To make the meaning of time even more obscure, we must factor Einstein’s Theory of Special Relativity which states that gravity and speed affect time. Out in space, further from the centre of gravity, time speeds up. However, to be out in space in an orbit around Earth, you will be required to move at an exceptionally fast speed, which causes time to slow down. This time dilation is accounted for by the GPS satellites, because they are both far from Earth (causing a faster time frame) and moving fast (causing a slower time frame). If we didn’t account for this, then over time, the GPS on your phone or in your car would misplace you on the map.
So, what does all this mean? Well, in our daily lives, it doesn’t mean too much. If the Sun rises or sets a few minutes later or earlier each day (celestial objects rise earlier by about 4 minutes per day, which means about two hours earlier per month. Therefore, the evening constellations change season by season), most people won’t even notice. Sometimes the biggest stressor regarding time is when our phone’s time isn’t in sync with the clock in our car…this is the one that bugs me the most!).
My suggestion is that you enjoy the changing seasons, get in the habit of watching the phases of the Moon change, notice the daylight hours increase now that we are passing the winter solstice, and enjoy all the time that you have here on Earth. The celestial clocks will move as they will. And since we are all on the same planet, we are all able to work with the same amount of time. So, make the most of it! There is nothing we can do to stop the advancement of time, so use what time that you have, to learn, appreciate, grow and enjoy the time, while you have it.