What’s In a Day? A Year?

Understanding time throughout the Solar System

Sarah Marie
7 min readDec 28, 2021


Fireworks explode in the center of the frame. The author has used this image to show the ending of one year and the beginning of new one.
Celebrating the culmination of a year and the beginning of a new one with fireworks. Photo by Roven Images on Unsplash

Despite the fact that many of us cannot believe it, and for reasons more than normal, 2021 is officially coming to an end. But what is a year? What is a day?

Most of you by this point should have an answer ready to go like a gunslinger in a Western.

There are 24 hours in a day.

365 days in a year.

Possibly even with a “duh” followed after it. We know these answers. We were taught these answers long ago and as adults, we certainly understand at least the first one, and hopefully the second.

A day is the time it takes from the rising of the sun to its setting, through the night, until the sun rises again. Pretty simple stuff that we experience every day. You don’t necessarily need someone to explain that to you.

But a year is somewhat harder. In fact, the history of the modern calendar is long and complicated with multiple revisions. People alive during these revisions had to deal with their calendar completely changing on them, to the point that a week just disappeared in 1582 as the new Gregorian calendar was enacted on Friday, October 15, 1582, even though the previous day had been Thursday, October fourth based on the old, Julian calendar.

So what was the issue? What were we calculating to make a year?

Well, it all goes back to astronomy and Earth’s relative location to the Sun.

Again, many of us can recite these astronomical definitions of day and night. A day is the time it takes for the Earth to do one full rotation of its axis.

Think of a basketball spinning on someone’s finger, but slow it down and put a light on one side of it a little bit away. From the perspective of any point on that basketball, there will be times when it is in full light, times when it is in full darkness, and areas in between.

A gif of the Earth rotating with light from the sun (off frame) illuminating the left side. This shows how we have day and night.
Gif showing the Earth rotating with light coming from left side, showing how day and night occur in an astronomical sense. From our perspective here on Earth, we see the Sun rise and set because we are actually spinning from darkness into light. Note: the image does not depict the tilt of the Earth’s axis to better show this detail. Gif Source

We see the sun rising and setting, not because it orbits us, but because we turn on our axis and therefore we see the sun appear and disappear as we rotate…



Sarah Marie

Author & Freelance Writer | Top Writer in Space | A little bit of everything: Science, books, personal development, fiction, poetry, hobbies, and art