2. What is a day, really?

Background Image by Moise Nicu via Wikimedia Commons.

The day is an ancient concept.  Imagine going back in time 5000 years.  You’re an early astronomer pondering nature’s cycles.  What would the day mean to you?

The answer would probably depend on where you live.  For communities that live close to the equator, the two basic periods, day and night, seem to be equally long.  Every day, the sun rises sometime after most people wake up.  A day could then be the time from one sunrise to the next.

But let’s say you don’t live close to the equator.  Your days and nights are not equally long. There are long days and short nights sometimes and short days and long nights at other times.  This means the sunrise time changes from day to day.

Your people have known this for some time but you’re about to put things together and make an astonishing discovery.

You begin to wonder if the midday time, the time halfway between sunrise and sunset is fixed.

You realize you’ve never paid attention to exactly where the Sun is in the middle of the day.  Your mother told you to never stare at the Sun and it sounded like good advice.  Plus there are no reference points in the sky that you can use to mark the position of the Sun.

But you don’t need to look at the Sun.  That’s what shadows are for.

It’s morning.  The Sun rose some time ago and is already too bright to look at.  You look down at your shadow.  It’s long now.  It will get shorter as the Sun moves higher in the sky.  At midday, halfway between sunrise and sunset, the shadow will be shortest.  Then, as the Sun moves down, the shadow will get longer.  If you were to look at it some time before sunset, it will be just as long as it is now.

Your shadow is trying to tell you something.  Image by Viktor van Werkhooven via Wikimedia Commons.

You plant a stick in the ground away from buildings and trees so that its shadow will remain unobstructed.  You have just invented the world’s first astronomical instrument.  It will come to be known as a gnomon.

You don’t need to sit outside all day looking at the shadow of your stick.  You can just glance at it periodically.  After some time, it has become shorter and changed direction.  Some time later, it has become even shorter and rotated some more.  The next time you look at it, it has rotated further but it is now longer.  It is past midday.

You try to figure out how long the shortest shadow would have been and in which direction it would have pointed.  You draw a line on the ground tracing this hypothetical shortest shadow.  Tomorrow you can come out and see if your guess was right.

On the next day, you see that your initial guess was off by a bit.  You trace the shortest shadow with a deeper line than you previously drew.

After a few days, it’s clear to you that the shortest shadow of the day always falls on this line.  You draw it deeper, with a spear.  You want it to last a long time.

This line will later be referred to as the meridian.  The Sun crosses the meridian at midday, a time that will later be referred to as noon.  People will use the term “ante-meridian” (or “AM”) to refer to the hours before noon and the term “post-meridian” (or “PM”) to refer to those after noon.

You have constructed a basic clock.  The direction the shadow of your stick moves will eventually be referred to as clockwise.

The only time your clock can tell is noon, but it’s a start.  From now on, you can define the length of the day precisely.  It is the interval from noon to noon.

IMG_5291 - Copy
A gnomon, among other things.  The Egyptian obelisk in St. Peter’s Square, Vatican City.

You with me so far?  Is this getting complicated?  Or does it sound too simple to be true?  

There is a famous quote often attributed to Albert Einstein: 

“Everything should be made as simple as possible but not simpler.”  

But where is the line between simple and oversimplified?  It’s not fixed.  It can vary from person to person and even from moment to moment.  It depends on how curious you choose to be. 

As a science teacher, I often struggle with where this line is, or should be, for the class as a whole.  Whenever I find myself in this predicament, I am reminded of an incident that took place during a biology class when I was in college.  While lecturing, the instructor stopped in mid-sentence and said “Wait, that was a lie.”  He then hesitated and said “Well, it’s almost true.”  Some students laughed.  Then he said “You know, education is a process where you get lied to a little less as you move along.”

Have I lied to you so far?  Oh yeah.  Sorry.  

For instance, when I said the midday time would later be referred to as noon, what I didn’t say was that the actual meaning of the word “noon” is “ninth.”  It originally referred to the ninth hour after sunrise or around 3 pm.  It would gradually be moved to 12 pm during the 12th to the 14th centuries.

This was more of an omission than a lie, really.  And it’s not my fault words change meaning over time.  I don’t feel bad about it at all.

The other lies were intentional.  I told them to keep this post short and simple.  Otherwise, I would have had to introduce additional concepts.    

I’ll tell you what these lies were next time.  If you think you know, post a comment.


Author: Bhasker Moorthy

I'm a Professor of Astronomy at Harper College in Palatine, IL, USA. My email address is bmoorthy [at] harpercollege.edu.

8 thoughts on “2. What is a day, really?”

  1. Professor Moorthy,
    A day is noon to noon….are you lying to us? If so, I am sure there is a good reason.

    From our friends of the Oxford Dictionaries:
    DAY-a period of twenty-four hours as a unit of time, reckoned from one MIDNIGHT to the next, corresponding to a rotation of the earth on its axis.

    Is there a reason to count a day from noon to noon instead of midnight to midnight?


    1. No shadow is cast at midnight…. ancient people probably defined the day from noon to noon because they were up and not sleeping and because they could see the change occur during the day and not at night…. how it changed to midnight to midnight sounds like another post that needs to be written.


  2. That wasn’t one of the intentional lies.

    There is the length of the day and then there is the time we choose to mark the beginning of the day. The length of a day is based on the Sun’s position at noon. The 24 hour day is based on the Sun, not on anything that’s visible at night. To keep time during the night, the ancient Egyptians used groups of stars called decans. A new decan rose roughly once an hour. So there were roughly 12 decans per night. (By the way, the reasons they’re called decans is that a new one rises at dawn every 10 days.) Then water clocks were invented which could be used during daytime and nighttime. This allowed for continuous timekeeping. Since there are, on average, the same number of daytime hours as nighttime hours, we ended up with a 24 hour day. So the concept of the hour could have come from nighttime timekeeping even though the day is based on the Sun.

    As for when each new day should begin, we could have chosen the mean sunrise time so that the first 12 hours will be during the day and the last 12 at night (not exactly because of seasonal variations but roughly). Noon is a better choice since it’s more consistent than the sunrise and sunset time. But then it would be annoying to have a new day start at noon. Morning and afternoon are on two different days. It’s much more convenient for all of our waking hours to fall within the same day. So eventually we settled on midnight.


  3. The stick and the shadow traces back long ago from around the Aztecs, etc with sundials. In fact, some people still use them today, but really only for decoration. It is interesting to see how the “times” have changed, ie. noon meaning nine, changing to 3 pm, etc. Our days do normally consist of being up for twelve hours, or more just not noon to noon, but rather 9 am to 9 pm, etc. Good post!


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