By Kyle Jeter

Pop quiz! Let’s think about a subject that most of us take for granted: how we measure time. We’ll start with an easy question.   

How long does the Earth take to rotate once on its axis?     

If you answered, “24 hours”, that’s not too bad. I think you deserve partial credit.   However, we can be more precise. It may surprise you to learn that the Earth rotates on its axis in about 23 hours and 56 minutes which astronomers refer to as a sidereal day.  

Our typical 24-hour day is based on the position of the Sun. Ancient people used sundials to mark the highest position of the Sun in the sky each day, and then eventually, civilizations began dividing this solar day into 24 equal segments.   

So why the 4-minute difference?  

Because while the Earth is spinning on its axis, it is also orbiting the Sun. In a sense, the Earth must spin an “extra”  four minutes to get the Sun aligned again at noon.  

On other planets, such as Mercury and Venus, the difference between the sidereal day and solar day is much more pronounced. On Mercury, for instance, a solar day is two Mercury years long!

Let’s try another question.  How long does it take for the Earth to revolve once around the Sun?  

Did you guess 365 days?  

Again, not bad. Perhaps you even said 365 and one-fourth days if you remembered that we add an extra day to the calendar every four years, a “leap year”  to account for that pesky one-fourth of a day.  

But guess what? That answer isn’t perfectly accurate either. An even more precise answer would be around 365.242 (you get the picture – it’s not exact).  

So how did timekeepers in the past account for the difference between 365.25 and 365.242? 

Here’s an obscure fun fact for you. We don’t actually have a leap year every four years, after all.   During Century years (1400, 1500, etc.), there is no leap year when the Century year is divisible by the number four. So, remarkably, the year 2000, for example, wasn’t a leap year! (Because the number 2000 is divisible by four). The year 2400 won’t be either in case you want to plan accordingly. 

Even with all of that, our current calendar still isn’t perfect. Adjustments in the future will still have to be made to account for such an awkward, inexact number. 

Next question: How many days does it take for the Moon to orbit the Earth?  

Did you guess 30? 31? 

Sorry. It may surprise you to learn that the Moon orbits the Earth in a little over 27 days. It only takes basic high school physics to calculate this number based on the mass of the Earth and the distance between the centers of mass of the Earth and Moon. 27.3 days is what we call a sidereal month. 

The 12 months we know and love (“moonths” in Old English) are known as synodic months. They are based on the familiar phases of the Moon, the time from one New Moon until the next New Moon. A synodic month is about 29.5 days long—the length of time you’d expect. 

Why is there a difference? While the Moon is orbiting the Earth in 27.3 days, the Earth is busy orbiting the Sun. So, in a sense, the Moon needs a couple of extra days to “catch up” to the moving Earth and be aligned with the Earth and Sun again. That’s why the calendar months are, on average, about 30 days long. 

Therefore, days, months, and years are based on the physical and orbital properties of the Earth. But the concept of a “week” isn’t. 

This leads us to the final question: Why are there seven days in a week?  

Ancient people lacked modern equipment such as telescopes and binoculars, of course. But they still understood that the five planets they could see with the naked eye behaved differently in the sky than the stars. 

The term planet comes from the Greek word “planetas,” which translates roughly to “wanderer.”  This is because planets drift, or wander, through the constellations. Their brightness changes significantly as well (as they orbit closer and farther from the Earth and the Sun).   

Planets occasionally appear to drift backward against the background stars, exhibiting so-called retrograde motion (important note – the planets always orbit the Sun in the same direction, though, despite this illusion). 

So, ancient people knew that the five planets visible to them were different entities than stars for all the abovementioned reasons. They were special.

And, it probably goes without saying that all ancient cultures around the world were intensely interested in the motions of the Sun and the Moon.  

So there were seven total objects in the sky of particular interest to ancient people.   That’s why we have seven days in our week.  

Think about it – what’s the first day of the calendar week? SUN-day, of course! And that’s immediately followed by the second most important object in the sky – MOON-day.   

The rest of the days of the week are devoted to the five naked-eye planets. In English, these days are associated with Nordic gods (Thor’s Day, for instance). But it’s easier to see the connection to the planets using Romance languages such as Spanish. In Spanish, Tuesday is Martes – Mars Day.   Wednesday is Miercoles – Mercury Day. Thursday is Jueves – Jupiter Day. And Friday, or Viernes, is Venus Day. In English, it’s evident that the final day of the week, Saturday, is named for the gorgeously-ringed planet.  

Well, time’s up!

Whether you aced your pop quiz or learned a few new things along the way, I appreciate you spending a little of your own time reading my article. Because the most important lesson about time is this – it’s the most valuable commodity any of us has.

I hope you enjoy every minute, friends, on this next trip around the Sun.

Kyle Jeter has been reading about Astronomy since he was five years old and has never stopped learning. Since 1994, he has lived in Coral Springs and worked at Marjory Stoneman Douglas. He has a daughter, Kayla, and a son named Kyle. Jeter started the Astronomy program at the high school in 1997. Follow him on Instagram at @jeterk1971 and subscribe on YouTube.

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