Galaxy guide: Can you pass this quiz on measuring time?
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 so bad. I think you deserve partial credit. However, we can be more specific. You may be surprised to learn that the Earth spins on its axis in about 23 hours and 56 minutes, what astronomers call a sidereal daytime.
Our typical 24-hour day is based on the position of the Sun. Ancient peoples used sundials to mark the Sun’s highest position in the sky each day, then eventually civilizations began to divide this solar day into 24 equal segments.
So why the difference of 4 minutes?
Because while the Earth rotates on its axis, it also rotates around the Sun. In a sense, the Earth must rotate four “more” minutes for the Sun to be aligned again at noon.
On other planets, such as Mercury and Venus, the difference between the sidereal day and the solar day is much more pronounced. On Mercury, for example, a solar day lasts two Mercury years!
Let’s try another question. How long does it take the Earth to orbit the Sun once?
Did you guess 365 days?
Again, not bad. Maybe you even said 365 and a quarter days if you remember that we add an extra day to the calendar every four years, a “leap year” to account for that annoying quarter day.
But guess what? This answer is not entirely accurate either. An even more accurate answer would be around 365.242 (you see the picture – it’s not exact).
So how did timekeepers of the past explain the difference between 365.25 and 365.242?
Here is an obscure fun fact for you. We don’t really have a leap year all 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, was not a leap year! (Because the number 2000 is divisible by four). Neither will the year 2400 in case you want to plan accordingly.
Even with all of this, our current schedule is still not perfect. Adjustments in the future will still need to be made to account for such an inconvenient and inaccurate number.
Next question: How many days does it take for the Moon to orbit the Earth?
Did you guess 30? 31?
Sorry. You might be surprised to learn that the Moon orbits the Earth in just over 27 days. All it takes is 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 the Moon. 27.3 days is what we call a sidereal month.
The 12 months we know and love (“month” in Old English) are known as synodic month. They are based on the familiar phases of the Moon, the time from one New Moon until the next New Moon. A synodic month lasts about 29.5 days, the length you would expect.
Why is there a difference? While the Moon orbits the Earth in 27.3 days, the Earth is busy orbiting the Sun. So, in a sense, the Moon needs a few more days to “catch up” to the moving Earth and be aligned with the Earth and the Sun again. This is why calendar months last on average about 30 days.
Therefore, days, months and years are based on the physical and orbital properties of the Earth. But the concept of “week” is not.
This brings 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 figured out 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 roughly translates to “wanderer”. This is because the planets drift or wander through the constellations. Their luminosity also changes significantly (as they orbit further and further away from Earth and the Sun).
Planets occasionally appear drift backward relative to background stars, exhibiting a so-called retrograde motion (important note – the planets always revolve around the Sun in the same direction, despite this illusion).
Thus, the ancient peoples knew that the five planets visible to them were different entities from the stars for all the aforementioned reasons. They were special.
And, it probably goes without saying that all the ancient cultures of the world were intensely interested in the movements of the Sun and the Moon.
So there were a total of seven objects in the sky of particular interest to ancient peoples. This is why we have seven days in our week.
Think about it – what is the first day of the calendar week? SUN-day, of course! And this is immediately followed by the second most prominent object in the sky – the day of the MOON.
The rest of the weekdays are dedicated to the five naked-eye planets. In English, these days are associated with Norse gods (Thor’s Day, for example). But it’s easier to see the connection to the planets using Romance languages like Spanish. In Spanish, Tuesday is Martes – Mars Day. Wednesday is Miercoles – Day of Mercury. Thursday is Jueves – Day of Jupiter. And Friday, or Viernes, is the day of Venus. In English, it is obvious that the last day of the week, Saturday, is named after the beautifully ringed planet.
Well, the time is up!
Whether you passed your pop quiz or learned a few new things along the way, I appreciate you spending some of your time reading my article. Because the most important lesson about time is this: it’s the most precious asset each of us has.
I hope you enjoy every minute, my friends, on this upcoming trip around the Sun.
Kyle Throw has been reading about astronomy since the age of five 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 in high school in 1997. Follow him on Instagram at @jeterk1971 and subscribe on Youtube.
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