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Janus submitted a new PF Insights post
Measuring How Many Days Are in a Year
Continue reading the Original PF Insights Post.
Measuring How Many Days Are in a Year
Continue reading the Original PF Insights Post.
Well thank god they realized the slowing rotation rate of the Earth in the 60's and changed the SI definition a second.mfb said:To make things worse, the rotation rate of Earth is not constant. The effect can be up to one second per year.
Over longer timescales, the days get longer by about 2.3 milliseconds each century, currently with a lower rate of 1.7 ms as some continental masses still move around related to the last ice age.
To keep atomic clocks in sync with the position of the sun, we frequently need leap seconds, and we will need more and more in the future.
Well atomic clocks would run more stable than Earth either way. But frequently tuning them would be really messy and make astronomic measurements weird ("and then we have to account for the longer seconds in 1994 to compare the results").PWiz said:Well thank god they realized the slowing rotation rate of the Earth in the 60's and changed the SI definition a second.
You need constants with dimensions to fix units. Dimensionless constants (in SI) do not allow to define things like kilograms.I guess we just have to wait for the Kilogram, Ampere and Kelvin to get a much needed change in definition to one in terms of (preferably dimensionless) fundamental constants.
Ahh, right, silly me. But incorporation of the fundamental constants in SI definitions is definitely the need of the hour.mfb said:You need constants with dimensions to fix units. Dimensionless constants (in SI) do not allow to define things like kilograms.
Jim60 said:In the past they must have thought there was only 360 days in a year, that’s probably why we’ve got 360 degrees in a circle?
Jim60 said:Finding how many days in a year by using the sighting of the crescent moon must have been impossible?
If you count 6 times 29 and 6 times 30, add them together, it comes to 354 days.
That’s 11.25 days less than the accepted figure of 365.25…
Did they know how to add fractions?
It must have been after Newton and Kepler when they finally got it about right?
The Greeks knew that the year was just a little shorter than 365.25 days. When Julius Caesar came to power in Rome, he instituted a reform of the calendar, since the old Roman lunar calendar had only about 355 days in a year, and the Roman months had gotten badly out of step with the seasons over time.Jim60 said:Finding how many days in a year by using the sighting of the crescent moon must have been impossible?
If you count 6 times 29 and 6 times 30, add them together, it comes to 354 days.
That’s 11.25 days less than the accepted figure of 365.25…
Did they know how to add fractions?
It must have been after Newton and Kepler when they finally got it about right?
The Babylonians used the moon as a clock because it was important to their religious practices, not because they wanted to be good astronomers, which they were.Jim60 said:Why did they use the moon as a clock if they knew the year was 365.25 days long, and more importantly, how did they work it out?
It must have been a remarkable achievement for a Babylonian. What was his name?
It wasn't the Romans who did. It was the Greeks among others who made the observations necessary.Jim60 said:Thanks very much for all the replies.
Has anyone tried timing the Sun or Sirius to get the length of a year?
If anybody can give me some tips, I’ll give it a go myself.
Getting 365.25... Day’s without any modern equipment like an accurate clock and a precision mounted telescope seems impossible, how did the Romans achieve it?
As far as I know, the sun rises in the east and sets in the west every day.Jim60 said:Would it be easier to start counting the days from March 20th the spring equinox, as the Sun would rise in the east and set in the west on that day?
After that day the Sun would start rising more northerly until about June 19th, then after that day the Sun would start to rise more easterly again.
Eventually the Sun would be back to where it started on about September 22nd
After September 22nd the Sun would start to rise more southerly until around December 21st.Then the Sun would return to where it started on March 20th 365.25 days later.
Hope this waffle can be understood.
Have I missed something?
There is nothing special about a sunrise exactly east and sunset exactly west - unless you use something like the equatorial ring, but that needs alignment before (and then you could also use it for different days, with different alignment).Jim60 said:Would it be easier to start counting the days from March 20th the spring equinox, as the Sun would rise in the east and set in the west on that day?
Jim60 said:That equatorial ring looks like it does the same job as the equatorial mount on a telescope, where it compensates for the 23 degrees tilt of the Earth?
I presume you would check the Suns position with that while it’s on the meridian at mid day?
The purpose of measuring how many days are in a year is to accurately track and record the passage of time. This information is important for various scientific, cultural, and practical purposes.
On average, there are 365.24 days in a year. This is known as a tropical year, which is the time it takes for the Earth to complete one orbit around the sun.
The number of days in a year is not a whole number because the Earth's orbit around the sun is not a perfect circle. It is slightly elliptical, causing the Earth to travel at different speeds throughout the year. This results in a slightly longer year.
The length of a year is measured in various ways, including using astronomical observations, mathematical calculations, and atomic clocks. These methods all take into account the Earth's rotation and orbit around the sun.
Accurately measuring the length of a year is important for various reasons. It allows us to track and predict seasonal changes, plan agricultural and cultural events, and ensure the accuracy of calendars and timekeeping systems. It also helps in understanding and studying the Earth's movements and its relationship with other celestial bodies.