So, if we say something is 1 billion years old, is that (365 spins on earth’s axis x 1 billion)? Is it (1 trip around the sun x 1 billion), or a different measurement? The answers to those change depending on how it’s calculated.
In other words, if I say I lived one year, that means 24 hr/day for 365 days/year in todays terms. Over time the earth’s orbit of the sun becomes faster and slower changing the meaning of a year. Also, as the earth spins faster and slower on its axis, a day in terms of hours is different relative to today. It breaks my brain.
What about the needs for adjustments for leap years? How does this influence radiometric dating? If a molecule degrades by 1 measurement every 300,000 years, the first 150,000 years are going to be different than the last half. If you want to pinpoint the halfway mark, where is it?
In: Planetary Science
The half-life of uranium doesn’t care how long a day is. All that matters is the strict passage of time. For example, we measure the age of the Earth as a little bit over 4.5 billion years, **as we currently measure years**. Even if Earth’s orbit was different way back then and it went around the sun more quickly or more slowly, that doesn’t change its absolute age.
And, with leap years being an artefact of the calendar we use, scientists go for a more… well… scientific approach. Officially, the length of the year is 365.26 days. So that’s what scientists use. They’re not going to try to calculate exactly what point in the Julian calendar something occurred in.
Forget about leap years. Any estimate saying something is 1bn years old means something along: „we think it is between 500m and 1500m years old. But we might be wrong“.
By how much did the time the earth takes to a trip around the sun change over time? 5% 10%? If so this is irrelevant for the estimates of timescales you mention
It’s one trip around the Sun times one billion.
If you want a good unit of time to standardize on, use the second. We can base that off of some physical process that we can assume is constant over geologic time.
Today, a (solar) day is 86400 sec. A long time ago, it was some smaller number of seconds. The year (measured in seconds) has been more stable than the day. Radiometric processes will be the same today as they always have been.
It would be 1 billion years ago with the current lenght of a year. I am not 100% sure what definition of yar is used but I would assume it is a Tropical year that is close to 265.25 days. That way leap days get included. The accuracy of measurement is not down to days. the making of error is typically +-2 to 5% for radiocarbon dating so at 1000 years it is 20 years before or later at 2% and 50 years at 5%. Radiometric dating methods in geological time scale can have a margin of error of millions of years.
It is by the way not 365 rotations of Earth’s axis, Earth rotates relative to far away stars once in 23 hours and a bit more the 56 minutes the extra 4 minutes is because Earth has moved a bit relative to the sun and Earth needs to rotate more then 1 revolution for the sun to return to the same point in the sky.
Consider a nonrotation earth that orbits the sun, the sun would move around once in the sky because of the orbit. 4 minutes times 365 days is 24 hours and a half our. It would be very close to 24 hours if we used the exact length. So Earth rotates close to 366 revolutions in 365 days.
A year is not an orbit of the earth around the sun either. It is a cycle of the season on earth like the time between two march equinoxes. The diffrence is because the precision of earth axis and is around 20 minutes per year. It might not sound like a lot but it is a day in 72 years. It result is exactly the location of stars changes and it can be a lot over time. The Zodiac is based on the stars behind the sun when you observe it from earth. If you compare typical dates for the signs of the zodiac to where the sun really is you will se they typically do not match, it is closer to a shift of one sign because the dates are from over 1000 years ago.
Generally, we use years as a time unit for simplicity and ease of communication. It is assumed to be a trip around the sun, about 365.25 days (365.2422 if you need precision). When we are talking about things that happened 200 millions years ago it is an approximation. Other time units are more suited for large periods of time like: era, eon, epoch, age…
For dating, any scientific methods will use the second as the standard unit of time. It is then converted to years to be more readable by dividing by 60x60x24x365.2422 or a similar conversion.
We now use the caesium standard, where a second is defined by a fixed number of periods of a fancy atomic clock. We then use this second to define a minute, hour, day, and eventually year.
(more specifically, a second is defined by a constant number of transition period of caesium-133 hyperfine ground states)
So to answer your question, the age of something has nothing to do with how many times the earth rotates or orbits around the sun, but the number of ticks on a clock. A thing is 1 billion years old if it is created (1 billion x 365.24 x 86400) seconds ago.
The orbit of the earth does not affect how fast time passes, it’s just something we use to divide up periods of time.
Lets use an example in our solar system – Mars.
Mars has a different day length and year length – a Mars day is 24 hours and 37 minutes and a Mars year is 668.6 Mars days.
However you can still measure an Earth year on Mars. It wouldn’t really mean much for someone on Mars, being 355.5 Mars days, but it’s still 1 Earth year.
No imagine instead of Mars you have Earth in the distant past. If you take a current Earth year and apply it to past Earth it won’t match up with 365 past Earth’s days or maybe even its year, but it’s still a period of time we can measure.
So basically we are taking the current day/year length, ignoring the fact these values have changed, and just using it as a fixed length of time.
It’s the same question that arises if you ask about the age of the universe. After all, you were talking about measuring time in years before the earth, our sun or our galaxy even existed.
It’s one of the reasons scientists have turned to a definition of time tied to the vibrations of a cesium Adam
For practical day-to-day purposes, days and years are extremely practical because they measure observable physical phenomena. Hours, minutes and seconds are arbitrary divisions of days, but they are useful for practical purposes because everyone agrees on essentially what those units of time represent.
But because days and years are not absolute across time, it was necessary to create a measure of time that, if our understanding of the universe is correct, would apply at any time or location in the universe
Think of year as more of a scientific measurement than an actual year. It measured time that is a certain number of seconds that is easily quantifiable and always the same. They are not measuring how many times the earth has been around the sun, they are calculating how many total seconds have passed that it has existed.
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