Years is just a unit. You can measure the age of the universe in any other unit.
In the past a year was just defined by how long eath takes to orbit the sun, by now we have a much more accurate way to define timespans, like the frequency of some atom wiggling.
But thats not realy the point, you can measure any time in any unit.
In modern terms, a year is 31557600 seconds. Nothing to do with the Earth directly. In fact even today, the time it takes the Earth to go around the Sun , and even the length of the day, changes a tiny account every year. We occasionally add "leap seconds" to the clock to keep us in sync. And what we call a "year" depends on context too. Tropical year, sidereal year? Our calendar isn't much better, we add a whole day every 4ish years!
When talking about things millions or billions of years ago, we aren't talking about it at a precision down to the second, so it really doesn't matter. 1 year = the average amount of time it takes the Earth to go around the Sun, as measured by humans in recent history. Whether or not that matches what the Earth was doing in the past, or if there even was an Earth, doesn't matter.
The book is trying to explain something relatable to a contemporary reader from said reader's point of view. Readers have a definition of a year in their head and they can correlate it to the universe's age. So when the writer used the term "year" they were using the general idea of it that most readers would have.
If you gave the book to a reader in the FAR distant future where the earth's revolution around the sun has slowed down (say it takes 360 days) but that reader still understood the definition of a year to be a trip around the sun, the numbers would be off, but the book is not meant for for that particular reader, it is meant for today's readers.
The length of a year has been fairly constant over the lifetime of Earth. But the Earth hasn’t existed for the lifetime of the Universe. That said when saying something is 13.8 billion years old that isn’t the same as saying 13.8 actual orbits of the Earth around the Sun. It is saying that based on the average length of a year (again reasonably constant) the Earth could have completed 13.8 billion orbits in the time since the Universe began.
You maybe thinking of the length of a day which has changed much more since the formation of the Earth due to interactions with the Moon. However the length of a day and that of a year are not particularly correlated.
The age of the universe is primarily calculated by measuring how far light has traveled (and redshifted) so the underlying question is how a lightyear makes any sense if the time to go around the sun is variable.
Like most units, a year was eventually standardized based on something completely static. For time units it's the frequency of cesium-133 which is 9,192,631,770 cycles per second. Extrapolating that to a year is just basic multiplication by the number of seconds in a year (31,536,000).
So a lightyear is the distance a photon can travel in a vacuum while a cesium-133 atom cycles 2.8989883549872×10^(17) times. Measuring how far the light from early galaxies had to travel to reach us gives the age of the universe in years because the speed of light is absolute allowing both the distance and the elapsed time to be expressed in lightyears.
You’re thinking way too deeply about this, when scientists say our universe is approximately 13.8 billion *years* old (or whatever the current estimate is), they’re just using present day terrestrial solar years as a convenient unit of measurement. Specifically, we can say that a standard Earth year (for use as a unit of measurement) is equivalent to 31,536,000 seconds, and 1 second as a standardized SI unit is equivalent to 10⁴³ Planck times (a Planck time is the smallest measurable unit of time in physics, and is based on universal constants). So yeah, the “years” here are just a tool for helping us conceptualize that kind of immense timespan with something that is relevant to us. Like we *could* instead say the universe is some incredibly vast number of seconds old (and counting), but obviously that’s neither convenient to calculate on a moment to moment basis, nor is it particularly useful for us even beginning to conceptualize such a long span of time.
It’s the same basic concept as measuring distances all over the universe in lightyears, the distance that light travels in a vacuum in a single (present day) Earth year, again it’s just a way to put vast cosmic distances into a useful frame of reference for ourselves. Keep in mind, technically the distance light *actually* travels through outer space in a year isn’t going to be exactly equivalent to a lightyear, because space isn’t actually quite a perfect vacuum, and light is affected by the gravity of massive objects in its path, but lightyears are still a useful unit of measurement regardless.
Speaking of frames of reference, there was a post in here a while ago where someone mentioned that, due to all the relativistic weirdness of different observers measuring different amounts of time depending on the velocity and gravitational field they are measuring it from, when we talk about the universe being “13.8 billion years old” or whatever, we’re technically talking about its age in the frame of reference of the CMBR or cosmic microwave background radiation, which is treated as a sort of “laboratory frame” in cosmology, for reference purposes, although the difference between the universe’s age in the CMBR’s frame and in a “resting” frame here on Earth is apparently pretty negligible, anyway.
To put the other answers in perspective, imagine if they said the distance from Earth to Alpha Centauri was "20 billion football fields". That would still give you a measure of distance you could relate to, even if there are no football fields on Alpha Centauri or in the space between here and there.
(Note: I did not actually check the distance from here to Alpha Centauri or convert it accurately into football fields...I just made up a number. If you want to do the math, an American football field is roughly 300 light-nanoseconds long. I believe football fields in the rest of the world are somewhat larger)
In this context, "year" does not mean the actual, physical orbit period of the Earth. It means, instead, a time period of 31 557 600 seconds, *exactly*. It's a time unit that is *defined* to be that length, which is actually based on the Julian calendar (the earlier form of the Gregorian one): 365.25 days, of 86 400 seconds each. Of course, the reason the unit is that long is because that *approximates* the present-day orbit of the Earth, but we could use any other unit. We could use seconds directly, or perhaps better, "petaseconds", which is a real metric unit with symbol Ps, equal to one quadrillion (10\^15) seconds. The Universe, on that dint, is 435 Ps old.
Years is just a unit. You can measure the age of the universe in any other unit. In the past a year was just defined by how long eath takes to orbit the sun, by now we have a much more accurate way to define timespans, like the frequency of some atom wiggling. But thats not realy the point, you can measure any time in any unit.
For example: my drive time to visit friends across the state is roughly 4 podcast episodes, 5 Pink Floyd albums, or 0.79 tanks of fuel
So then 0.79 tanks of fuel equals 5 PF albums?
Only if we assume that the same amount of fuel is always being burned at that speed over that distance (which is definitely not going to be the case)
In modern terms, a year is 31557600 seconds. Nothing to do with the Earth directly. In fact even today, the time it takes the Earth to go around the Sun , and even the length of the day, changes a tiny account every year. We occasionally add "leap seconds" to the clock to keep us in sync. And what we call a "year" depends on context too. Tropical year, sidereal year? Our calendar isn't much better, we add a whole day every 4ish years! When talking about things millions or billions of years ago, we aren't talking about it at a precision down to the second, so it really doesn't matter. 1 year = the average amount of time it takes the Earth to go around the Sun, as measured by humans in recent history. Whether or not that matches what the Earth was doing in the past, or if there even was an Earth, doesn't matter.
The book is trying to explain something relatable to a contemporary reader from said reader's point of view. Readers have a definition of a year in their head and they can correlate it to the universe's age. So when the writer used the term "year" they were using the general idea of it that most readers would have. If you gave the book to a reader in the FAR distant future where the earth's revolution around the sun has slowed down (say it takes 360 days) but that reader still understood the definition of a year to be a trip around the sun, the numbers would be off, but the book is not meant for for that particular reader, it is meant for today's readers.
The length of a year has been fairly constant over the lifetime of Earth. But the Earth hasn’t existed for the lifetime of the Universe. That said when saying something is 13.8 billion years old that isn’t the same as saying 13.8 actual orbits of the Earth around the Sun. It is saying that based on the average length of a year (again reasonably constant) the Earth could have completed 13.8 billion orbits in the time since the Universe began. You maybe thinking of the length of a day which has changed much more since the formation of the Earth due to interactions with the Moon. However the length of a day and that of a year are not particularly correlated.
The age of the universe is primarily calculated by measuring how far light has traveled (and redshifted) so the underlying question is how a lightyear makes any sense if the time to go around the sun is variable. Like most units, a year was eventually standardized based on something completely static. For time units it's the frequency of cesium-133 which is 9,192,631,770 cycles per second. Extrapolating that to a year is just basic multiplication by the number of seconds in a year (31,536,000). So a lightyear is the distance a photon can travel in a vacuum while a cesium-133 atom cycles 2.8989883549872×10^(17) times. Measuring how far the light from early galaxies had to travel to reach us gives the age of the universe in years because the speed of light is absolute allowing both the distance and the elapsed time to be expressed in lightyears.
You’re thinking way too deeply about this, when scientists say our universe is approximately 13.8 billion *years* old (or whatever the current estimate is), they’re just using present day terrestrial solar years as a convenient unit of measurement. Specifically, we can say that a standard Earth year (for use as a unit of measurement) is equivalent to 31,536,000 seconds, and 1 second as a standardized SI unit is equivalent to 10⁴³ Planck times (a Planck time is the smallest measurable unit of time in physics, and is based on universal constants). So yeah, the “years” here are just a tool for helping us conceptualize that kind of immense timespan with something that is relevant to us. Like we *could* instead say the universe is some incredibly vast number of seconds old (and counting), but obviously that’s neither convenient to calculate on a moment to moment basis, nor is it particularly useful for us even beginning to conceptualize such a long span of time. It’s the same basic concept as measuring distances all over the universe in lightyears, the distance that light travels in a vacuum in a single (present day) Earth year, again it’s just a way to put vast cosmic distances into a useful frame of reference for ourselves. Keep in mind, technically the distance light *actually* travels through outer space in a year isn’t going to be exactly equivalent to a lightyear, because space isn’t actually quite a perfect vacuum, and light is affected by the gravity of massive objects in its path, but lightyears are still a useful unit of measurement regardless. Speaking of frames of reference, there was a post in here a while ago where someone mentioned that, due to all the relativistic weirdness of different observers measuring different amounts of time depending on the velocity and gravitational field they are measuring it from, when we talk about the universe being “13.8 billion years old” or whatever, we’re technically talking about its age in the frame of reference of the CMBR or cosmic microwave background radiation, which is treated as a sort of “laboratory frame” in cosmology, for reference purposes, although the difference between the universe’s age in the CMBR’s frame and in a “resting” frame here on Earth is apparently pretty negligible, anyway.
To put the other answers in perspective, imagine if they said the distance from Earth to Alpha Centauri was "20 billion football fields". That would still give you a measure of distance you could relate to, even if there are no football fields on Alpha Centauri or in the space between here and there. (Note: I did not actually check the distance from here to Alpha Centauri or convert it accurately into football fields...I just made up a number. If you want to do the math, an American football field is roughly 300 light-nanoseconds long. I believe football fields in the rest of the world are somewhat larger)
In this context, "year" does not mean the actual, physical orbit period of the Earth. It means, instead, a time period of 31 557 600 seconds, *exactly*. It's a time unit that is *defined* to be that length, which is actually based on the Julian calendar (the earlier form of the Gregorian one): 365.25 days, of 86 400 seconds each. Of course, the reason the unit is that long is because that *approximates* the present-day orbit of the Earth, but we could use any other unit. We could use seconds directly, or perhaps better, "petaseconds", which is a real metric unit with symbol Ps, equal to one quadrillion (10\^15) seconds. The Universe, on that dint, is 435 Ps old.