How was 'years' defined in the book of Genesis?

Genesis 5 clearly shows the number of years people lived from Adam to Noah. In the modern world, we know that 1 year equals to the number of days the earth revolves around the sun, but this knowledge was just claimed in the 17th century by Galileo while the book of Genesis should have been written much earlier. As another point of view, the calendar system that most parts of the world use today dated back to 45 BC through the introduction of Julian calendar. This may be the first time a wide group of society agreed on how long 1 year is. How did the author of Genesis define how long 1 year was? **EDIT:** While [Why did people live so long before the Flood](https://christianity.stackexchange.com/questions/2972/why-did-people-live-so-long-before-the-flood) concerned about the ages (specific to Genesis 5), my question wonders more on the way a year was defined in general, possibly valid across the Pentateuch, by giving Genesis 5 as one example. It does not ask why men lived with such long ages, which had answers such as: bodily well-being was enhanced; a greater degree of nourishment; water before the flood protected human ages; etc. These answers in that discussion did not address my question. Hence my question could have been, "How did Moses define 1 year in Pentateuch?" The problem with this phrase is that it could mistakenly viewed as a duplicate with other discussions, such as [who wrote the Pentateuch](https://christianity.stackexchange.com/questions/1037/who-wrote-the-book-of-genesis/2510#2510) , or whether the Pentateuch was written by the same author. In contrast, my question emphasizes the definition of a "year" written across the Pentateuch. To avoid conflicting discussions, herewith I specify one example by pointing to Genesis 5. An example of satisfying explanation is from one comment below, that one didn't need a calendar to know the years, but used winter and summer.