How do we know how old the Earth is? The age of the Earth was a relatively contested figure for a while, with early scientists struggling to date it with any level of precision. The first and most-well known way to set a lower bound on the age of the earth is through radioactive dating. Simply, find as many rocks as possible, look at radioactive isotopes within those rocks, and compare the quantity of radioactive source to the quantity of members of its decay chain. This method is useful for establishing a lower bound for age, since in theory, the rocks must have formed after earth did. Whichever rock we can find that is dated to the furthest past date is the lower bound for the age of the earth. There are a few problems with this. First, radioactive decay can be unreliable, especially as gases and such are able to escape. Second, it’s not able to easily establish an upper bound, since the rocks formed after earth did. Third, it is possible rocks may have been transferred to earth from asteroids or some other cosmic event, thereby making questionable some of the assumptions of the process. Beyond dating earth alone, scientists must turn outwards and examine both nearby and far away systems. By dating nearby systems such as the moon and mars, it is possible to ascertain that they formed around 4.5 billion years ago, similar to estimates from the earth. Similarly, by examining planets in different stages of their development, it is possible for scientists to determine how long the life cycle can take for similar planets. In this way, astronomy can lend significant insights to geology and the study of our own planetary formation.