We are familiar with the traditional birth certificate research. We use those for documenting the birth date of an ancestor, their parents' names, and their location in time. But you do not have to go too far back in your research timeline to a point when formal birth records were required. The question in your research very quickly becomes... How do you determine a birth date for your ancestor in the absence of a birth certificate?
Birth certificates redesigned as a personal infographic: Other details including astrological, astronomical and etymological facts are also included, plus mapping coordinates for location of birth are added to the digital version.
US Vital Records: Where to Get Birth, Death and Marriage Certificate Copies