The Cepheids and the calculation behind it was possible because of the Hubble space telescope HST. The Hubble space telescope was launched in the year 1990, the telescope revolutionizes the way man thought of the universe and of the science that could enable them to explore the Universe. It represents one of the many advances that were made in the context of astrosciences. The HST orbits the earth and primarily ensures that it is placed in such a way so as to distort the light that reaches the planet and hence gives a view of the universe which is more than that can be receives via a ground telescope. The HST therefore enabled people to see more than what could be seen from the ground and hence determined scientists to come to conclusions on how the certain questions about the earth and the stars such as the images of earth, the existence of dark energy etc could be solved. The HST enabled the calculation of the pulsations more accurately and to understand how the cluster of stars in the M100 worked (Hubble had previously established that there were single stars gleaming).
Before the HST was built the expansion rate came to be called as the Hubble constant. This was estimated to be approximately 160 km/sec per million-light-years. This value was not well accepted then because people already had an understanding of the Milky way and it was seen that what Hubble gave them only seemed to indicate that the past estimates was erroneous.