Final answer:
The cosmic distance ladder starts with parallax, the most direct measurement of stellar distances, which was first successful in 1838 and underpins later methods like the H-R diagram and Cepheid variables.
Step-by-step explanation:
The cosmic distance ladder begins with the most direct measurements of distance using the method of parallax, which is the foundation of all stellar distance estimates. This method involves triangulation of distances from a baseline created by Earth's annual motion around the Sun. By measuring the shift in a star's position relative to distant background stars from opposite sides of Earth's orbit, astronomers can determine the parallax of the star and thus its distance. The first successful parallax measurements date back to 1838, and they have since been critical for calibrating other methods used to measure distances in the Universe, such as the Hertzsprung-Russell (H-R) diagram and the period-luminosity relationship for Cepheid variables.