Question

This passage is adapted from an article about the history of modern astronomy.

In the late nineteenth and early twentieth centuries, the Harvard College Observatory was a center of astronomical innovation, though much of its most tedious work was performed by a dedicated group of women. Known colloquially as the "Harvard Computers," these women were hired by the observatory director, Edward Charles Pickering, to analyze and catalog thousands of photographic plates of the night sky. Among these researchers was Henrietta Swan Leavitt, a graduate of Radcliffe College who joined the observatory in 1895. Despite facing progressive hearing loss and receiving little public recognition during her lifetime, Leavitt would make a discovery that fundamentally transformed our understanding of the scale of the universe.

Leavitt was assigned to study variable stars—stars whose brightness changes over time. In particular, she focused her attention on the Small Magellanic Cloud, a dwarf galaxy visible in the Southern Hemisphere. Using a magnifying glass to examine the glass photographic plates, Leavitt identified thousands of variable stars, including a specific class known as Cepheid variables. Cepheids are pulsating stars that brighten and dim in a highly predictable, cyclical pattern. Leavitt meticulously recorded the minimum and maximum brightness of each variable, as well as the precise length of its cycle, or period.

In 1908, Leavitt published her initial catalog of 1,777 variable stars in the Annals of the Astronomical Observatory of Harvard College. In this paper, she noted a curious trend: a handful of the brightest Cepheid variables appeared to have the longest periods. Recognizing the significance of this observation, Leavitt continued her investigations. By 1912, she had compiled data for 25 Cepheid variables in the Small Magellanic Cloud, confirming a direct mathematical relationship: the longer a star’s period of pulsation, the greater its intrinsic brightness, or luminosity.

The crucial element of Leavitt’s discovery lay in the location of the stars she observed. Because all the Cepheids in her study were located within the Small Magellanic Cloud, they were all roughly the same distance from Earth. Therefore, any difference in their apparent brightness as seen from Earth was not a function of their distance, but rather reflected a real difference in their actual light output. By establishing this "period-luminosity relation," Leavitt provided astronomers with a revolutionary tool. If the period of a distant Cepheid could be measured, its absolute luminosity could be calculated. By comparing this absolute luminosity to the star’s apparent brightness, astronomers could determine exactly how far away the star—and the galaxy hosting it—was.

Prior to Leavitt’s breakthrough, astronomers relied almost exclusively on stellar parallax to calculate distances. This geometric method, which measures the apparent shift of a nearby star against more distant background stars as Earth orbits the Sun, was highly accurate but severely limited. Due to the limits of early twentieth-century telescopes, parallax could only be used to measure distances to stars within approximately 100 light-years of Earth. Beyond this narrow bubble, the universe was a vast, unmeasurable expanse, and astronomers actively debated whether the Milky Way constituted the entirety of the cosmos.

Leavitt’s period-luminosity relation, often referred to as the cosmic "standard candle," shattered these boundaries. In 1924, astronomer Edwin Hubble located Cepheid variables in the Andromeda Nebula. Using Leavitt’s relationship, Hubble calculated that Andromeda was roughly 900,000 light-years away—far outside the boundaries of the Milky Way. This single calculation proved that Andromeda was not a cloud of gas within our galaxy, but an independent galaxy of its own. Hubble’s subsequent discovery of the expanding universe was built directly upon the foundation of Leavitt’s meticulous work with the glass plates of Harvard.

According to the passage, prior to Henrietta Swan Leavitt's discovery of the period-luminosity relation, what method did astronomers primarily use to calculate astronomical distances?