How did Aristarchus try to estimate the distance from the Earth to the Sun and Moon?

A fundamental technique in understanding the universe, geometric estimations in astronomy involve using observational data to deduce distances and sizes of celestial objects. Ancient Greek astronomer Aristarchus applied this technique by observing the angle between the Moon and Sun during a half-moon. He assumed a right-angle triangle could be formed at that moment, which is a key concept in trigonometry known as the trigonometric parallax.

He knew that the sum of angles in a triangle equals 180 degrees and, by estimating the missing angle between the Moon and Sun, he attempted to calculate the distance from the Earth to these celestial bodies. Despite his incorrect estimation, the concept of using geometric relationships remains a crucial point in understanding celestial distances and continues to underpin modern astronomical measurements.

Astronomy is among the oldest of the natural sciences with its roots tracing back to ancient civilizations who charted the heavens. Aristarchus of Samos, who lived around 310 – 230 BCE, made significant contributions to the field. His approach to estimating celestial distances laid foundational stones in astronomy and influenced many future scientists.

Although his numerical estimates were incorrect, his methodology was revolutionary for the time. It illustrated that Earth was not the center of the Universe, which was a bold, heliocentric proposition that would later be refined and defended by astronomers such as Copernicus and Galileo. Aristarchus's work embodies the history of astronomy's gradual shift from a qualitative to a quantitative science.

Determining the scale of space is a central challenge in astronomy. Aristarchus's attempt to measure the distance from Earth to the Moon and Sun relied on simple geometry. His method is an early example of deducing celestial distances, a pursuit that has significantly evolved.

Today, we use more precise techniques such as radar ranging, standard candles, redshift, and the parallax effect, which measures the apparent shift of an object against a distant background from different positions. Celestial distance measurement, or astrometry, allows not only to map the cosmos but also to infer other critical astronomical properties, such as the composition, mass, and luminosity of celestial bodies, enriching our understanding of the universe.