Highlighting 10 major astronomical milestones of the past century in celebration of the Royal Greenwich Observatory’s 350th anniversary

Founded in 1675 by King Charles II, the Royal Observatory Greenwich was established to advance navigation and improve maritime safety during Britain’s age of exploration. Its astronomers produced some of the most accurate star charts and timekeeping methods of the era, laying the groundwork for determining longitude at sea—a breakthrough that revolutionized global trade and exploration. The observatory later became the reference point for the Prime Meridian (0° longitude), anchoring the system of global time zones we rely on today. As both a historic site and a symbol of scientific progress, the Royal Observatory continues to inspire curiosity about the universe.

Dreaming of the stars is a pursuit as old as humanity itself. From ancient sailors who navigated by the night sky to today’s citizens guided by space-based satellites, the quest to understand the cosmos has shaped human history. Modern space exploration would not be possible without the foundational discoveries of astronomy. In celebration of the Royal Greenwich Observatory’s 350th anniversary, we reflect on 10 of the most significant astronomical milestones of the past century.

1. The Great Debate

In 1924, Harlow Shapley and Heber Curtis engaged in “The Great Debate” regarding whether the Milky Way was the “center of the universe.” Harlow Shapley advocated that the Milky Way was larger than initially presumed and encompassed more “island universes” or nebulae, whereas Heber Curtis argued the “island universes” reached beyond our Milky Way and were separate galaxies.

Although the 1924 “Great Debate” ended inconclusively, Edwin Hubble took to the stars and precisely measured the distance of the Andromeda Nebula using Cepheid variables and a new powerful 100-inch telescope located at the Mount Wilson Observatory. In 1925, Edwin Hubble’s measurements conclusively ended the “Great Debate,” showing that the Milky Way was but one of many galaxies.

2. An Expanding Universe

Edwin Hubble’s efforts did not end with the discovery of multiple galaxies. His work continued, and in 1929, he proved to the world that the universe is expanding by measuring the shifts of red light in distant galaxies.

His work became known as Hubble’s Law, which states that the farthest galaxies are moving away faster than the closest ones. In 2018, Hubble’s Law was renamed to Hubble-Lemaître law to recognize the contributions of Georges Lemaître, who theorized on the expansion of the universe in the 1920s.

3. Pluto’s Saga

Clyde Tombaugh discovered the ninth planet, Pluto, in our solar system in 1930. However, in the early 2000s, astronomers began surveying the far reaches of our solar system for any other planet-sized objects. They found four. These four objects led to a heated scientific debate of “what is a planet?” with the International Astronomical Union ruling that an object has to orbit the Sun, be sufficiently large with a spherical shape, and clear its region of space. Pluto and the four recently discovered objects did not meet all three criteria, leading to Pluto being demoted to a dwarf planet in 2006.

4. A New Way to Study

In 1933, while troubleshooting static on his transatlantic voice communications, Karl Jankey realized static shifted across the sky. This realization led him to consult an astronomer who linked the static to a cloud of cosmic dust, indicating that some objects in space produce radio waves. This led to the creation of radio astronomy and a new method for exploring the cosmos.

5. Launch of the Space Age

Due to a predicted increase in solar activity during the years 1957-1958, the International Council of Scientific Unions passed a resolution to establish the International Geophysical Year, encouraging scientists around the world to build artificial satellites with a goal to map the Earth’s surface. The Soviet Union answered the call first with the launch of Sputnik I on October 4, 1957, with the United States following on January 31, 1958, with its Explorer I satellite.

6. The Big Bang’s Wallpaper

In 1965, Arno Penzias and Robert Wilson detected a signal that they could not attribute to anything in the sky, yet the signal appeared to emanate from everywhere. This signal, later called the Cosmic Microwave Background radiation, is theorized to be evidence supporting the Big Bang Theory.

The Big Bang Theory proposes that the universe began with a powerful hot phase, which took hundreds of thousands of years to cool. This initial explosion created an imprint on everything that still exists, almost like wallpaper, which can now only be detected at microwave frequencies.

7. Birth of Global Navigation Satellite System (GNSS)

Since the launch of Sputnik I in 1957, scientists have wondered if satellites could be used for navigation. Researchers at Johns Hopkins Applied Physics Lab initially tested this hypothesis by using the Doppler effect of the radio signal emitted by Sputnik I to track its orbit.

In 1964, the US Navy navigated its subs by tracking its own satellites. However, this solution was limited and only a precursor to what is today known as GNSS, which uses a satellite constellation for navigation across the globe. There are several GNSS systems, including the European Space Agency’s Galileo, the United States’ Global Positioning System, and the Russian GLONASS.

8. Satellites Push Beyond Earth

Prior to the 1980s, satellite development focused on understanding Earth-related phenomena, e.g., weather and geology, or celestial bodies close to Earth, such as the moon, with little interest in looking at the edges of space.

This changed with the launch of the Hubble Space Telescope in 1990 and again with the James Webb Space Telescope launched in 2021. These satellite-based telescopes revolutionized astronomy and created more opportunities for detailed observational analysis of distant stars, exoplanets, and provided insights into the evolution of galaxies.

9. Exoplanets

Given the vast size of the universe, Astronomers believed other planets beyond our solar system must exist, but the confirmation of the existence of exoplanets did not come until 1992.

Two astronomers, Aleksander Wolszczan and Dale Frail, identified a planet near a pulsar they were analyzing. The pulsar appeared to pause its pulses in regular intervals, leading the astronomers to conclude that two planets were orbiting around the pulsar. Once this breakthrough occurred, the discovery of exoplanets exploded, with over 5800 exoplanets catalogued to date.

10. First Images of a Black Hole

In 2019, the Event Horizon Telescope initiative, an international collaboration linking radio telescopes across the globe, captured the first-ever images of a black hole in the galaxy M87. These images provide direct visual evidence of a black hole and its shadow and confirm theoretical predictions.

The Royal Greenwich Observatory Inspires Future Generations of Astronomers

In the past 50 years, the Royal Greenwich Observatory’s focus changed from research to public education and preservation of historical instruments. This change supports the development of the next generation of astronomers and scientists. As technology advances and new probes reach the ever-expanding universe, astronomy requires new minds and innovation. The Royal Greenwich Observatory provides a great environment for education and engagement, offering the planetarium, museum, and the historical significance of the prime meridian.

About the Author: Christina Balocki

Christina Balocki holds a Master of International Public Policy from Johns Hopkins SAIS and a Master of Science in International Relations with a focus on National Security Affairs from Troy University. She currently serves as an officer in the Air National Guard. The views expressed in this article are those of the author and do not necessarily reflect the official policy or position of the Air National Guard, the Air Force, the Department of Defense, or the US Government.

Image: Shutterstock

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