Eratosthenes
Compared the Sun's angle at noon in Syene and Alexandria and worked out Earth's circumference to within a few per cent.
Why it matters: First good measurement of our planet — with a stick and geometry.
📄 Described by Cleomedes, On the Circular Motions of the Celestial Bodies.
Aryabhata
In the Aryabhatiya he proposed that Earth rotates on its axis and explained eclipses by the shadows of Earth and Moon.
Why it matters: A landmark of Indian astronomy and mathematics.
📄 Aryabhata. Aryabhatiya, 499 CE.
Nicolaus Copernicus
Placed the Sun, not Earth, at the centre of the planets' orbits.
Why it matters: The Copernican revolution.
📄 Copernicus N. De revolutionibus orbium coelestium. Nuremberg, 1543.
Johannes Kepler
Using Tycho Brahe's precise observations: orbits are ellipses; planets sweep equal areas in equal times; period² ∝ distance³.
Why it matters: Exact rules for how planets move.
📄 Kepler J. Astronomia Nova (1609); Harmonices Mundi (1619).
Galileo Galilei
Saw Jupiter's four big moons, the phases of Venus, mountains on the Moon and countless stars in the Milky Way.
Why it matters: Evidence that not everything circles the Earth.
📄 Galilei G. Sidereus Nuncius. Venice, 1610.
Isaac Newton
The same force that drops an apple holds the Moon and planets in orbit; it explained Kepler's laws.
Why it matters: One law for Earth and sky.
📄 Newton I. Philosophiæ Naturalis Principia Mathematica. London, 1687.
William Herschel (with Caroline Herschel)
The first planet found with a telescope. Caroline Herschel went on to discover eight comets.
Why it matters: Doubled the known size of the Solar System.
📄 Herschel W. Phil. Trans. R. Soc. 71:492 (1781).
Friedrich Bessel
Measured the parallax of 61 Cygni: ≈ 10 light-years away.
Why it matters: Showed how vast the distances to stars are.
📄 Bessel FW. Astronomische Nachrichten 16 (1838).
Urbain Le Verrier & Johann Galle
Le Verrier calculated where an unseen planet must be from Uranus's wobbles; Galle found it within 1° of the prediction.
Why it matters: A triumph of Newton's gravity.
📄 Galle JG. Monthly Notices RAS 7:153 (1846).
Henrietta Swan Leavitt
Found that Cepheid variable stars' brightness and pulsation period are linked — a way to measure huge distances.
Why it matters: The key that unlocked the size of the Universe.
📄 Leavitt HS, Pickering EC. Harvard College Observatory Circular 173 (1912).
Cecilia Payne
Her PhD thesis showed the Sun and stars are made mostly of hydrogen and helium.
Why it matters: Changed our picture of what the Universe is made of.
📄 Payne CH. Stellar Atmospheres. Harvard, 1925.
Edwin Hubble (after Lemaître, 1927)
Galaxies are moving away from us, faster the farther they are. (In 1924 he had shown Andromeda is a separate galaxy.)
Why it matters: Evidence for the Big Bang.
📄 Hubble E. PNAS 15:168 (1929); Lemaître G. Ann. Soc. Sci. Bruxelles 47A:49 (1927).
Clyde Tombaugh
Found by comparing photographic plates at Lowell Observatory.
Why it matters: First Kuiper Belt object found (reclassified as a dwarf planet in 2006).
📄 Lowell Observatory announcement, 13 March 1930.
Hans Bethe
Worked out the nuclear fusion reactions that power the Sun and stars.
Why it matters: Nobel Prize 1967.
📄 Bethe HA. Phys. Rev. 55:434 (1939).
Soviet Union
The first artificial satellite.
Why it matters: Began the space age.
📄 Launched 4 October 1957.
Arno Penzias & Robert Wilson
Found the cosmic microwave background — faint radiation from every direction, left over from the hot early Universe.
Why it matters: Strong evidence for the Big Bang (Nobel 1978).
📄 Penzias AA, Wilson RW. Astrophys. J. 142:419 (1965).
Jocelyn Bell Burnell (with Antony Hewish)
Spotted regular radio pulses every 1.34 s — a spinning neutron star.
Why it matters: Proved neutron stars exist.
📄 Hewish A, Bell SJ et al. Nature 217:709 (1968).
NASA Apollo 11
Neil Armstrong and Buzz Aldrin walked on the Moon on 20–21 July 1969; 12 people walked there by 1972.
Why it matters: Brought back 382 kg of Moon rock in all the Apollo missions.
📄 NASA Apollo 11 Mission Report (1969).
NASA
Toured Jupiter, Saturn, Uranus and Neptune; both have now left the heliosphere for interstellar space (2012, 2018).
Why it matters: Most of what we first knew about the outer planets.
📄 Stone EC et al. Science 341:150 (2013).
NASA/ESA
Above the blurring atmosphere, it measured the Universe's expansion rate and age (≈ 13.8 billion years).
Why it matters: Over 1.5 million observations.
📄 Launched 24 April 1990.
Michel Mayor & Didier Queloz
Found 51 Pegasi b, a ‘hot Jupiter’, from the star's tiny wobble.
Why it matters: Opened the study of exoplanets (Nobel 2019). Over 6,000 are now confirmed.
📄 Mayor M, Queloz D. Nature 378:355 (1995).
Perlmutter; Schmidt & Riess
Distant supernovae showed the expansion is speeding up — driven by mysterious ‘dark energy’.
Why it matters: Nobel 2011.
📄 Riess AG et al. Astron. J. 116:1009 (1998); Perlmutter S et al. ApJ 517:565 (1999).
LIGO & Virgo
Detected ripples in space-time from two black holes merging 1.3 billion light-years away — predicted by Einstein in 1916.
Why it matters: A new way to observe the Universe (Nobel 2017).
📄 Abbott BP et al. Phys. Rev. Lett. 116:061102 (2016).
Event Horizon Telescope
A shadow ringed by glowing gas around M87*, 6.5 billion times the Sun's mass; Sagittarius A* in our galaxy followed in 2022.
Why it matters: Saw a black hole's shadow directly.
📄 EHT Collaboration. ApJL 875:L1 (2019).
NASA/ESA/CSA
A 6.5 m infrared telescope 1.5 million km from Earth; sees galaxies from a few hundred million years after the Big Bang.
Why it matters: Finds surprisingly bright early galaxies and studies exoplanet atmospheres.
📄 Launched 25 December 2021; first images July 2022.
ISRO
India became the fourth country to soft-land on the Moon and the first near its south pole (23 Aug 2023). Aditya-L1 began watching the Sun in 2024.
Why it matters: Opens the water-ice-rich polar regions to exploration.
📄 ISRO mission reports, 2023.