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• Born in Italy.
• He did studies of comets and meteor streams.
• He discovered the asteroid Hesperia.
• He drew maps of Mars, the best of his day.
• He revised the names of features on Mars. He found long, straight lines on the planet, and called them canali.

Bernhard Schmidt (1879-1935)

• Born in Estonia.
• He made mirrors and lenses for telescopes.
• He devised a means to photograph wide areas of the sky with a single plate, with the definition sharp across the whoe of the field.

Johann Schröter (1745-1816)

• Born in Germany.
• He made many drawings of the Moon, and made measurements of the heights of its mountains.
• He made maps of Mars and Venus.
• He recorded the faint luminosity on the night side of Venus.
• He and members of his committee discovered the first four asteroids.

Karl Schwartzschild (1873-1916)

• Born in Germany.
• He was the first to discuss black holes, and presented theories about them.

Pietro Angelo Secchi (1818-1878)

• Born in Italy.
• He originated the classification of stars by spectrum analysis.

Harlow Shapley (1885-1972)

• Born in USA.
• He showed that Cepheid variables are pulsating stars, not eclipsing binaries.
• He calculated the diameter of our galaxy.
• He discovered that globulars are not spread uniformly across the sky and are most common in the south.
• He identified two dwarf galaxies in Sculptor and Fornax.

Vesto Melvin Slipher (1875-1969)

• Born in USA.
• He measured the rotation periods of the planets.
• He measured the speeds of spiral nebulae, discovering that they must lay beyond the Milky Way.
• He and his assistants located Pluto.

Friedrich Georg Wilhelm Struwe (1793-1864)

• Born in Germany.
• He founded the study of double stars.
• He published a catalogue of over 3,000 binary stars.
• He carried out one of the first determinations of stellar distance.