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On this day in 1906 – April 28th – Bart Bok was born.

Bartholomeus Jan "Bart" Bok was a Dutch-born American astronomer, teacher, and lecturer. He's best known for his work on the structure and evolution of the Milky Way galaxy, but his name lives on in the dark nebulae known as Bok globules.

On this day in 2013 – April 29th – the Herschel Space Observatory's mission ended.

The HSO was launched by the European Space Agency to observe the Universe in infrared. It was named for William (who discovered infrared) and Caroline Herschel. The mission lasted longer than expected, but ended finally when it ran out of coolant.

On this day in 1961 – May 5th – the Freedom 7 suborbital flight took place.

Alan Shepherd became the first American in space.

On this day in 2016 – May 9th – the planet Mercury transited the Sun.

The next transit will occur on November 11, 2019.

On this day in 1900 – May 10th – Cecilia Payne-Gaposchkin was born.

Cecilia Payne-Goposchkin was an English-born astrophysicist. Applying laws of atomic physics to the study of the temperature and density of stellar bodies, she was the first person to conclude that hydrogen and helium are the two most common elements in the Universe.

On this day in 1916 – May 11th – Einstein's The Foundations of the General Theory of Relativity was published.

The General Theory of Relativity changed the way we understand the Universe, turning upside down old ideas of physics and cosmology.

On this day in 1930 – May 12th – the Adler Planetarium opened in Chicago.

The Adler Planetarium was the first planetarium in the western hemisphere.

On this day in 1973 - May 14 - Skylab was launched.

Skylab was the first American space station.

On this day in 2009 - May 14 - both Planck and Herschel were launched.

Both space observatories were operated by the European Space Agency (ESA). Planck mapped the cosmic microwave background (CMB) with great precision. Herschel was the largest infrared telescope ever sent into space.

On this day in 1857 - May 15 - Williamina Fleming was born.

Williamina Fleming was one of the computers of Harvard College Observatory. She discovered several hundred variable stars, supervised other computers, and prepared material for publication. She was a member of the Astronomical and Astrophysical Society of America and an honorary fellow of the Royal Astronomical Society in Britain.

On this day in 1969 – May 18 – Apollo 10 was launched.

Apollo 10 was the dress rehearsal for the Apollo 11 Moon mission. The crew carried out all of the procedures that Apollo 11 was expected to do, except for actually landing on the Moon. On May 22nd, they took the lunar module down to about 50,000 feet from the lunar surface.