On this day in 1598 - April 17th - the Italian astronomer Giovanni Riccioli was born.
Riccioli's best known for his Almagestum Novum published in 1651. It included his Moon map and over 1500 folio pages densely packed with text, tables and illustrations. The work became a standard technical reference book for astronomers all over Europe and included the names of prominent lunar features still in current use.
On this day in 2014 - April 18th - NASA's Lunar Atmosphere and Dust Environment Explorer (LADEE) was intentionally crashed into Moon at end of its mission.
LADEE, orbiting the Moon's equator, studied the lunar exosphere and dust in the Moon's vicinity. It was crashed on the far side of the Moon to ensure that it didn't damage historically important landing sites on the near side.
On this day in 1858 - April 23rd - Max Planck was born.
Planck was a German theoretical physicist best known as the originator of quantum theory. His work won him the Nobel Prize in Physics in 1918. The European Space Agency (ESA) named a space observatory for him - it mapped the cosmic microwave background from 2009-2013.
On this day in 1920 - April 26th - the Shapley-Curtis debate took place on the nature and distance of spiral nebulae.
Even less than a century ago, we didn't know what the "spiral nebulae" were. Were they objects in our Milky Way galaxy and therefore comparatively close? Or were they "island universes", i.e., other galaxies, and therefore at a great distance?
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