1621 in science

Astronomy

• Johann Schreck, also known as Johannes Schreck, Terrenz or Terrentius, introduces the telescope to China.

Botany

• The University of Oxford Botanic Garden, the oldest botanical garden in Great Britain, is founded as a physic garden by Henry Danvers, 1st Earl of Danby.

Medicine

• Robert Burton publishes his treatise The Anatomy of Melancholy.

Physics

• Willebrord Snellius formulates Snell's law on refraction.

Technology

• A simple microscope is developed.
• Cornelius Vermuyden begins reclamation of Canvey Island in England.

Births

• January 27 – Thomas Willis, English physician who contributes to knowledge of the nervous and cardiovascular systems

Deaths

• July 2 – Thomas Harriot, English ethnographer, astronomer and mathematician
• September 1 – Bahāʾ al-dīn al-ʿĀmilī, Arab philosopher and astronomer
• Jan Jesenius, Slovak physician