De webpagina's van Erik Springelkamp
Panorama Loquard
Panorama Ems
Panorama Klein Maarslag
Panorama Englum
http://en.wikipedia.org/wiki/Jacobus_Cornelius_Kapteyn
http://en.wikipedia.org/wiki/Zernike
http://en.wikipedia.org/wiki/Gerardus_%27t_Hooft
http://en.wikipedia.org/wiki/Jan_Oort
http://en.wikipedia.org/wiki/Willem_de_Sitter