Jespersen، James؛ Fitz-Randolph، Jane. From Sundials to Clocks: Understanding Time and Frequency(PDF). National Institute of Standards and Technology Monograph 155 (Report) (ط. 1999). U.S. Department of Commerce Technology Administration and National Institute of Standards and Technology. ص. 188–190. مؤرشف من الأصل(PDF) في 2017-08-30.
Jespersen، James؛ Fitz-Randolph، Jane. From Sundials to Clocks: Understanding Time and Frequency(PDF). National Institute of Standards and Technology Monograph 155 (Report) (ط. 1999). U.S. Department of Commerce Technology Administration and National Institute of Standards and Technology. ص. 188–190. مؤرشف من الأصل(PDF) في 2017-08-30.