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Panoramic view of the clock hall at PTB with the four caesium clocks CS1, CS2, CSF1 and CSF2.

When does the sun rise?

It is not so easy to set the alarm clock so precisely that it rings as punctually as the cock on the manure when the sun rises, for the moment of the first daylight changes from day to day. But from the rotation of the Earth and its revolution round the sun the time of the first and of the last sunbeam can be calculated. The diagram shows these times for Braunschweig, situated at 52°17' north, 10°27' east.

Between the last Sunday in March and the last Sunday in October, Central European Summer Time (CEST) is the legal time, so you have to add two hours to the times given in the graph in Universal Time Coordinated (UTC). Before and after this period, the Central European Time (CET) is valid: You have to add one hour. For a place which is situated one degree to the east (west) of Braunschweig, four minutes have to be taken away (added). In the north-south direction (along a meridian), it is not possible to simply state a numerical value, as the differences are dependent on the season. On the days of the equinoxes (first day of spring and autumn), the times of sunrise and sunset along a meridian are the same. On the first day of summer, in a place at a latitude of 54 degrees north (near Rostock), the sun rises 33 minutes earlier than in a place at the same longitude but at a latitude of 48 degrees north (the parallel which runs through Freiburg). The minimal differences from calendar year to calendar year have been neglected in our curve.

Hannah Hillmann and Björn Beckmann, students in a job-finding training measure at the PTB, have put the data published in The Astronomical Almanac into this graph.