The Day

Shadow Clock When positioned correctly, the value on a sundial in DeKalb tells A. universal time. B. local sidereal time. C. local mean solar time. D. local apparent solar time E. central standard time.

Carpe Diem A local observer sees the sun rise and move from east to west across the sky. –Same for the stars at night. This is due to the spin rotation of the earth on its axis. –Locally as if the sun circled the earth –Stars apparently move at night The time when the sun crosses the local meridian is local noon.

Solar Day The apparent solar day is defined as the time between noons. The day is divided into 24 equal hours. –Not particularly precise The mean solar day is more accurate. Length of the day (1998)  February h  March h − 18.1 s  May 1424 h  June h s  July h  September h − 21.3 s  November 3 24 h  December h s

Sundial A sundial is an ancient and simple device to measure the apparent solar time. –Gnomon oriented N-S –Angle matches latitude

Universal Time Mean solar time is the same throughout the year and is subdivided. –60 minutes/hour –60 seconds/minute Noon for universal time (UT or UTC) is the hour angle of the sun plus 12 hours. Joachim Alves Gaspar

Time Zones Time zones are shifts from UT so that noon is approximately local noon. –Central Standard Time (CST): UT – 6:00

Sidereal Time The sun appears to move compared to the stars. –Earth’s orbit around the sun Sidereal time matches motion of the stars. –One extra solar day per year –4 minute shorter day Wikipedia

Official Second Astronomy is no longer used to define time. –Set to vibrations of cesium atoms in 1967 –Accuracy for GPS satellites UTC noon drifts relative to perfect atomic clocks. –Day increases 4 ms each century –Error of about 1 second each year (friction, glaciers, earthquakes. Clocks add leap seconds some years.