AST 111 Lecture Notes - Lecture 3: Leap Year, Gregorian Calendar, Precession
Document Summary
The north star will change over time because the earth precesses. Astronomers are responsible for time and date. Time is determined by a meridian: between the north and sound. When the sun hits the meridian, it is local noon : am= ante meridian, pm= post meridian. Even though the sun works, it is not a uniform measure because: the earth"s orbit around the sun isn"t circular, the sun"s path in the sky is tilted. The earth moves faster on its orbit when it is close to the sun. Mean (average) solar time: watch time , average time the earth takes to orbit the sun, the time zones were developed for ease of commerce and conversation, arizona doesn"t honor daylight savings time. Sidereal time: time by the stars, time kept in our book, 23. 9345 hours/day. Julian calendar: every year divisible by 4 has a leap year, 11 minute/year error.
