AST 111 Lecture Notes - Lecture 3: Leap Year, Gregorian Calendar, Precession
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AST 111: Lect 3: Knowing the heavens
-The north star will change over time because the earth precesses
-Astronomy and Time/Date
- Astronomers are responsible for time and date
- Time is determined by a meridian
oBetween the north and sound
- When the sun hits the meridian, it is “local noon”
oAM= ante meridian
oPM= post meridian
- Even though the sun works, it is not a uniform measure because:
oThe earth’s orbit around the sun isn’t circular
oThe 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
o“watch time”
oaverage time the earth takes to orbit the sun
oThe time zones were developed for ease of commerce and conversation
oArizona doesn’t honor daylight savings time
- Sidereal Time
oTime by the stars
oTime kept in our book
o23.9345 hours/day
Calendars
- Julian Calendar
oEvery year divisible by 4 has a leap year
o11 minute/year error
- Gregorian Calendar
oEnd of the century years must be divisible by 400
oOnly one day for every 3300 tropical years of error