1. The Year

2. Changing Days The length of the day varies over time. –Sunrises at different places on the horizon –Changes in patterns of stars – Major weather changes The pattern repeats and this regular motion is the year. The pattern is due to the orbit of the earth around the sun.

3. Solstice and Equinox The length of days changes through the year. The longest and shortest days are called a solstice. –Winter (short), summer (long) When the day and night are of equal length is an equinox. –Vernal (March), autumnal (September)

4. Tilted Axis The combination of the orbit and tilted axis causes seasons.

5. Figure-8 Due to the orbit and tilt of the earth the sun is in a different spot each day. –Noon moves This pattern is called the analemma. Greenwich Observatory analemma by JPL Jack Fishburn

6. Julian Calendar A normal year is just over 365 days. –Close to 365.2425 –24 extra days per century –2 extra days per millennium In 46 BC Julius Caesar decreed a calendar. –From Egypt (Sogigula) –Leap day every four years –8 days too many every 1000 years

7. Gregorian Calendar The Julian calendar was wrong by a week after 1000 years. –Useless for planting crops The Gregorian calendar has no leap day on a century not divisible by 400 (such as 1700, 1800, 1900, 2100, 2200). Adopted in different years by different countries. Spain and Catholic Europe 1582 (skip 10 days) England 1751 (skip 11 days) Russia 1918 (skip 13 days)

8. Precession The spinning earth slowly precesses like a wobbling top. –Celestial poles shift over 26,000 years