Download The Year

The Year
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.
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)
Tilted Axis
• The combination of the orbit and tilted axis causes seasons.
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.
Jack Fishburn
Greenwich
Observatory
analemma by JPL
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
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)
Precession
• The spinning earth slowly precesses like a wobbling top.
– Celestial poles shift over 26,000 years