How Earth Moves, Why are there Seasons, Calendars

This is a great video summarizing, how the earth, moves, why we have the seasons, why we have the Julian and Gregorian calendars and why we lost 11 days in 1752.

• Earth spins - The earth spins counterclockwise.  A way to remember  this is to think of the US as an animal running with its head leading.
• Meridians - Earth is split into lines
• Longitude (vertical)
• Latitude (horizontal)
• Tropic cancer(top) -due to tile of the earth.
•  most northerly circle of latitude on earth at which the sun can be directly overhead (ie it turns )
• Occurs on June solstice (when northern hemisphere is tilted towards the sun to its maximum effect).
• Tropic of capricorn(bottom)
• Most southerly circle of latitude on earth at which the sun can be directly overhead
• Sun is highest to you (your noon ) when it is on your median (all shadows point to either north or south pole)
• Subsolar point
• This is when the sun is directly overhead so there are no shadows -shadows fall straight down so they disappear
• In Hawaii this is called Lahaina noon
• It occurs twice year,
• Local apparent solar noon: This is when the sun is overhead on your meridian
• Sidereal day (pertaining to the stars): It takes 23.9 hours for the earth (eg a meridian to spin) relative to a far away star.
•  (eg This is live driving a car and looking at a far away mountain, which doesn't seem to  move)
• This is a day relative to a far away stars
• Solar day :Earth moves counterclockwise around sun - more than sidereal day is required for a meridian to point back at  the sun.
• Solar day varies.
• Solargraph - picture of sun's path across the sky (normal distribution tall to shallow)
• Should be straight line at noon local apparent solar noon but is 8 (top small, bottom long)
• So your clock appears slow and fast relative to this
• Equation of time - compare clock time to solar time
• Why is there difference between our clocks and solar time? 
• Earth on an eliptical orbit around the sun so speed changes - close and faster in January so a solar day is longer (turn more) compared to further away
• Earth is tilted so the subsolar point goes  up and down
• Sep 18 solar day - shortest 1min less
• Dec 22 solar day- longest 1min more
• Tilt causes seasons
• More heat energy laid down per area.
• Also tilted half spends more time receiving sunlight
• Solar year - The solar year  365.25 solar days 
• Julian Calendar-
• This calendar added a leap day every 4 years (Feb 29)
• Introduced in 46 BC by Julius Ceaser
• Leap days move calendar too far. Its not  365.25 but 365.2421 and it fluctuates year to year
• so in 1582 - The calendar was 10 days behind the season compared to where they used to be.
• Gregorian   calendar 

• Created by Pope Gregory the 13th.
• Catholic church wanted Easter to occur exactly when it occurred centuries ago.
• This makes 3 fewer leap days every 400 years. There is a leap day added every 4 years leap except if divisible by 100 unless if it also evenly divisible by 400
• In 1582 the calender went from oct 4 to 15 for countries who listened to the Pope  (i.e. they lost 10 days)
• England and her colonies (including the US) followed in  Sep 1752. They went from the 2nd to the 14th (ie lost 3-13) so they lost 11 days. They also changed the 1st day of the year from March to Jan 1st.
• This calendar still causes a difference every 3216 years.
• However the oceans, internal  liquids and celestial bodies are all affecting the Earth's movement.
• Speeds
• Earth spins 1670 km/hour around its axis.
• Earth spins sun 108k km/hour around sun.
• Solar system moves 70k km/h in direction of vega
• Solar system is part of galaxy milky way. It is at a  60% tilt and moving clockwise at 792k km/hour
• Galaxy moving through universe at 2.1million km/hour towards the great attractor