The Day of the Sun's Return, The Winter Solstice

1
The Day of the Sun's Return, The Winter Solstice

• Montgomery College Planetarium
• at Takoma Park/Silver Spring
• http//montgomerycollege.edu/Departments/planet
• By Dr. Harold Williams

2
One People One Sky Two Seasons
3
(No Transcript)
4
(No Transcript)
5
(No Transcript)
6
(No Transcript)
7
(No Transcript)
8
(No Transcript)
9
(No Transcript)
10
(No Transcript)
11
(No Transcript)
12
(No Transcript)
13
Length of Seasons

• Summer 93 days 15 hours 29 minutes
• Autumn 89 days 20 hours
• Winter 88 days 23 hours 54 minutes
• Spring 92 days 18 hours 26 minutes
• Sun and Earth closest, perihelion
• Around January 3 or 4
• Sun and Earth furtherest, aphelion
• Around July 4

14
Obliquity of the ecliptic

• 23 ½ degrees, approximately, between day axis of
  rotation and year axis of revolutionary orbit
  about the sun
• Causes the seasons
• Seasons are not caused by the fact that the
  earths orbit is not a perfect circle, but its
  orbit is slightly elliptical.
• This ellipticity of the earths orbit does cause
  the seasons which are caused by the tilt,
  obliquity of the ecliptic, to be slightly unequal
  in length.

15
Seasonal Years

• Mean tropical year is 365.242 189 670 SI days
• vernal equinox 365.24237404 0.00000010338a
  days
• northern solstice 365.24162603 0.00000000650a
  days
• autumn equinox 365.24201767 - 0.00000023150a
  days
• southern solstice 365.24274049 - 0.00000012446a
  days
• Average tropical year is 365.2422 currently.
• a in Julian years from 2000

16
Winter Solstice Links

• http//en.wikipedia.org/wiki/Shortest_day

17
How to tell time

• http//en.wikipedia.org/wiki/Equation_of_time

18
Equation of Time