1. The Day of the Sun's Return, The Winter Solstice
Montgomery College Planetarium
at Takoma Park/Silver Spring
By Dr. Harold Williams

2. One People … One Sky …Two Seasons

7. The spectacular Trifid nebula (Messier 20) is one of the best known in the sky. It is a striking mixture of brilliant red light emitted from excited hydrogen gas and the soft blue glow of a reflection nebula. The blue arises from starlight, scattered by dust particles between the stars. The size of the particles is minute, similar to those of smoke, which also has a bluish hue. However, the scattered light is not a pure blue, and if we see it through a medium that is yellow (i.e. absorbs blue light) some green colouration remains. This effect can be seen in some of the darker parts of the faint reflection nebula that surrounds the Trifid, where partial absorption of light by foreground dust provides a yellow 'filter', producing shades of green. This image has been photographically enhanced using an unsharp mask.

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: ×a days
northern solstice: ×a days
autumn equinox: − ×a days
southern solstice: − ×a days
Average tropical year is currently.
a in Julian years from 2000
(a in Julian years from 2000)

16. Winter Solstice Links

17. How to tell time

18. Equation of Time