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

One People … One Sky …Two Seasons http://www.astronomerswithoutborders.org/images/stories/SwB/SwB4_print.jpg

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.

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

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.

Seasonal Years Mean tropical year is 365.242 189 670 SI days vernal equinox: 365.24237404 + 0.00000010338×a days northern solstice: 365.24162603 + 0.00000000650×a days autumn equinox: 365.24201767 − 0.00000023150×a days southern solstice: 365.24274049 − 0.00000012446×a days Average tropical year is 365.2422 currently. a in Julian years from 2000 http://en.wikipedia.org/wiki/Tropical_year (a in Julian years from 2000)

Winter Solstice Links http://en.wikipedia.org/wiki/Shortest_day

How to tell time http://en.wikipedia.org/wiki/Equation_of_time

Equation of Time