When the Sky Falls

Montgomery College Planetarium

at Takoma Park/Silver Spring

http://montgomerycollege.edu/Departments/planet

By Dr. Harold Williams

When the Sky Falls

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, perihelionAround January 3 or 4Sun and Earth furtherest, aphelionAround 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 eliptical.

• This elipticity 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.