Seasons Earth’s Tilt –Tilt of rotational axis with respect to its orbit around sun Solar Insolation –More Direct Sun Light –Longer Duration of Day Light Hours Orbit Variations –tilt –eccentricity –precession

Celestial Sphere

Tilt Earth’s rotational axis remains fixed with respect to the stars.

Astro-calisthenics Earth’s –rotation –revolution –tilt Orientation (Polaris) Diurnal Changes (Daily) Monthly Changes Yearly Changes

Celestial Orientation The sun is in the direction of the constellation of Aries on the Vernal Equinox, the first day of spring.

“Bowl” Diagram Ecliptic = Earth’s orbit (sun’s apparent motion) N S W NCP Zenith Celestial Equator Ecliptic

EQUINOX Equinoxes (Equal Day and Equal Night) Spring (Vernal Equinox) March 21 st -ish Fall (Autumnal Equinox) September 21 st -ish Sun Located on Celestial Equator (Earth’s Equator) Tropic of Cancer Tropic of Capricorn Equator Prime Meridian

SOLSTICE Summer (June 21 st -ish) Sun Located at the Tropic of Cancer Winter (December 21 st -ish) Sun Located at the Tropic of Capricorn Tropic of Cancer Tropic of Capricorn Equator Prime Meridian

Seasons Sphere The tilt of the earth on its rotational axis when compared to its orbit about the sun, differs by 23.5 degrees.

Winter Solstice

Local Noon Shadow

December Solar Insolation

Summer Solstice

Local Noon Shadow

June Solar Insolation

Equinox Shadow Sun rises due East, Sun sets due West

Combined Annual Average Solar Insolation

Average Day Light Hours

Summer and Winter For a given Hemisphere –Summer Sun shines more directly and more intensely Sun shines for many more hours on average –Winter Sun shines more obliquely and much less intensely Sun shines for fewer hours on average

All-Sky Star Chart Note the path of the sun on the sky (ecliptic)

All-Sky Star Chart Note the path of the sun on the sky (ecliptic) Celestial Equator and Vernal Equinox RA DEC

Earth’s Orbit Counter Clockwise (When viewed from “above”?) –Above = toward Polaris, North Pole, N. Hemisphere –Rotation (Earth’s spin around axis) –Revolution (Earth’s orbit around Sun)

Angular Momentum LAngular Momentum: A combination of... mMass vSpeed of Rotation rMass Location (with respect to rotational axis) Conservation Examples: Spins of Dancers or Ice Skaters Those Funky Coin Vortexes in Stores Tops and Gyroscopes Riding a Bicycle

Precession 1 complete cycle takes 26,000 years

Precession The “North” Star Changes Identity

Egyptians There is an alignment between the entrance to the Great Pyramid and Thuban, the closest star to the rotational axis of the earth in 4420 B.C. Betelguese, which marked the Vernal Equinox is also aligned with the southern shaft in the King’s chamber. And others…

Orbital Variations Over Millennia Perihelion Point Tilt Angle Precession

Summary Earth’s pole and equator are tilted 23.5 degrees away from the plane of its orbit around the sun. This tilt is the key factor in determining seasons via the concentration and duration of sunlight. Other factors can influence seasons but are smaller effects acting over long periods of time (i.e. precession) Earth’s precession is caused by conservation of angular momentum.