Transcript Seasons
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) Zenith Ecliptic NCP Celestial Equator N S W EQUINOX Equinoxes (Equal Day and Equal Night) Spring (Vernal Equinox) March 21st-ish Fall (Autumnal Equinox) September 21st-ish Sun Located on Celestial Equator (Earth’s Equator) Tropic of Cancer Equator Tropic of Capricorn Prime Meridian SOLSTICE Summer (June 21st-ish) Sun Located at the Tropic of Cancer Winter (December 21st-ish) Sun Located at the Tropic of Capricorn Tropic of Cancer Equator Tropic of Capricorn 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 DEC RA 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 L m v r Angular Momentum: A combination of... Mass Speed of Rotation Mass 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.