seasons earth’s tilt –tilt of rotational axis with respect to its orbit around sun solar...

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

TiltEarth’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 OrientationThe sun is in

the direction

of the

constellation

of Aries

on the Vernal

Equinox,

the first day

of spring.

“Bowl” DiagramEcliptic = Earth’s orbit (sun’s apparent motion)

N S

W

NCP

Zenith

Celestial Equator

Ecliptic

EQUINOXEquinoxes (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

Tropic of Capricorn

Equator

Prime Meridian

SOLSTICESummer (June 21st-ish)

Sun Located at the Tropic of Cancer

Winter (December 21st-ish)

Sun Located at the Tropic of Capricorn

Tropic of Cancer

Tropic of Capricorn

Equator

Prime Meridian

Seasons SphereThe 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 ShadowSun 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 ChartNote the path of the sun on the sky (ecliptic)

All-Sky Star ChartNote 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 MomentumL Angular Momentum: A combination of...

m Mass

v Speed of Rotation

r 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.