Download 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.