Seasons
on
Earth
Butler Middle School
8th Grade Science
Bob, Paul, Ms Adams, Oct 25, 2012
The Observed Data: 4 Seasons
Autumn or Fall
Winter
Spring
Summer
We can explain the 4 seasons using Models
of the Earth and Sun
Scientific Model
• Representation of an object for study and
understanding (very common in Science)
•
Models can be physical and/or conceptual
- Model leg in 7th grade life science
- Computer model of a large weather
system
Spherical Earth Model
Equator
Earth rotates on
N-S axis once
per day
Earth-Sun Model
(evolved over 100’s maybe 1000‘s of years)
Earth revolves around the sun once per year + a
little bit
Earth’s Orbital Plane
Points to Polaris
(North Star)
to Polaris
orbital plane
7
perpendicular
to orbital plane
Rotate and Revolve
• Rotation: spinning like a top
- Earth rotates on its axis once per 24hr
- Causes day & night
• Revolution: 1 object moving around a 2nd object
- Earth revolves around sun once per year:
365 days + 1/4 day
• Leap Year
- Every 4 years February has an extra day
• 4 years = (365 + 365 + 365 + 366) = 1461 days
Comparing Diameters
Earth Sun
Sun ≈ 109 x Earth
Sun illuminates Earth
The Sun is a huge ball of fire
- sends energy (light & heat) to the earth
- 1/2 of earth is always illuminated - day
- 1/2 of earth is always in the dark - night
Energy distribution from Sun
S
slanted hit
large area
(cooler)
U
N
direct hit
small area
(warmer)
Energy distribution from Sun
Understanding the Seasons
• It’s all in the tilt
• Summer:
- Earth is tilted toward the sun, we get
a more direct hit, so it’s warmer
• Winter:
•
- Earth is tilted away from the sun, we get
a slanted hit, so it’s cooler
No tilt means 1 Season!
Understanding Summer and Winter
Summer in the North
☺
S
☺
night
Winter in the North
day
☹
☹
U
N
South
South
From summer to winter earth
revolves half-way around Sun
Special Days
Vernal Equinox
March 20
Winter
Solstice
Dec 21
Summer
Solstice
June 21
Autumnal Equinox
Sept 22
Earth Sun Model
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