The Reason for the Seasons

! What causes seasons? Is the Earth closest to the Sun in (northern) summer? When does summer occur south of the equator? Why are days longer in the summer?

! How do the ecliptic and the celestial equator define the Earth’s axial tilt? How many degrees is the Earth’s tilt?

! What are the solstices and equinoxes? Their dates? The total hours of daylight on those dates?

! Can you draw a diagram to find the Sun’s angle from zenith as it crosses your meridian on the solstices and the equinoxes at any latitude?

! What latitudes define the tropics and the arctic? Why? ! Note the [xtra] beside lectures 1-7 on the schedule.

These may be helpful to learn hard concepts

Learning Objectives

What Causes the Seasons?The Earth is closer to the Sun in the summer? Right?

! No. No. NoNoNo. NO.! The Earth’s orbit is not

perfectly circular, but it is actually closest to the Sun in January

! Also, summer in the Earth’s Northern Hemisphere is winter in the Earth’s Southern Hemisphere and vice versa

What Causes the Seasons?So what does cause the seasons?

! It’s the tilt of the Earth’s rotation axis! Note, the direction of the tilt does not

change over the year

Sun's Daily Paths! In the summer, the Sun is north of the celestial

equator. At northern latitudes (e.g., in Laramie):! Days are longer

than 12 hours! The Sun is high

in the sky! In the winter, the

Sun is south of the celestial equator! Days are shorter than 12 hours! The Sun is low in the sky

Approximate Solstices & Equinoxes in the Earth’s Northern Hemisphere! Winter Solstice – December 21

!night longer than day in the Northern Hemisphere! Vernal (Spring) Equinox – March 21

!12 hours of daylight, 12 hours of night! Summer Solstice – June 21

!day longer than night in the Northern Hemisphere! Autumnal (Fall) Equinox – September 21

!12 hours of daylight, 12 hours of night

Approximate Solstices & Equinoxes in the Earth’s Southern Hemisphere! Winter Solstice – June 21

!night longer than day in the Southern Hemisphere! Vernal (Spring) Equinox – September 21

!12 hours of daylight, 12 hours of night! Summer Solstice – December 21

!day longer than night in the Southern Hemisphere! Autumnal (Fall) Equinox – March 21

!12 hours of daylight, 12 hours of night

Poll Everywhere: Thoughts! The tilt of the Earth to its orbital plane (i.e. the

angle between the celestial equator and the ecliptic) is ~23.5o (this is formally called Earth’s axial tilt)

! The celestial equator is at 0o declination! So, the Sun moves from +23.5o declination

(23.5oN, at the time of the Summer Solstice in the Northern Hemisphere) to -23.5o (23.5oS, the Winter Solstice in the Northern Hemisphere)

! Laramie is near +40o (40oN) geographic latitude, so around +40o declination is at zenith

Important bad drawing Latitude, seasons, how high does theSun get as it crosses your meridian

Practice QuestionThe Earth is tilted ~23.5o to the ecliptic, so the Sun moves from +23.5o to -23.5o

declination. Laramie is near 40o latitude. On June 21, in Laramie, when the Sun crosses the meridian (is at its highest point), how far is the Sun from zenith?

16.5o

23.5o

40o

63.5o

The Tropics! Over the year, the Sun varies from 23.5º north

to 23.5º south of the celestial equator! If you live between 23.5ºN and 23.5ºS latitude,

the Sun can reach zenith – directly overhead! the Sun never reaches zenith in Laramie at 40ºN

! The latitudes between 23.5ºN and 23.5ºS latitude are called the tropics!23.5ºN – Tropic of Cancer!23.5ºS – Tropic of Capricorn

The Arctic and Antarctic! North of 66.5ºN you can see 23.5ºN above

the horizon by looking north! Below 66.5ºS you can see 23.5ºS by looking south!

! At these latitudes the Sun sometimesnever sets!

! North of 66.5ºN !The Arctic

! South of 66.5ºS!The Antarctic Midnight Sun in Barrow, Alaska

Tropics and Arctics

Equator

Tropic of Cancer

Tropic of Capricorn

Arctic Circle

Antarctic Circle

Next Time

Eclipses and the Lunar Cycle