4.54 billion years4.60 billion years

First life3.8 billion-year-old

First hominids 6-7 mya

Age of universe: 13.7 bya

Decoding Evidence Activity• 4 stations• Group of 2• Provide a brief

description of each evidence

• Anticipation:Collectively, what conclusion can you deduce about Earth from these evidence?

How do we know the Earth is round?

https://www.ted.com/talks/how_simple_ideas_lead_to_scientific_discoveries (1:13 to 4:06)

Earth’s axis of rotation

https://www.youtube.com/watch?v=9n04SEzuvXo

viewed from North pole Earth rotates

counter-clockwise

viewed from South pole

Earth rotates clockwise

Is 23.5o tilted

SUN

• Earth rotates eastward toward the Sun

How rotation differs from revolution?Rotation Revolution

The turning of an object around an imaginary axis running through itE.g. Earth’s rotation takes 24 hrs

The rotation of an object around an external axis (e.g. another object)E.g. Earth’s revolution around the Sun is 365.24 daysLeap year anyone?

When it is summer in the northern hemisphere, it is winter in the southern hemisphere. Why?

Watch: MIT video https://www.youtube.com/watch?v=Pgq0LThW7QA

Reason for seasons

• Answer: because of Earth’s tilt

• In summer months: Northern hemisphere toward the Sun

• In winter months: Northern hemisphere away from Sun

• The opposite happens to Southern hemisphere

Earth’s orbit around the sun is elliptical (oval or egg-shaped)• Our winter = Earth closest to sun• Our summer = Earth furthest from the

sun

Please read Activity 7-2 p 285 for tomorrowBring your textbook

Hands-on exploration: Activity 7-2 p 285Examining the effect of Earth’s tilt on the differential amount of sunlight received on Earth’s surfaces

Answer question 1-4 for HW

Extra slides if time permit

Why Earth keeps on spinning?

• Has to do with the way it was formed

• Formed from spinning gas and dust

• Inertia makes our planet keep on spinning unless there is a force to stop it

Axis of rotation

https://www.youtube.com/watch?v=TQxeutcYP6I

A tour beneath our feet

• https://www.youtube.com/watch?v=NAHY6965o08

Solstices• Solstice occurs when the

Sun shine directly on one of the two tropics because the tilt of the Earth’s axis reaches its maximum angle compared to the Sun

• During the June Solstice the rays of the Sun shine directly on the Tropic of Cancer; longest day

Occur on June 21 and Dec 21

• During the December Solstice the Sun’s rays shine on the Tropic of Capricorn; longest night

Equinox• As the Earth moves

around its orbit, it reaches two points during the year where the tilt of its axis causes it to be straight relative to the Sun.

• Thus, the Sun will shine directly on the equator

• On Mar 20 and Sep 22