Seasons on Earth
and some stuff about the Moon too
Latitude - lines parallel with the equator
Where do we expect to see coldest/warmest average temperatures on the Earth?
---> Why?
Seasons and tropical/temperate/arctic...
...are caused by the same thing!
demo: Flashlight aimed straight at board vs at angle - see area, intensity
Our view of the Moon
Sunlight spread over a larger area at an angle affects Moon brigntness to us.
Crescent Moon: relatively dim; Moon appears to us to be near the Sun in the sky.
Sketch a quarter and full moon with respect to Earth and Sun. Quarter = we see half lit)
Moon Brightness
Guess: how bright is the quarter moon relative to the full moon?
(these photos are taken with different camera settings; you can't depend on them to answer - use your experience!
You might have guessed half as bright...
You would be guessing wrong. Full moon- WAY brighter.
how much? 11 times brighter!
In fact, about 3 days before or after a full moon, the moon is 95% lit, but it is only 50% as bright from our perspective due to the angle of sunlight being reflected.
Full moon happens tonight!
Watch its brightness and the
shadow it casts tonight (or last night)
vs. the next few nights;
it will drop off suddenly
More Moon stuff
The moon rotates exactly once as it revolves; this is due to tidal forces
It has one side always facing Earth
(we actually get to see just over 1/2 of it at different times in its cycle)
The moon's days are ___ Earth days long
The Earth will eventually become tidally locked with the moon; longer days and only one side facing the moon.
Axial Tilt and Equinox
Earth’s axial tilt is oblique (not perpendicular) to its orbit significantly; 23.5 degrees.
http://solarelectricityhandbook.com/solar-angle-calculator.html
Equinox: sun appears directly over Equator
We see: everyone experiences 12h day.
Earth’s axis is perpendicular to sunlight
Happens in spring and fall (optional assign.)
Equinox
Discuss: how would the sun appear if you were standing at the N or S pole on this day? How much light would hit the ground? (equator is zero latitude, N and S pole are 90 degrees latitude.)
Solstice
Solstice: Sun appears directly over tropic of Cancer (N) or Capricorn (S)
(quiz: what latitude lines are these?)
We see: Sun appears to be highest or lowest in sky, and longest or shortest day of year.
Earth’s tilt relative to sunlight reaches a maximum.
Double - whammy: day length at a maximum or minimum.
Solstice at the poles?
Predict what you would see at the summer or winter solstice at the poles of Earth.
(discuss)
Angle of sun in sky? _____ degrees
Earth's orbit
Elliptical, yes, but only slightly.
perihelion - January
closest to sun
Aphelion - July
furthest from sun
This should produce more
extreme seasons in S.
hemisphere
They don't have this....
due to water