Science
ademola
Nov2_earth_moon_sun1.pptxThe Earth, Moon and Sun
Reason for the seaons
Phases of the Moon
Earth’s orbit: A common misperception…
The motion of the Earth in its orbit around the Sun is an ellipse, not a circle. That means that at some points in the orbit the Earth is closer to the Sun than at others.
But….
Circle Ellipse
Some people think the earth is closer to the sun in July than in January
TOTALLY NOT TRUE!!!!
Earth’s Revolution…
The difference in distance is only about 3.5%...not enough to account for the seasonal changes! And we’re closest to the sun in January!!
Note: 1 AU is defined as the average of these two numbers
From my demo
Earth’s curved surface
Earth’s Curved surface
This tells you why its cold in Alaska and the North Pole and hot at the equator
But not why winter is in January and summer July
Angle of the sun
Why does it change throughout the year?
Why is the sun high in the sky in summer? (more direct energy)
Why is the sun low in the winter (more indirect)
Another question: Earth’s Rotation…
Earth’s rotation on its axis generates periods of daylight and night.
But…. We know length of day and night
changes throughout the year. Why?
Part of Answer: Tilt of axis
The amount of daylight/nighttime and
angle of the sun in winter and summer are both determined by the amount of Earth’s tilt.
The Earth’s axis is tilted 23.5o from the plane of the Earth’s orbit around the Sun.
Self test: Do you know what the Earth’s axis is? What latitudes does it
connect?
Answer:
Axis is the line around which the Earth rotates.
It connects 90N and 90S, i.e. the NP and SP,
passing through the center of the earth.
Northern Summer: NP tilted toward sun
Southern hemisphere seasons are reverse from Northern Hemisphere. Summer
for Australians begins around Christmas.
SP tilted away from sun
Polar night = 6 months
Polar day= 6 months
Earth’s axis tilt: Northern Summer
Most direct rays at 23.4oN latitude
Axis points to same place in space*, but Earth moves around the sun
* Where does the axis point to……….. The North Celestial Pole (Polaris, in Ursa Minor)
Earth’s Orbit around the sun…
So what causes the daily and seasonal changes?
Earth’s curvature and its tilt !
Earth’s Revolution
Since Earth’s axis always tilts in the
same direction, at a given point on
the Earth the rays change in
directness and duration as it moves
around the sun.
Earth’s Revolution
Most direct in NH
Most direct in SH
Length of Day
When your hemisphere is tilted toward the sun, your daylight is longer
24 hour day light
24 hour day night
Terminology
Why is it called “Equinox” (for spring and autumn)?
Sun is most direct on equator at that time
Equinox
Equator
Equal
Equate
All the same root meaning. “same” “even”. Night and
Day are always the same (12 hours long) on the equator.
And at equinox night and day are the same everywhere
Solstice: the beginning of summer and winter
Consequences of orbit and tilt
1. Northern summer is southern winter
Christmas is summer vacation in Argentina
2. There are two magic latitudes where the sun is overhead on
summer solstice.
23.5N (tropic of cancer): June 22
23.5S (tropic of capricorn): Dec 22
3. Where is the sun overhead on spring and autumn equinox?
Good videos to watch
Earth Tilt 1: Reasons for the Seasons
https ://www.youtube.com/watch?v=Pgq0LThW7QA
Earth Tilt 2: Land of the Midnight Sun
https://www.youtube.com/watch?v=eUsWUiVCq5U
Good demo’s but incomplete in one important aspect. All they talk about is
the tilt of the axis. In reality, its curvature of the earth’s surface and its
orbit around the sun which combine with the tilt.
Actually… axis does change tilt!
2.4 degrees, every 41,000 years. One major reason for ice ages!
The term is obliquity. Oblique means “high angle”
Example with Mars (same concept)
Warmer summer mean
more polar ice
can melt
Cooler summers mean
less polar ice
can melt
Same idea
And the more ice melts away in summer
Obliquity Cycle
Currently, obliquity is 23.44. About in the middle. It is decreasing- meaning
high latitude summers have been getting cooler for the last few
thousand years.
Original predictions were that we were headed for a new glacial
maximum (in a few thousand years), but now, man made effects
(increased greenhouse effect) have delayed or prevented this.
One subtle effect of obliquity changes- it can introduce some leeway in
defining the habitable zone.
A planet with high obliquity can remain habitable a bit further from
the star/sun than a planet with low obliqiuty
Lots of other small motions
Think of earth as a top: it can wobble as it spins
Also the orbit will change sometimes being more circular and
sometimes being more like an oval
Why does the Earth’s tilt change so little?
Mars really goes through a change this big:
Enormous changes in climate. If Earth did this, at low obliquity,
without summer weather north of 30-40 latitude, N. America and Europe would be
barren: ice covered or grasslands.
Our overall seasons remain fairly stable compared with a planet that has big changes.
Thank our weird moon
Its huge….. Compared to earth. If orbiting on its own might be a full
fledged terrestrial planet. Its gravity stabilizes our axis and prevents it
from tilting over. Or tilting straight up.
Other planets moons
Mars’ moons
are barely
big chunks of
rocks 5-10 miles across!
Jupiter and Saturn’s moons are big
Earth and moon are almost “twin planets”
But much much tinier than their parent planets
Galilean satellites from backyard telescope
Full moon
(overexposed)
Even from your backyard, you can see they’re tiny compared with Jupiter
Formation of moon: giant impact hypothesis 4.5 billion years ago
This explains why the moon’s composition resembles earth’s mantle. Also
may have given us the 23o axis tilt to begin with- knocked Earth over.
Earth gets melted, debris coalesces and starts orbiting earth
Moon’s orbit around the earth
This diagram is looking down at the Earth’s
North Pole and the moon’s orbit
What is period of moon’s orbit? 27-29.5 days
(origin of the word “month”, i.e. one “moon” cycle)
Good video explanation
https://www.youtube.com/watch?v=AQ5vty8f9Xc
Its called “Moon Phases: Crash Course Astronomy #4” (listed on the bottom)
Lunar Phases and Eclipses
To be continued Wednesday………… phases of moon, plus effects of
Moon’s gravity on Earth
