Download Unit I – The Seasons

Unit I – The Seasons
ASTR 101
Prof. Dave Hanes
Revisiting the Situation
Earlier, we visualized the Earth sitting alone in the void, with
remote stars in all directions. But the Earth is actually very close
to one star: the Sun. (The next nearest star is about 400,000
times farther away!)
www.maniacworld.com
The Sun gives us light and warmth, and supports life.
Two Motions: Rotation First
As we know already, the Earth spins (rotates) on its axis
once every 24 hours.
This is why we see the sun rise and set once a day, and
why the pattern of stars moves across the sky at night.
Watch this (sped-up!) computer simulation:
http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/E-Spin.mp4
Revolution Next
The entire Earth moves through space, orbiting
(revolving around) the Sun – spinning on its axis
as it does so.
A complete orbit takes one year.
Watch this computer simulation of the movement:
http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/E-Move.mp4
The Zodiac
Does the Earth itself really move, orbiting around a static
Sun? (Wouldn’t we feel that motion?) Or does the Sun
move around the static Earth, as the ancient Greeks
thought? We will return to this question later.
Whichever is correct, the
result is the same! We will
see different constellations
of the Zodiac at different
times of year.
There’s Another Yearly
Cycle to Consider
There are seasonal changes.
Why? There are two obvious
possibilities:
n 
n 
The Earth is sometimes closer, sometimes farther away,
in its orbit around the Sun.
The orientation of the Earth in its orbit matters somehow.
Varying Distances Can Matter
Some comets experience huge differences
of heat and cold.
Mars also does so, but
it’s not so extreme.
But Consider Australian Winter..
- it coincides with Canadian summer!
In fact, we are closest to the sun in early January,
during the depths of our Northern winter! This perihelion
point is about 3% closer than our average distance.
Consider Instead the Effects of Inclination
A lowered sun (or flashlight!) spreads its light over
a larger area, with reduced heating effects
Hence the cool Arctic regions!
-- and the cool of the evening as
the Sun goes down in the West.
So: The Second (Correct) Possibility
Noon, Dec 21
Noon, Jun 21
The seasonal heating and cooling arises because the sun is lower
in the sky in the winter, higher in the sky in the summer.
(Here, ‘Kingston’ images from simulation software.) But why?
The Critical Point
ON ANY GIVEN DAY, the Sun is just like any other star, except
that it is extraordinarily bright. It lights up the whole sky.
During our Winter, it acts like a Southern star, and follows a
relatively low daily ‘arc’ across the southern skies. During our
Summer, it is a Northern star, and climbs much higher into
the sky.
FROM ONE DAY TO THE NEXT, however, it appears to drift a
little in its north-south position. Over the course of time, the
changes accumulate.
Why? The ‘Tip’ of the Earth’s Axis
In Kingston
The sun’s path
in June is much
higher in the sky
than it is in
December.
There are also
more hours
of daylight.
Some Special Dates
June 21: the Sun ends its steady climb northward, will start
moving southward again. This is the summer ‘solstice’ [= ‘sun
stopped’]. Maximum daylight hours.
Dec 21: the Sun ends its steady drift southward, will start
moving northward again. Winter ‘solstice,’ minimum daylight
hours.
March 21 and Sept 21: Sun is directly overhead at the Equator.
Everyone on Earth sees 12 hours of daylight, 12 hours of night.
These are the spring and autumnal ‘equinoxes’ [= ‘equal night’].
Two Special Locations
An Extreme ‘Tip’
Two More Conservation Laws
Linear Momentum (Briefly)
&
Angular Momentum
(Important Here!)
Linear Momentum
The fullback’s linear momentum is given by his
mass x velocity. (The equation doesn’t matter.)
Linear Momentum is Conserved!
www.colourbox.com
Where Does
It Come From?
Newton’s Cradle
- conserving energy and linear momentum
http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/NCrad.mp4
More Important (for now!)
Angular Momentum
Likewise a measure of ‘momentum’ – the
product of mass x velocity
But also includes an element of turning or
rotation
Angular Momentum for a Rotating Object
(the equation doesn’t matter!)
Angular Momentum is Conserved
Suppose material moves inward,
towards the axis of rotation. This
makes ‘r’ smaller. To conserve
angular momentum, ‘v’ has to
increase!
www.daviddarling.info
Look at Me
from Above
Redistributing weights
http://www.youtube.com/watch?v=kKkMt-HURZU&feature=youtu.be
Changing the direction
of spin
http://www.youtube.com/watch?v=usffSVUKCUg&feature=youtu.be
The Importance: This Yields Stability
Spinning objects tend to maintain their orientation!
http://www.youtube.com/watch?v=qBu80LT-hO4&feature=em
Other Examples
Spinning bullets (from ‘rifled’ barrels):
https://www.youtube.com/watch?v=otpFNL3yem4
https://www.youtube.com/watch?v=QfDoQwIAaXg
Spiralling footballs (to maintain streamlined orientation):
https://www.youtube.com/watch?v=6xO9oAcKhBk
Hence the Reliable Seasons
As the Earth orbits the Sun, its spin axis stays
pointing in the same direction (towards Polaris!).
It does not ‘flop about’. Stability!
This Does Not Last Forever!
External Forces Do Matter
The axis of the Earth points close to Polaris,
and will do so for many centuries
But it does slowly change (‘precess’)
because of the tiny gravitational tugs of
Jupiter and other objects.
The Long-Term Result:
A Different North Star!
Other Consequences
The days have a constant duration because
the Earth cannot spontaneously slow
down or speed up dramatically.
The orbit of the Earth around the Sun is
likewise stable. The observed motion of
the sun and the length of the year are
reliable and predictable.