Unit I – The Seasons
ASTR 101
Prof. Dave Hanes

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. times farther away!)
(The next nearest star is about 400,000 www.maniacworld.com
The Sun gives us light and warmth, and supports life.

As we know already, the Earth once every 24 hours.
spins (rotates) on its axis
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

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

Does the Earth itself
Sun? (Wouldn’t we really move, orbiting around a static 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:

The Earth is sometimes closer, sometimes farther away, in its orbit around the Sun.

The orientation of the Earth in its orbit matters somehow.

Some comets experience huge differences of heat and cold.
Mars also does so, but it’s not so extreme.

- 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?

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 relatively low daily ‘arc’ across the southern skies. During our
Summer, it is a the sky.
Northern
Southern star, and follows a star, and climbs much higher into
FROM ONE DAY TO THE NEXT, however, it appears to changes accumulate.
drift a little in its north-south position. Over the course of time, the

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.

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

Linear Momentum (Briefly)
&
Angular Momentum
(Important Here!)

The fullback’s linear momentum is given by his mass x velocity . (The equation doesn’t matter.)

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

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
Suppose material moves inward, towards the axis of rotation. This makes ‘r’ smaller. To conserve angular momentum, ‘v’ has to increase!
www.daviddarling.info

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

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

As the Earth orbits the Sun, its spin axis stays pointing in the same direction (towards Polaris!).
It does not ‘flop about’. Stability!

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 days have a constant duration down or speed up dramatically. because the Earth cannot spontaneously slow
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.