Unit I – The Seasons

ASTR 101Prof. Dave Hanes

Revisiting the SituationEarlier, 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 willsee 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.

Varying Distances Can Matter

Some comets experience huge differences of heat and cold.

Mars also does so, butit’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 pathin June is

muchhigher in the

skythan it is inDecember.

There are alsomore hoursof 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. Thismakes ‘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=otpFNL3yem4https://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.