====== 1. Introduction and questions ======

  - Why do seasons change? Why the seasons are opposite in the northern and southern hemispheres?
  - What is the universe? What is the meaning of the word 'universe'?
  - How fast does earth rotate and revolve? Does the solar system move as well?
  - Why don't we feel that the earth is moving?
  - What is physics? What is the relation between astronomy and physics?
  - What is science? What is the difference between science (astronomy) and pseudoscience (astrology)?

===== - Seasons =====
Why do seasons change? Why the seasons are opposite in the northern and southern hemispheres?

Seasons are so important for us culturally that we seldom think about the reason behind seasons. The change from Summer to Winter is so striking that the best poets have always used it as a metaphor for life. For example Shakespeare:

<poem>
"For never-resting Time leads summer on
To hideous winter, and confounds him there," -- Sonnet 5
কারণ অবিশ্রাম সময় গ্রীষ্মকে তাড়ায়ে
কুৎসিত শীতের কারাগারে করে রাখে বন্দি;

"Then let not winter’s ragged hand deface
In thee thy summer ere thou be distilled:" -- Sonnet 6
শীতের রুক্ষ হাত না ভাঙুক তোমার গ্রীষ্ম
যতক্ষণ না তৈরি হয় তোমার নির্যাস:
</poem>

And painters are no exception as you see below.

[{{:courses:phy100:four_seasons_1208__a_b_c_d__copy.jpg?nolink&700|//Four Seasons// (winter, spring, summer, autumn), Yehouda Chaki, Canada.}}]

But why do seasons change?

Earth **rotates** around its axis once in 24 hours and **revolves** around the sun once in approximately 365 days.

The revolution is one of the reasons behind the change of seasons, but not because earth's orbit is elliptical. For all practical purposes, earth revolves around the sun in an almost circular orbit. So earth's distance from the sun (and the amount of light earth receives) does not change much in a year.

The reason behind change of seasons is:
  - The tilt of the axis of rotation of earth.
  - The revolution of earth around the sun.

{{https://upload.wikimedia.org/wikipedia/commons/thumb/8/8b/North_season.jpg/1024px-North_season.jpg?nolink}}

This image shows four important positions of earth in a year. The purple line is the axis of rotation. The seasons are opposite in the northern and southern hemispheres.

During northern winter (yellow font, around Dec 21) and southern summer (white font), the northern hemisphere is less exposed to the sun compared to the south.

During northern summer (yellow font, around June 21) and southern winter (white font), the northern hemisphere is more exposed to the sun compared to the south.

The north pole has an almost 6-months long night during the northern winter and vice versa.

During northern spring (around Mar 20) and southern autumn, the two hemispheres are almost equally exposed to the sun.

The days of Dec 21 and Jun 21 are called **solstices** while Mar 20 and Sep 22 are called the **equinoxes**.

==== - Light falling at an angle ====
But what do we mean by **more** and **less** exposed? It is all about the the angle at which sunlight falls. If light falls directly (at an angle of 90 degrees), the temperature is higher causing summer. If light falls at an angle much less than 90 degrees, temperature is lower causing winter.

Use the animation below to get a handle of this.

https://astro.unl.edu/naap/motion1/animations/seasons_ecliptic.html

{{ :courses:phy100:screencast_from_02-21-2023_05_19_04_pm.webm?nolink&700 |}}

I have selected a location in the northern hemisphere by dragging the person (upper right panel). Now, when I mode the earth (on the left panel) you can see the angle at which light falls on that person on the lower right panel. You can see that the light falls at a higher angle when the earth is on left side of the sun and vice versa.
===== - Universe =====
What is the universe? What is the meaning of the word ‘universe’?

The word 'universe' comes from Latin 'uni' (one) + 'versus' (to turn). We see a lot of different and disconnected things around us, but when we imagine all these things to be part of a single 'object', that single object is called the 'universe'. Because we have turned a lot of things into **one** thing.

In English 'verse' means 'poem' and indeed there is a connection with poem as well. The modern meaning of 'verse' came from the same sense of 'turning'. Originally Latin 'versus' was used for agriculture to describe the turning of soil. Poets turns (changes) the meaning of words in field of language as farmers turn soils in the field.

The word 'university' is also derived from 'universe' and it has the same sense of 'turning into one'. In universities all knowledge are turned into a single understanding of human existence. And also in universities people from many different families are turned into one people containing one **national** identity.
===== - Motion =====
How fast does earth revolve?

Earth revolves around the sun at a speed of 30 kilometers per second.

Compare that with the speed of a bus (60 kilometers per **hour**).

More interesting comparison would be with a rocket. Typical speed of a rocket is 12 kilometers per second.

So earth revolves faster than a rocket.


Browse the solar system here: https://solarsystem.nasa.gov (we will use this animation throughout the course).
===== - Emotion =====
Why don't we feel that the earth is moving?

We do not feel because earth revolves (30 km per second) and rotates (once in 24 hours) at constant speed. The **acceleration** is very small.

For the same reason inside an elevator, you feel something only when the elevator starts and stops, but you do not feel the motion in the middle. This is because the elevator has some acceleration (change of speed) only when it starts from rest or comes to rest. The elevator runs at constant speed in the middle and so you do not have any emotion of motion.

The same could be said about an aeroplane; you feel the motion more only during take-off and landing, but nothing much during a steady flight.

To be sure, an elevator or aeroplane still gives you emotions of motion because of friction with the surroundings which is not the case for earth.
===== - Physics =====
What is physics? What is the relation between astronomy and physics?

Physics is the most fundamental physical science. The physical reality is made of space, time, matter and energy. Physics deals with the position and motion of matter and energy within the framework of space and time. Its subject matters spans from the very smallest (quarks, electrons, atoms) to the very largest (quasars, )
===== - Science =====
What is science? What is the difference between science (astronomy) and pseudoscience (astrology)?

"Science is the **practice** that provides us with the most **reliable** (most warranted) statements that can be made, at the time being, on subject matter covered by the **community** of knowledge disciplines." -- Sven Ove Hansson.

{{ :courses:phy100:foursciences.png?nolink |}}

<poem>
"Not from the stars do I my judgement pluck,
And yet methinks I have astronomy,
But not to tell of good or evil luck,
Of plagues, of dearths, or seasons’ quality;

Nor can I fortune to brief minutes tell,
Pointing to each his thunder, rain, and wind,
Or say with princes if it shall go well
By oft predict that I in heaven find.

But from thine eyes my knowledge I derive,
And, constant stars, in them I read such art
As truth and beauty shall together thrive
If from thyself to store thou wouldst convert:

Or else of thee this I prognosticate,
Thy end is truth’s and beauty’s doom and date." -- Sonnet 14, Shakespeare
</poem>