Goals: The student will|
- Recognize the daily motion of the Sun across the sky, defining the main directions of east, west, south and north
- Recognize (without elaboration) that this daily motion is also what makes a sundial function.
- Recognize (without elaboration) that the locations of sunrise and sunset (for viewers north of the equator) migrate southward in the winter and northward in the summer, changing the length of the day and causing summers to be warmer, winters colder.
- Learn about the elevation of the Sun above the horizon, another factor in making summers warm and winters cold.
- Learn how the first calendars were based on changes in the Sun's noontime elevation, and on locations of sunrise and sunset.
Terms learned: East, west, south, north, horizon, equinox, winter solstice, summer solstice, elevation of the Sun )above the horizon).
Stories and extras: Psalm 19 (A poetic impression of the sky and the Sun's motion)
Guiding questions and additional tidbits
(Suggested answers in parentheses, brackets for comments by the teacher or "optional")
Start the class with a discussion of the main directions: North, south, east and west.
Anyone can tell what a map is?
(A map is a drawing of the way the land is shaped--rivers, mountains, cities, roads. Pictures from a high-flying airplane or from satellites look like maps, except maps outline features such as roads, railroads, towns, parks and so forth, using color to help show them.)
When you use a map on a hike or a drive, you must always place it so that its directions are the same as those of the land around you.
(Otherwise, directions on the map are different from the corresponding directions on the ground)
How to you match directions? (North is always on top of the map).
Today we talk about North, and related directions. Where is north in this room?
Ask around until everyone agrees. (A small compass may help resolve the issue; The magnetic compass is also mentioned later.)
So where is south? Where is east? West? Are these 4 directions always at right angles?
(Yes. If you face north, east is on your right, west on your left, south is behind you. Have a student stand in front and demonstrate this as you talk.)
After this, present the rest of the material. The quoted part of psalm 19 may be read by someone at a proper moment The questions below may be used in the presentation, the review afterwards or both
The 4 main directions are associated with the positions of the Sun in the sky. Two of the positions are associated with the horizon. Who can tell what is the horizon?
The horizon is the boundary between the sky and the flat shape land--ignore buildings and nearby trees, etc. You see it best from a ship on the ocean, where the dividing line between sky and water is very well defined.)
Where does the Sun usually rise?
Exactly in the east, every day?
(Except for twice a year; We will come to that!)
Where does the Sun usually set?
Exactly in the east, every day?
(the class should be able to answer that!)
As the Sun rises higher in the sky, then angle between its direction and the direction to the point on the horizon closest to it [drawing may help] is called its elevation angle.
Noon is the time when the elevation of the Sun is highest, halfway between sunrise and sunset. Afte noon, the elevation of the Sun again decreases, and it becomes zero when it meets the horizon at sunset.
[[That was the original definition of noon. Today we adjust the clock for time zones, and shift it by one hour each summer, so the clock does not exactly tell "real noon," just some time near it.]
What is the direction to the Sun at noon, when its elevation is highest (in the USA)?
Exactly south of us?
If we use the Sun's elevation to define noon--yes.
You watch the shadow of a flagpole on a sunny day. As the Sun moves, the shadow is always in the opposite direction. Where does it point at sunrise?
The sun rises in the east, so the shadow points west.
The sun sets in the west, so the shadow points east.
The noon sun is in the south, so the shadow points north.
How does the shadow rotate--clockwise or counter-clockwise?
[Have a student at the blackboard draw a cross with the 4 directions, labeled N (top), E (right), W (left), S (bottom).
The shadow rotates from W (sunrise), to N to E (sunset). Which direction is that?
[This is how a sundial functions, too. Some people believe that when clocks were face built, the direction in which the hands rotate was chosen to be clockwise, because that was the sense in which the shadow rotated on sundials.]
South of the equator, say in Argentina or Australia--the Sun is to the north at noon.
[If time allows, repeat the above exercise to show that there, too, the shadow rotates clockwise, even though at noon it now points south.]
(Near the equator, it may pass to the south or to the north, depending on season, even though on any day at noon, it will be quite close to overhead. That's why the climate there is so hot! All these things may be studied again later in more detail.)
When is the shadow of a flagpole shortest?
At noon, when the Sun's elevation is highest. (May need a blackboard drawing, of the shadow when the elevation is high and when it is low.)
People use a magnetic compass to find where north is. Is that the same definition we use?
(It is almost an accident that Earth has a magnetic region in its center (spacecraft tell us the planet Venus does not have one) and that its magnetic poles are very near the poles on the axis around which the Earth turns.
Sailors who use the compass for navigating on the ocean know there exists a small difference in direction between "magnetic north" and the "true north" which they need. Special maps give the difference, which must be taken into account.
Seasons of the year
Twice a year the Sun rises (almost) exactly in the East and sets (almost) exactly in the west. On these days day and night are both equally long--12 hours--so they are called equinox dates.
One marks the beginning of spring and is therefore called "spring equinox" or among scientists "vernal equinox" (from the latin word for spring).
Anyone knows when it is?
(Usually on or near March 21. It may shift a day or two as the Earth's orbit is modified by the Moon and by planets.)
The other marks the beginning of fall and is therefore called "fall equinox" or among scientists "autumnal equinox".
Anyone knows when it is?
(On or near September 22)
Imagine the "apparent path" of the Sun around us, in the sky. (Actually it's the Earth that rotates, but that is how it appears to us.) At equinox, exactly half the path is above the horizon--see the image on the web page, or copy it in part on the blackboard.
You can imagine the other half of the path below the horizon, the path of the Sun during the night. At equinox, both halves are equal, so day and night are equal, too.
In winter the Sun rises further in the south, and sets further south too. It makes a shorter hop across the sky, lasting less than 12 hours. Nights are longer than days, one reason (there is another) why it gets so cold
In summer the Sun rises further north, and sets further north too.
Anyone cares to continue?
It makes a longer hop across the sky, lasting longer than 12 hours. Nights are shorter than days, one reason why it gets so warm in summer.
In the fall, days get shorter and shorter until around December 21, the Winter solstice. the longest night of the year. That is traditionally taken as the start of winter.
Then nights start getting shorter and days longer. They are equal around March 21, and finally, the shortest night is on the summer solstice, around June 21. That is traditionally the start of summer.
The other reason for cold winters and hot summers is the elevation of the Sun. In winter, the Sun makes short hops across the southern sky and never gets far from the horizon. Its rays therefore fall very diagonally, and spread out over a larger area. That reduces their ability to heat the ground.
Anyone cares to continue?
In summer, the Sun makes "big hops" across the sky and gets quite far from the horizon. Its rays therefore fall very steeply, and do not spread out the way they do in winter. That increases their ability to heat the ground.
Suppose you are a priest in ancient Babylon, trying to create a calendar. How will you do it?
Watch where the Sun rises, day after day. Mark a position on the horizon, and say that on the following days, the Sun rises further and further south. This means we are approaching winter.
Count the days until sunrise again occurs at that point, with the Sun again moving towards winter. The number of days you have counted give one year.