(1) Stargazers and Skywatchers
Part of a high school course on astronomy, Newtonian mechanics and spaceflight
by David P. Stern
This lesson plan supplements the material on the web page|
"Stargazers and Skywatchers," on disk Ssky.htm, on the web
Goals: The student will|
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?
How to you match directions? (North is always on top of the map).
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?
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?
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?
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.
What is the direction to the Sun at noon, when its elevation is highest (in the USA)?
Exactly south of us?
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?
How does the shadow rotate--clockwise or counter-clockwise?
The shadow rotates from W (sunrise), to N (at noon, when the Sun passes south), 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 passes to the north at noon.
[If time allows, repeat the above exercise to show that there the shadow rotates counter-clockwise. The shadow now rotates from W (sunrise), to S (at noon, when the Sun passes north), to E (sunset).]
(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?
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).
The other marks the beginning of fall and is therefore called "fall equinox" or among scientists "autumnal equinox".
Anyone knows when it is?
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.
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.
In summer.. Anyone cares to continue?
Suppose you are a priest in ancient Babylon, trying to create a calendar. How will you do it?
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.
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Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: stargaze("at" symbol)phy6.org .
Last updated: 26 August 2004