(1)   Stargazers and Skywatchers     

That is the initial web-page (no. #1) of
"From Stargazers to Starships" home page and index: on disk Sintro.htm, on the web
          http://www.phy6.org/stargaze/Sintro.htm

• 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.

    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.)

   (Otherwise, directions on the map are different from the corresponding directions on the ground)

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 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.)

(In the east)

No.
(Except for twice a year; We will come to that!)

(In the west)

(the class should be able to answer that!)

[[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.]

It is south of us.

If we use the Sun's elevation to define noon--yes.

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.

[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 (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.]

(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.)

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.)

No!
(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.

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.)

(On or near September 22)

    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.

    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.

    In summer.. 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.

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.