(3)   Seasons of the Year     

"From Stargazers to Starships" home page and index: on disk Sintro.htm, on the web
          http://www.phy6.org/stargaze/Sintro.htm

• Know how the inclination of the Earth's rotation axis--the 23.5 degree angle between it and the perpendicular to the ecliptic--causes seasons of the year, by varying the length of the local day and the angle at which the Sun's rays arrive on the surface of the Earth.

• Recognize that seasons in the southern hemisphere (at middle latitudes) occur at opposite parts of the year from seasons in the US and Europe.

• Realize that the seasons in the midle-latitudes in US and Europe are quite different from seasons experienced near the equator.

• Know about the polar day, when the Sun never sets but just marches around the horizon, and the polar night, when it never rises (though one may get "white nights" of twilight, when the Sun is just below the horizon).

Terms: Inclination of the Earth's axis, polar night, polar day, solstice, equinox--also summer solstice (on or near June 21), winter solstice (on or near December 21), spring (or vernal) equinox (on or near March 21) and fall (or autumnal) equinox (on or near September 22).

Stories and extras: The oceans moderate and delay summer and winter. Earth is closest to the Sun in early January.

  And by the way: Shakespeare wrote a delightful play "Midsummer Night's Dream." When is "midsummer night"? What is so special about it?

(It is the shortest night of the year, about June 21. In Baltic and Nordic countries this is often a time for celebration.)

    After this, present the material. The questions below may be used in the presentation, the review afterwards or in both

-- If the rotation axis of the Earth were perpendicular to the ecliptic, would day and night be equally long?

Yes

i = 23.5 degrees

The northern half of Earth leans towards the Sun when the northern hemisphere has summer, away from the Sun when it has winter.

Because when the North Pole leans towards the Sun (producing summer north of the equator), the south pole automatically leans away from it (producing winter south of the equator)--and vice versa [demonstrate with a globe, a ball or with fruit]

In northern summer, the pole is always in sunlight, 24 hours in a day. The same holds for points near the North Pole for an appreciable part of summer. This area defines the Arctic Circle.

It seems to be moving around the horizon.

[The North Pole is not a good choice for a science base, because it is in mid-ocean, covered with floating sea ice. However, the South Pole is located on the thick Antarctic ice cap and in fact a large scientific base is located there. In the middle of southern summer (e.g. December), that is a good place for observing the Sun, and the science station does in fact include a solar observatory. One reason for it is the stability of the cold atmosphere (the image does not jitter), but another is that in mid-summer it can observe the Sun continuously for weeks at a time!]

It does not appear, it never rises. The location is in continuing darkness--the "polar night."

In or near the pole, during mid-summer, the days are 24 hours long. In mid-winter, they are zero hours long. At lower latitudes, summer days are longer while winter days are shorter.

Solstices are the times when the Earth axis leans directly in the direction to the Sun--one pole leans sunward, the other leans away from the Sun. North of the equator, midsummer day is around June 21. Midwinter day falls around December 21. South of the equator it is the other way around. Dates may vary by a day or so).

The shortest day is the day of the winter solstice December 21, in the US. The shortest night is the day of the summer solstice (June 21).

Equinoxes are the times in the Earth's orbit when the lean of the Earth's axis is neither towards the Sun nor away from it, but exactly sideways [demonstrate with a globe, a ball or an apple]. The axis of the Earth is then perpendicular to the direction of the Sun.

Show that the dividing line between light (=day) and darkness (=night) then runs from pole to pole (and back to the first pole).

The equinoxes occur around March 21 ("spring equinox" or "vernal equinox") and September 22 ("fall equinox" or "autumnal equinox.").

Night ("nox") is about equal in length to the day.

The oceans are slow to heat up and cool resulting in a moderating influence over temperature variations.

The Earth is closest to the Sun between January 3 and 5. The difference is a few percent, too small to be seen.

(Don't bring up the following question--it is too distracting--unless some student asks it. If you count the days from the fall equinox to the spring equinox, the (northern) winter half of the year, you find its one day longer than half the year, while the summer half is one day shorter. Why?

    The answer is discussed in section #12a, and has to do with the fact that the orbit of the Earth around the Sun is not a circle but an ellipse, it is slightly elongated. In winter we are closer to the Sun, in the "smaller half" of the ellipse, and also, Earth then moves a little faster. Both effects make the "winter half" of the year (northern winter) slightly shorter.)

    (Some students may make guesses)

Let us calculate. The Sun is about 23500 Earth radii away from Earth. So....?

(Students should realize from the drawing on the board, that the difference in the distance of the northern and southern hemisphere is about 1 Earth radius or less: much smaller than the difference in the Sun's distance due to the ellipticity of the Earth's orbit.

So that ellipticity should have a bigger effect on the heat we receive. Yet it clearly isn't the cause of the seasons; if it was, the northern and southern hemispheres would be heated or chilled at the same time!)