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Goals: The student will
- Know about the apparent motion of the Sun, Moon and planets across the sphere of "fixed" stars ("celestial sphere") and understand that the reason all these object travel along very similar paths is that the solar system is flat and disk-shaped.
- Know that the Earth orbits the Sun counterclockwise (when seen from north of the orbit), which is also the direction in which the other planets and the Moon travel around the Sun, In addition, the Earth also rotates in a counterclockwise sense around its own axis.
- Know that the ecliptic on the celestial sphere (a line) marks the path of the Sun during the year.
- Understand the causes of eclipses and the reason they can only happen when the Moon is also in the ecliptic.
Terms: Zodiac, ecliptic (the line across the celestial sphere),the plane of the ecliptic (flat surface), zodiac, solar eclipse, lunar eclipse [umbra, penumbra].
Guiding questions and additional tidbits
(Suggested answers in parentheses, brackets for comments by the teacher or "optional")
Start by asking students if any of them had seen an eclipse of the Sun or Moon, and to describe it. Ask to explain what causes it, and say that today we will study the paths of the Sun and Moon across the sky, and the reason the Moon sometimes comes in front of the Sun (eclipse of the Sun), or enters the shadow of the Earth (eclipse of the Moon)--and why such events tend to be rare.
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Some students may have a difficulty of visualizing motions in 3 dimensions: they should understand that when we look along a flat surface--say, a sheet of paper--we see a line in our field of view.
After this, present the material. The questions below may be used in the presentation, the review afterwards or both
-- How does the Sun appear to move across the sky during the day--say, in the US?
On the average the Sun rises in the east, passes to the south at noon--at which time it is also at the greatest distance above the horizon--and sets in the west.
--Does it move any differently seen from Australia or Argentina?
Yes. Viewed from countries south of the equator, the sun passes north at noon.
Teacher's comment: This was already discussed in section 1, "Stargazers and Skywatchers." Today's class, however, goes further, to discuss what causes that apparent motion.
-- If we could see the stars in the daytime, how would we see the Sun change its position among them?
The Sun would slowly move in a big circle around the celestial sphere, called "the ecliptic." It would take one year to complete its circuit.
-- Why do we see the Sun only along that line and nowhere else in the sky?
The orbit of the Earth around the Sun is in a flat plane, known as "the plane of the ecliptic." We can only see the Sun along one of the lines that lie in that plane. We will never, ever, see it in a direction perpendicular to that plane (or indeed, at any direction making a non-zero angle with it!) Demonstrate with apple and plum, or two similar fruits, on a tabletop--or with a diagram on the board.
-- What is the connection between "ecliptic" and eclipses??
An eclipse cannot happen unless the Moon is also in the plane of the ecliptic Here is why--illustrate again with 3 fruits or objects. Put the "Sun" and "Earth" on a flat table, and explain that the "Moon" moves near that surface (which is the plane of the ecliptic), but not exactly on it. In one side of its orbit it goes a bit above it [demonstrate], in the opposite side it goes below [though never very far in either direction].
Eclipses need the Earth, the Moon and the Sun in a straight line, and therefore can only happen where the two planes intersect.
Two types of eclipse exist:
(a) In an eclipse of the Sun, the Moon comes between the Earth and the Sun. We are in its shadow. Usually, only a small area on Earth is fully shaded, because the Moon appears to us only slightly bigger than the Sun (and even that not always). Only the (rather small) part of Earth seeing the Moon exactly over the position of the Sun is in full darkness. For the rest of Earth, the Moon is a bit to the side and only part of the Sun is covered.
Sun-Moon-Earth are then lined up. If the Sun and the Earth are in the plane of the ecliptic, the Moon, which is between them, also must be in the plane.
(b) In an eclipse of the Moon, the Earth comes between the Moon and the Sun. The Moon is in the shadow of the Earth. Since that shadow is big (almost the size of the Earth) and the Moon is much smaller than the Earth, often the entire Moon is darkened.
Sun-Earth-Moon must then be in a straight line, in that order. If Sun and Earth are both in the plane of the ecliptic, not only the line between them must be in that plane, but also the continuation of that line. The Moon must be somewhere on that continuation.
-- Although the planets move across the sky, they are always observed near the ecliptic. What does this tell us?
Each planet orbits the Sun somewhat like the Earth, in a plane with the Sun near the center
The fact that the planets are always seen near the ecliptic means that their planes are all near the ecliptic. The solar system is a flat disk, and we view it edge-on. Make sure to have this sentence on the board, and that students copy it.
-- The position of the Moon in the sky is always near the ecliptic. What does this tell us?
The orbital plane of the Moon is also close to the orbital plane of Earth. It shares the flatness of the solar system!
Further comment by the teacher: That plane is however tilted by a few degrees to the ecliptic. If this did not happen--if the orbit of the Moon was also exactly in the plane of the ecliptic--we would see an eclipse of the Sun and one of the Moon every month, essentially every orbit of the Moon. Because of the small angle (think of two sheets of paper intersecting by 5 degrees), when the Moon passes between us and the Sun, its position in the sky is usually a few degrees above or below
that of the Sun, and we don't get any solar eclipse. Similarly, when passing on the night side, it usually passes north or south of the Earth's shadow.
-- One evening, after sunset, we saw two bright stars and the Moon lined up diagonally, on a line which reaches the horizon just about where the Sun has set. What does this mean?
These stars are planets, and that line is close to the ecliptic.
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