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(2) The Path of the Sun, the Eclipticand the apparent motions of the Sun, Moon and planets across the sky. |
Part of a high school course on astronomy, Newtonian mechanics and spaceflight
by David P. Stern
This lesson plan supplements: "The Path of the Sun, the Ecliptic," section #2: on disk Secliptc.htm, on the web http://www.phy6.org/stargaze/Secliptc.htm
"From Stargazers to Starships" home page and index: on disk Sintro.htm, on the web |
Goals: The student will
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.
-- How does the Sun appear to move across the sky during the day--say, in the US?
-- If we could see the stars in the daytime, how would we see the Sun change its position among them?
-- Why do we see the Sun only along that line and nowhere else in the sky?
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??
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:
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.
-- Although the planets move across the sky, they are always observed near the ecliptic. What does this tell us?
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?
-- 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?
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Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: stargaze("at" symbol)phy6.org .
Last updated: 28 August 2004