The Earth and its Seasons:
Observations: It is colder in the winter and warmer in
the summer.
Do you think the earth is closer to the sun in the summer?
What do you know about the position and path of the sun
in the summer, compared with the winter?
The Earth and its Seasons:
In Arizona, it is colder in the winter and warmer in the
The sun is higher in the sky in the summer than in the
Sunrise and sunset points change with the seasons
And… the seasons are reversed in the southern hemisphere.
How can we explain this? We pause for a
demonstration…with globe and light source.
March 21: first day of spring, sun crosses the celestial
equator going north
June 21: first day of summer, sun as far north of the
equator as it can go
Sept 21: first day of autumn, sun crosses the celestial
equator going south
Dec 21: first day of winter, sun as far south of the
equator as it can go
Now, why does sunrise and set point change?
Project: plotting the path of the sun during one day. Volunteers?
(We will take the model outside later)
Next let’s explore the matter of the distance between
earth and sun
Lecture-tutorial, p. 93, Part 1: Earth-Sun Distance
Question 1: Is the direction that the earth’s axis points
changing over the year?
Next: Eclipses of the sun and moon
Solar eclipse: moon comes in front of the sun, for a very few
minutes, and only for a small area on earth.
Lunar eclipse: moon passes into the shadow of the
earth, slowly, over several hours, and half the earth
can see it
- sometimes the moon turns very dark, other times
its deep red
The Geometry of Eclipses
So why don’t we see an eclipse every two weeks?
Let’s Model the actual eclipse
The distance between the earth and moon is much larger than
the illustrations in most books, so eclipses happen only when
everything in lined up perfectly.
So, how often do they really occur? Let’s look at a list of
recent eclipses for an idea:
Now, we will go outside and model eclipses,
and see how to plot the path of the sun…