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 summer.

The sun is higher in the sky in the summer than in the winter

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

Review:

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…