November 26, 2013
Return homework 4.
Open Calendar and discuss next week & rest of semester.
Review
Last time I started going over the explanation for why there are seasonal changes
on Earth. Seasonal changes in temperature happen because the geographical
distribution of energy coming from the sun changes during the year.
Review points (1) and (2) on WORD notes. Mention (3) “tilted” on its axis.
Last time I went over the solar angle at noon. The sun is directly overhead at solar
noon at one latitude each day. This location is called the solar declination. See
graph on web page (and handouts). You should understand what is on this diagram.
I also said that you need to be able to compute the solar angle at noon, given the
solar declination and your latitude.
Example. Minneapolis, MN is at 45° North latitude. Find the range in solar angle at
noon over the year.
 Minimum solar angle at noon (day of maximum solar heating) is June
Solstice. Solar declination is 23.5° North latitude.
o Solar angle at noon is (45 – 23.5) = 21.5° South of straight up
 Maximum solar angle at noon (day of minimum solar heating) is December
Solstice. Solar declination is 23.5° South latitude.
o Solar angle at noon is (45 + 23.5) = 68.5° South of straight up.
Cover Items (4) – (9) on global warming discussion … other sheets
Go over Homework 5, Global Warming Essay
Discuss homework #5
Go over the WORD document on the assignment.
Most important that you decide where you stand on issue now.
Don’t want you to give both sides of the issue, we did that in class. Clearly state
what you think and discuss reasons why you think the way that you do. But you
must address the 5 points described in the homework assignment, even if they do
not influence your opinion.
Seasons.html (Handouts)
I am now going to describe why there are seasons on Earth and why the Latitude of
a place on Earth is the biggest factor in determining the climate of that location.
First I want to make sure that you all understand latitude on Earth. Show link to
latitude picture on web. Latitude most important for seasonal changes. Point
out where tropics and polar latitudes are. Misleading areas due to map
projection. Much more surface area low latitudes than middle than high.
Seasonal changes take place because the distribution of solar heating around the
globe changes during the year, following a 365 day cycle.
The amount of solar radiation striking the Earth … (ELMO WORD notes #1)
I will first describe the solar angle effect.
Solar angle (SA) is the angle to the Sun …, (WORD #2, ELMO)
And Then show linked figure 14 (Solar angle) at same time.
You know from your own experience that this is true. When does Sun feel
stronger, at sunrise and sunset when sun is on the horizon or in middle of day when
sun higher in sky?
Let’s look at how the solar angle varies around the globe at any given time
Show linked figure 15 (Solar radiation incident on the Earth)
 perpendicular (solar angle = 0°) at only one point on sphere at a time
 rotation of Earth means everyday only one latitude will have sun
directly overhead at solar noon.
What is left to explain is why the position of this point (or latitude) changes during
the year. To understand that you have to be able to visualize the geometry of
Earth’s orbit.
Use model … As the Earth goes around the Sun, its path traces out an ellipse.
The path of the orbit defines a plane (in notes called Ecliptic plane)
This piece of all thread is the Earth’s axis of rotation … spinning around this axis
causes day and night to happen
Image you are the Sun, looking at the Earth. You see it spinning around, where
does the Sun hit perpendicularly (at the Equator) … but it does not look like this
because the axis of rotation is not perpendicular to the plane of the orbit, but is
tilted at a 23.5 angle to it … Let me point it 23.5 toward you (the Sun) … Now
where does Sun hit perpendicularly? Not on Equator, but north of the Equator at
23.5 North latitude, called the tropic of Cancer. This is June 21, the day of the
summer solstice.
Now the key thing to understand about the orbit of the Earth is that this particular
orientation does not change as it goes around the Sun … Start to walk around room
… show Spring Equinox (axis not facing toward or away from Sun) … winter
solstice (axis pointing away) and Fall Equinox.
Show linked picture of Earth’s orbit (figure 16). Make sure to make point about
where in orbit axis points toward the sun (Summer solstice) and where it points
neither toward or away (Equinoxes) and where it is away (Winter solstice).
Seasons on Earth occur because … (Use WORD noted #3)
Turns out that most people in the US, including college graduates are not able to
explain the reason for seasons. Many give the answer that seasons happen b/c the
Earth gets closer to the sun in the summertime …
Many say, without thinking that
Relate to the statement “Earth is tilted on its axis …”
Solar declination (use definition on handout)
 only one latitude each day
 changes slowly from day to day, following a 365 day cycle
 ranges from 23.5 North latitude to 23.5 South latitude during year
Examine graph on handout.
Bring up linked figure of the Solar declination and Northern Hemisphere
Seasons. Show Tropic of cancer, solstices, etc. on figure and handout.
Tropics (use definition on handout)
Go through the example on the web page that Tucson never overhead.
What is solar angle at noon for places not located at solar declination? It is
easy to compute and you will have to do this on exam.
Solar angle at noon (use bottom of 1st page of handout to explain)
 Go through Tucson, equator, and 60° N examples (handout)
 Draw corresponding diagram for Tucson on ELMO
Go through cosine stuff (handout)
I won’t ask you to do calculations with cosines, but you should realize:
!!! (Make point)!!! Seasonal (yearly) changes in solar heating at noon are
smallest at equator and get larger toward the north and south poles
Changes in length of day is the other factor that changes the amount of solar
radiation energy that hits the surface. I am going to describe that variation up
graphically.
Use graph of length of day changes from web page.
 after defining Arctic and Antarctic regions, show animation links [open
these before class].
 Point out length of latitude circle in light vs dark.
Read over points on length of day from web page.
Changes in the direction to sunrise and sunset
Before covering this last section, I want to mention one other aspect of seasonal
changes that you may of may not have noticed. This has to do with the local
direction of sunrise and sunset … It is not directly east and west, except on two
days during the year (the spring and fall equinox days) … Every other day of the
year sunrise is not directly east and sunset is not directly west.
When the Earth’s axis of rotation is such that the northern hemisphere faces the
sun, then the direction to sunrise is north of east everywhere on earth and the
direction to sunset is north of west everywhere on Earth. Show this using the
animation for summer solstice. This is day of year when sunrise is the most
north of east. But north of east all the way from Spring Equinox to Fall Equinox,
reaching its maximum north of east on Summer solstice.
Show same thing for the winter solstice … south of east. Talk about calendars!!
And use of the yearly cycle of position to sunrise and sunset.
Seasons – Popular usage
Last section on page describes the relationship between the astronomical seasons,
the solstices and equinoxes, and the popular use of terms spring, summer, fall,
winter
 Start with linked figure for Northern Hemisphere seasons
 Describe why there is a “lag”. Oceans very important. Prime hurricane
season is late summer and early fall.
 Same thing each day … lag b/t solar noon and warmest time of day (figure)
 Finish by showing my linked figure 17. Why the long delay? OCEANS.
 Go over the example at the bottom of the section.