Download The Year and The Seasons

The Year and The Seasons
Night is a curious child, wandering
Between earth and sky, creeping
In windows and doors, daubing
The entire neighborhood
With purple paint.
- Frank Marshall Davis
Announcements
•  Remember that there is going to be a in-class
quiz this coming Thursday (you need #2 pencil):
•  Check additional infos at:
www.astro.umass.edu/~calzetti/astro100
•  Homework # 1 will start on Tuesday, Sept. 20th,
and will be due on Thursday Sept. 29th.
•  Log-in into your SPARK account
•  Select the ASTRON100 course
•  Click on the OWL Powerlink
•  You will now be into OWL, where the Homeworks will
be posted
Goals for Today
•  To start understanding the cause of
seasons.
•  If time permits, start discussing how we
keep time
Assigned reading
•  Units 6 and 7
One clarification: when viewed from the North Pole, the Earth rotates
counterclockwise on its axis and orbits counterclockwise around the Sun. It takes
1 year for the Sun to cross all constellations in the zodiac.
Earth orbits around Sun
Earth rotates
The Zodiac and the Horoscope
What does the date of your birth, the location of
the constellations in the sky, and the relative
location of the planets have to do with what s
going to happen to you today, tomorrow, or
next week?
1) Nothing.
2) This can only be determined by
paying a trained astrologer to
interpret the sky for me.
3) I read my horoscope today in the
newspaper, it s gotta be true!
The Zodiac and the Horoscope
What does the date of your birth, the location of
the constellations in the sky, and the relative
location of the planets have to do with what s
going to happen to you today, tomorrow, or
next week?
1) Nothing.
2) This can only be determined by
paying a trained astrologer to
interpret the sky for me.
3) I read my horoscope today in the
newspaper, it s gotta be true!
The Zodiac and the Horoscope
The doctor who helped your Mom in
giving birth to you has more
gravitational influence on you than any
planet, star, or constellation.
So, if you want to know your future….
…. Ask for the name of that doctor!
What causes the seasons?
What are the differences between days in
summer and winter?
Longer days
stronger Sun
in summer than in winter
We also know that the seasons change as the Earth orbits around
the Sun in the course of a year.
Are the seasons due to the
changing distance from the sun?
(Not to scale)
Distance doesn't matter here!
Seasons are due to the tilt of the earth.
Your Horizon
S
E
N
The Sun s path
during one day.
The Sun s location
and path six months
later.
Summer Solstice Light
Winter Solstice Light
The sun rises in the northeast
and sets in the northwest!
When is the sunrise due east?
The sun rises in the southeast and
sets in the southwest!
The sun is above our horizon longer in summer than in winter.
Survey Question
It is warmer, on average, in FL than in MA because:
1) the sun is higher in the sky, on
average, throughout the year
in Florida
2) the sun is higher in the sky, on
average, throughout the year
in MA
3) the Sun is lower in the sky in Jan.
in Florida
4) Earth s orbit puts Florida closer
to the Sun, on average
Survey Question
Suppose you are in Australia in August. Which
of the following is true:
1) It s winter and the Sun rises in
the Northeast
2) It s summer and the Sun rises in
the Northeast
3) It s winter and the Sun rises in
the Southeast
4) It s summer and the Sun rises
in the Southeast
You are dropped onto an unknown location on Earth
(with no GPS). Cleverly, you note that Polaris is about
30o above the horizon. What's your latitude?
a.
30o north
b.
30o south
c.
60o north
d.
60o south
Rotation
The Earth s rotation about its axis determines
the day/night cycle
Earth s Rotation Axis
23.5°
to the Sun
The spinning Earth also `wobbles , meaning
that the rotation axis changes direction
This wobbling is called PRECESSION.
The PRECESSION of the Earth s axis
has a 26,000 year Period.
Survey Question
Where will the Earth s rotation axis be pointed
in 13,000 years?
a) one degree away from its present
orientation
b) 23.5 degrees away from its
present orientation
c) 47 degrees away from its
present orientation
d) the same as now
Survey Question
Where will the Earth s rotation axis be pointed
in 13,000 years?
a) one degree away from its present
orientation
b) 23.5 degrees away from its
present orientation
c) 47 degrees away from its
present orientation
d) the same as now
Survey Question
What effect will that have on the zodiac?
a) it will be composed of entirely
new constellations
b) it will be composed of ½ new
constellations
c) it will have no effect.
Survey Question
What effect will that have on the zodiac?
a) it will be composed of entirely
new constellations
b) it will be composed of ½ new
constellations
c) it will have no effect.
However…
Precession alters the Earth s climate:
q Currently the Northern Hemisphere is closest to
the Sun during winter;
q In 13,000 year, it will be furthest from the Sun,
implying more severe winters.
q This may have had consequences in the past,
including the ice ages.
Discussion Questions/Issues
•  What would happen to the seasons if the earth
were tipped at 35 degrees, instead of 23.5
degrees?
•  How about if it were not tipped?
Morning and Evening Stars
(a.k.a. the planets Venus and Mercury)
Why is this view of Mercury only
possible to take at sunrise or
sunset?
Morning and Evening
Stars
We see Mercury and Venus
follow the Sun around in the sky.
They may go down after, or
come up before it. If they go
down after, we see them in the
evening.
This is because they have orbits inward of
ours. That means they can only be seen to a
certain maximum angle away from the Sun.