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ASTRO 101
Principles of Astronomy
Instructor: Jerome A. Orosz
(rhymes with
“boris”)
Contact:
• Telephone: 594-7118
• E-mail: [email protected]
• WWW:
http://mintaka.sdsu.edu/faculty/orosz/web/
• Office: Physics 241, hours T TH 3:30-5:00
Text:
“Discovering the Essential Universe,
Fifth Edition”
by
Neil F. Comins
Course WWW Page
http://mintaka.sdsu.edu/faculty/orosz/web/ast101_fall2012.html
Note the underline: … ast101_fall2012.html …
Also check out Nick Strobel’s Astronomy Notes:
http://www.astronomynotes.com/
No appointment needed!
Just drop by!
Where: Room 215, physics-astronomy building.
When:
• Monday:
• Tuesday:
• Wednesday:
• Thursday:
12-2, 4-6 PM
12-1 PM; 4-6 PM
12-2, 5-6 PM
4-6 PM
• Homework due September 6: Question 15
from Chapter 1 (Why is it warmer in the
summer than in winter?)
• Write down the answer on a sheet of paper
and hand it in before the end of class on
September 6.
Questions from Before
• What causes the seasons? The tilt of the
Earth’s axis relative to its orbital plane
gives rise to a changing pattern of solar
heating…
• What causes the phases of the moon? The
relative position of the Sun, Moon, and
Earth…
• Why don’t we see eclipses every month?
Coming up…
Questions for Today
• Why are the seasons opposite between the
northern and southern hemispheres?
• Why don’t we see eclipses every month?
• What is gravity?
Next:
Discovering the Night Sky
Coming Up:
• Introduction to the Sky
–
–
–
–
Constellations
Stellar Brightness
Stellar coordinates and the Celestial Sphere
The “clockwork” of the sky
• Day/night
• Phases of the moon
• The seasons
Next:
• The seasons
In Detail:
•
If we do some careful observations, we find:
1) The length of the daylight hours at a given spot
varies throughout the year: the Sun is out a longer
time when it is warmer (i.e. summer), and out a
shorter time when it is colder.
2) On a given day, the length of the daylight hours
depends on where you are on Earth, in particular
it depends on your latitude: e.g. in the summer,
the Sun is out longer and longer the further north
you go.
In Detail:
• Near the North
Pole, the Sun never
sets in the middle
of the summer (late
June).
• Likewise, the Sun
never rises in the
middle of the
winter (late
December).
In Detail:
• In most places on Earth, the weather patterns
go through distinct cycles:




Cold weather: winter, shorter daytime
Getting warmer: spring, equal daytime/nighttime
Warm weather: summer, longer daytime
Cooling off: fall, equal daytime/nighttime
• These “seasons” are associated with the
changing day/night lengths.
In Detail:
• When it is summer in the northern
hemisphere, it is winter in the southern
hemisphere, and the other way around.
What Causes the Seasons?
• Is the Earth closer to the Sun during
summer, and further away during winter?
(This was the most commonly given answer
during a poll taken at a recent Harvard
graduation).
• No! Otherwise the seasons would not be
opposite in the northern and southern
hemispheres.
What Causes the Seasons?
• The Earth moves around the Sun. A year is
defined as the time it takes to do this, about
365.25 solar days.
• This motion takes place in a plane in space,
called the ecliptic.
• The axis of the Earth’s rotation is inclined
from this plane by about 23.5 degrees from
the normal.
What Causes the Seasons?
• The axis of the Earth’s rotation points to
the same point in space (roughly the
location of the North Star).
• The result is the illumination pattern of the
Sun changes throughout the year.
What Causes the Seasons?
• Here is an edge-on view, from the plane of the
Earth’s orbit.
What Causes the Seasons?
• Here is a view from slightly above the Earth’s
orbital plane.
What Causes the Seasons?
• A slide from Nick
Strobel.
What Causes the Seasons?
• Because of the tilt of the Earth’s axis, the
altitude the Sun reaches changes during the
year: It gets higher above the horizon
during the summer than it does during the
winter.
What Causes the Seasons?
• Because of the tilt of the Earth’s axis, the
altitude the Sun reaches changes during the
year: It gets higher above the horizon
during the summer than it does during the
winter.
• Also, the length of the daytime hours
changes during the year: the daylight hours
are longer in the summer and shorter in
winter.
What Causes the Seasons?
• The altitude of the Sun matters: when the Sun is
near the horizon, it does not heat as efficiently as
it does when it is high above the horizon.
Image from Nick Strobel’s Astronomy Notes (http://www.astronomynotes.com/).
What Causes the Seasons?
• The Sun’s daily path across the sky
depends on the time of year…
What Causes the Seasons?
• Winter: The combination of a short
daytime and a Sun that is relatively low
above the horizon leads to much less
heating in the day, plus a longer period of
cooling at night. Overall, it is colder.
What Causes the Seasons?
• Summer: The combination of a long
daytime and a Sun that is relatively high
above the horizon leads to much more
heating in the day, plus a shorter period of
cooling at night. Overall, it is warmer.
What Causes the Seasons?
• Spring and Fall: The number of hour of
daylight is about equal to the number of
nighttime hours, leading to roughly equal
times of heating and cooling.
Next:
The Moon
The Phases of the Moon
• Next to the Sun, the Moon is the most noticeable
object in the sky.
• The lunar cycle is the basis of the month.
How Long is one Month?
How Long is one Month?
• It depends:
How Long is one Month?
• It depends:
 If you use the Sun as a reference, the Moon
takes 29.5 days to complete one orbit around
the Earth.
How Long is one Month?
• It depends:
 If you use the Sun as a reference, the Moon
takes 29.5 days to complete one orbit around
the Earth.
 If you use a star as a reference, the moon takes
27.3 days to go around the Earth.
How long is one Month?
• During the course of 27
days, the Earth has moved
around a substantial part of
its orbit about the Sun.
• It takes an extra 2 days for
the Moon to “catch up” with
the Sun.
How Many Months are in a Year?
• It depends:
– 365.25/29.5=12.4 if you use the Sun as the
reference.
– 365.25/27.3=13.4 if you use a star as the reference.
– 12 calendar months, with each calendar month
being slightly longer than one lunar cycle.
What Causes the Phases of the
Moon?
What Causes the Phases of the
Moon?
• The full Moon always rises just after sunset.
• The crescent Moon always points towards the Sun.
• A crescent Moon sets shortly after sunset, or rises just
before sunrise.
• The Moon is illuminated by reflected sunlight.
What Causes the Phases of the
Moon?
• The full Moon always rises just after sunset.
• A crescent Moon sets shortly after sunset.
What Causes the Phases of the
Moon?
• The full Moon always rises just after sunset.
• A crescent Moon sets shortly after sunset.
What Causes the Phases of the
Moon?
• The lit side of the
Moon always faces the
Sun.
• Because of the motion
of the Moon relative to
the Sun, we see
different amounts of lit
and dark sides over the
course of a month.
What Causes the Phases of the
Moon?
• The lit side of the
Moon always faces the
Sun.
• Because of the motion
of the Moon relative to
the Sun, we see
different amounts of lit
and dark sides over the
course of a month.
The Seven Day Week?
The Seven Day Week?
•
There are seven bright objects in the sky that are
not stars:
The Seven Day Week?
•
There are seven bright objects in the sky that are
not stars:
1) The Sun.
The Seven Day Week?
•
There are seven bright objects in the sky that are
not stars:
1) The Sun.
2) The Moon.
The Seven Day Week?
•
There are seven bright objects in the sky that are
not stars:
1) The Sun.
2) The Moon.
3) Mars.
The Seven Day Week?
•
There are seven bright objects in the sky that are
not stars:
1)
2)
3)
4)
The Sun.
The Moon.
Mars.
Mercury.
The Seven Day Week?
•
There are seven bright objects in the sky that are
not stars:
1)
2)
3)
4)
5)
The Sun.
The Moon.
Mars.
Mercury.
Jupiter.
The Seven Day Week?
•
There are seven bright objects in the sky that are
not stars:
1)
2)
3)
4)
5)
6)
The Sun.
The Moon.
Mars.
Mercury.
Jupiter.
Venus.
The Seven Day Week?
•
There are seven bright objects in the sky that are
not stars:
1)
2)
3)
4)
5)
6)
7)
The Sun.
The Moon.
Mars.
Mercury.
Jupiter.
Venus.
Saturn.
The Seven Day Week?
•
There are seven bright objects in the sky that are
not stars:
1)
2)
3)
4)
5)
6)
7)
The Sun.
The Moon.
Mars.
Mercury.
Jupiter.
Venus.
Saturn.
Sunday
The Seven Day Week?
•
There are seven bright objects in the sky that are
not stars:
1)
2)
3)
4)
5)
6)
7)
The Sun.
The Moon.
Mars.
Mercury.
Jupiter.
Venus.
Saturn.
Sunday
Monday
The Seven Day Week?
•
There are seven bright objects in the sky that are
not stars:
1)
2)
3)
4)
5)
6)
7)
The Sun.
The Moon.
Mars.
Mercury.
Jupiter.
Venus.
Saturn.
Sunday
Monday
Saturday
The Seven Day Week?
•
There are seven bright objects in the sky that are
not stars:
1)
2)
3)
4)
5)
6)
7)
The Sun.
The Moon.
Mars.
Mercury.
Jupiter.
Venus.
Saturn.
Sunday
Monday
Tui’s Day (Norse)
Woden’s Day (Norse)
Thor’s Day (Norse)
Freya’ Day (Norse)
Saturday
The Seven Day Week?
•
There are seven bright objects in the sky that are
not stars:
1)
2)
3)
4)
5)
6)
7)
The Sun.
The Moon.
Mars.
Mercury.
Jupiter.
Venus.
Saturn.
Sunday
Monday
Martes in Spanish
Miercoles in Spanish
Jueves in Spanish
Viernes in Spanish
Saturday
Next:
Lunar and Solar Eclipses