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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 is the shape of the Earth, and is it
stationary?
• We all know the answers to the above, but
can you give evidence for the correct
conclusion without using modern
technology?
– The stars change as you go north-south
– The shape of the Earth’s shadow during lunar
eclipse
– The manner in which ships disappear.
Questions for Today
• What causes the seasons?
• What causes the phases of the moon
• Why don’t we see eclipses every month?
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
The Celestial Sphere
• Imagine the sky as a hollow sphere with the
stars attached to it. This sphere rotates once
every 24 hours. This imaginary sphere is
called the celestial sphere.
• Even though we know it is not the case, it is
useful to imagine the Earth as being
stationary while the celestial sphere rotates
around it.
The Celestial Sphere
• The north celestial pole is directly above
the north pole on the Earth.
• The south celestial pole is directly above
the south pole on the Earth.
• The celestial equator is an extension of the
Earth’s equator on the sky.
• The zenith is the point directly over your
head. The horizon is the circle 90 degrees
from the zenith.
The Celestial Sphere
• The celestial poles and the celestial equator
are the same for everyone.
• The zenith and the horizon depend on where
you stand.
http://www.astronomynotes.com/nakedeye/s4.htm
Stellar Coordinates and Precession
• There are a few ways to specify the location of a
star (or planet) on the sky:
• Altitude/Azimuth:
– The altitude describes how many degrees the star is
above the horizon, the azimuth describes how far the
star is in the east-west direction from north.
– The altitude and azimuth of a star is constantly
changing owing to the motion of the star on the sky!
Stellar Coordinates and Precession
• There are a few ways to specify the location of a
star (or planet) on the sky:
• Equatorial system:
– Lines of longitude on the earth become right
ascension, measured in units of time. The RA
increases in the easterly direction.
– Lines on latitude on the earth become declination,
measured in units of degrees. DEC=90o at the north
celestial pole, 0o at the equator, and -90o at the south
celestial pole.
– http://www.astronomynotes.com/nakedeye/s6.htm
Stellar Coordinates and Precession
• The north celestial pole moves with respect to
the stars very slowly with time, taking 26,000
years to complete one full circle.
The Clockwork of the Universe
• There are many familiar astronomical cycles:




The Day/Night cycle.
The phases of the Moon (the lunar cycle).
The seasons of the year.
The seven day week???
The Day/Night Cycle
• Every day, the Sun rises in the east, and sets
in the west.
• At night, the stars move in fixed patterns.
The specific paths depend on where you
look…
The Day/Night Cycle
• The stars rotate about a fixed point in the northern
sky (for observers in the northern hemisphere).
Day/Night Cycle
•
•
We have the Sun rising and setting, and
the stars moving in fixed patterns about a
point in the sky.
Two possible reasons:
1) The Earth is fixed in space, and the Sun and
the “celestial sphere” move around it.
2) The Earth is spinning about its axis.
Day/Night Cycle
• The Earth is spinning on its axis, while it
and the other planets revolve around the
Sun.
• The apparent motion of the Sun defines the
day.
• The rotation of the Earth affects weather
patterns, ocean currents, the paths of
missiles etc.
How Long is a Day?
How Long is a Day?
• It depends:
How Long is a Day?
• It depends:
 If you measure the length of time between
successive “noons” (the time when the Sun is at
its highest point in the sky), the day is on
average 24 hours. This is a “mean solar day.”
How Long is a Day?
• It depends:
 If you measure the length of time between
successive “noons” (the time when the Sun is at
its highest point in the sky), the day is on
average 24 hours. This is a “mean solar day.”
 If you do the same thing, but with a star rather
than with the Sun, the day is about 23 hours
and 56 minutes. This is a “sidereal day”.
Solar vs. Sidereal
• Since the Earth is
moving around the
Sun, it has to rotate an
extra bit each day so
that the Sun appears in
the same point in the
sky.
Image from Nick Strobel’s Astronomy notes (http://www.astronomynotes.com/))
Solar vs. Sidereal
• 1-6_SideralTime.html
• Your favorite star rises 4 minutes earlier
each successive night.
• Eventually, it will rise when the Sun is still
out. Hence, with a few exceptions, a given
constellation is visible at night only during
certain times of the year.
How Fast does the Earth Rotate?
How Fast does the Earth Rotate?
1) Pretty fast: the circumference of the
Earth is about 24,000 miles at the equator,
so a point on the equator moves 1000
miles per hour (24,000 miles in 24 hours).
How Fast does the Earth Rotate?
1) Pretty fast: the circumference of the
Earth is about 24,000 miles at the equator,
so a point on the equator moves 1000
miles per hour (24,000 miles in 24 hours).
2) Pretty slow: just watch the hour hand on
the clock for a while. That hand moves
twice as fast as the earth!
What Time is it?
• A nice map of time zones:
http://www.worldtimezone.com/datetime.htm
The Cycle of the Sun
• The Sun would be in different constellations
during certain times of the year, if you could see
the stars in the day. Where the Sun is depends on
the season.
The Cycle of the Sun
• The Sun would be in different constellations
during certain times of the year, if you could see
the stars in the day. Where the Sun is depends on
the season.
Next:
• The seasons
Seasons in Detail:
•
If we do some careful observations, we find:
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.
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
Getting warmer: spring
Warm weather: summer
Cooling off: fall
In Detail:
• In most places on Earth, the weather patterns
go through distinct cycles:




Cold weather: winter
Getting warmer: spring
Warm weather: summer
Cooling off: fall
• These “seasons” are associated with the
changing day/night lengths.
In Detail:
• In most places on Earth, the weather patterns
go through distinct cycles:




Cold weather: winter, shorter daytime
Getting warmer: spring
Warm weather: summer
Cooling off: fall
• These “seasons” are associated with the
changing day/night lengths.
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
Cooling off: fall
• These “seasons” are associated with the
changing day/night lengths.
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
• These “seasons” are associated with the
changing day/night lengths.
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?