Transcript Here

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_fall2013.html
Note the underline: … ast101_fall2013.html …
Also check out Nick Strobel’s Astronomy Notes:
http://www.astronomynotes.com/
Fall 2013
No appointment needed!
Just drop by!
Where: Room 215, physics-astronomy building (PA-215).
When: All semester long, at the following days and times:
• Monday:
12 – 2 PM; 5 – 6 PM
• Tuesday:
12 – 2 PM; 5 – 6 PM
• Wednesday: 12 – 2 PM; 5 – 6 PM
• Thursday: 1 – 2 PM; 3 – 6 PM
• Homework due September 5: 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 5.
Homework
• Homework due September 12: Question 11
from Chapter 2 (In what ways did the
astronomical observations of Galileo support a
heliocentric cosmology?)
• Write down the answer on a sheet of paper and
hand it in before the end of class on September
12.
Homework
• Go to a planetarium show in PA 209:
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The days and times of the shows will be (all shows last less than 1 hour):
Thursday, September 5 : 5 PM
Friday, September 6 : 2 PM
Monday, September 9 : 1 PM and 5 PM
Tuesday, September 10 : 1 PM and 5 PM
Wednesday, September 11 : 5 PM
Thursday, September 12 : 5 PM
Friday, September 13 : 3 PM
• Get 10 points extra credit for homework part of grade.
• Sign up for a session outside PA 209.
• Hand in a sheet of paper with your name and the data and time of
the session.
Next:
Discovering the Night Sky
Coming Up:
• Introduction to the Sky
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Constellations
Stellar Brightness
Stellar coordinates and the Celestial Sphere
The “clockwork” of the sky
• Day/night
• Phases of the moon
• 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?
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’s 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
Lunar and Solar Eclipses
• But first, let’s discuss “angular size” and
“linear size”…
Angular Size
• The physical size is
measured in meters,
light-years, etc.
• The distance is
measured in the same
units.
• The angular size is
how large something
“looks” on the sky,
and is measured in
degrees.
Angular Size
• The angular size is
how large something
“looks” on the sky,
and is measured in
degrees.
• As you move the
same object further,
its angular size gets
smaller.
Angular Size
• The angular size is
how large something
“looks” on the sky,
and is measured in
degrees.
• If two objects are at
the same distance, the
larger one has the
larger angular size.
Angular Size
• Trick photography often involves playing with
different distances to create the illusion of large
or small objects:
http://www.tadbit.com/2008/03/06/top-10-holding-the-sun-pictures/
http://www.stinkyjournalism.org/latest-journalism-news-updates-45.php
Angular Size
• This figure illustrates how objects of very different
sizes can appear to have the same angular sizes. The
Sun is 400 times larger than the Moon, and 390 times
more distant.
Lunar and Solar Eclipses
• A solar eclipse is seen when the Moon
passes in front of the Sun, as seen from a
particular spot on the Earth.
• A lunar eclipse is seen then the Moon
passes into the Earth’s shadow.
Shadows
• If the light source is extended, then the shadow of an
object has two parts: the umbra is the “complete”
shadow, and the penumbra is the “partial shadow”.
Shadows
• If the light source is
extended, then the
shadow of an object has
two parts: the umbra is
the “complete” shadow,
and the penumbra is the
“partial shadow”.
Lunar Eclipses
• During a total lunar eclipse, the Moon passes through
Earth’s shadow.
Solar Eclipses
• The umbral shadow of
the Moon sweeps over
a narrow strip on the
Earth, and only people
in that shadow can see
the total solar eclipse.
Solar Eclipses
• The umbral shadow of
the Moon sweeps over
a narrow strip on the
Earth, and only people
in that shadow can see
the total solar eclipse.
Solar Eclipses
• The umbral shadow of
the Moon sweeps over
a narrow strip on the
Earth, and only people
in that shadow can see
the total solar eclipse.
• During totality the
faint outer atmosphere
of the Sun can be seen.
Annular Eclipses
• The angular sizes of
the Sun and Moon
vary slightly, so
sometimes the Moon
isn’t “big enough” to
cover the Sun
Lunar and Solar Eclipses
• Why isn’t there an eclipse every month? Because the orbit of the
Moon is inclined with respect to the orbital plane of the Earth
around the Sun.
How often do we see an Eclipse?
• Roughly every 18 months there is a total solar
eclipse visible somewhere on the Earth.
Next:
The Scientific Method
Gravity and the motions
of the planets