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ASTRONOMY 101
Introduction to Solarr System Astronomy
D Taylor
Greenwich HS, CT
Rowan University
University, NJ
©2008
Astronomy 2008
8: The Web Page
www.DTF
Fizzix.com
Astronomy
y Picture of the Day
y (APOD):
(
)
http://antwrp.gsffc.nasa.gov/apod/
The Celesttial Sphere
Thursday, September
S
11
The Celestial Spheere: Key Concepts
(1) The sky as seen from Earth is divided into 88
constellations.
(2) It is convenient to pretend thhe stars are attached to a
celestial sphere.
p
(3) The celestial sphere appearss to rotate about the celestial
poles (1 day).
(4) The Sun appears to move west to east relative to stars
(1 year)
year).
(5) The Moon appears to move west to east relative to stars
(1 month)
month).
(1) The sky is divided into 88 constellations
Mesopotamia, circa 3000 BC:
oldest know constellaations
Ptolemy,
Pt
l
2nd
2 d century
t
A
AD:
AD
48 constellations in northern
n
skyy
16th to 18th century AD
D:
unmapped regions off sky filled in
Constellations aree largely arbitrary
Other cultures, other constellations:
Example: Ursa Major = bear,
b
dipper, bull's leg,
grain scoop,
scoop wagon,
wagon plo
ow etc
ow,
etc.
Stars in a constellation usu
ually are not at the same
distance from us.
At a diff
differentt place
l
in
i ourr Galaxy,
G l
we would
ld see
different star patterns.
A modern star chart of Ursa Major:
(2) Stars are “attachedd” to a celestial sphere
Distances to stars are hard
h
to measure.
However, we can preteend all stars are at the
same distance
di t
from
f
u attached
us,
tt h d to
t a large
l
celestial sphere.
p
Position on the celestiaal sphere is known even
when the distance in unknown.
u
C l ti l Sphere:
Celestial
S h
A large
l
imagin
i
inary sphere
h centered
t d on Earth
E th
Special locations onn the celestial sphere
North Celestial Pole = pooint directly above Earth’s
N hP
North
Pole
l ((near the
h starr Polaris)
P l i)
South Celestial Pole = pooint directly above Earth
Earth’ss
South Pole (no nearby bright
b
star)
Celestial Equator = circle directly above Earth’s
Equator
Distances betweeen points on the
celestial sphere are measured in
d
degrees,
arcminute
i tes, and
d arcseconds
d
360 degrees in a circle
60 arcminutes in a degree
60 arcseconds in an arcmiinute
½ degree = angular size of
Sun & Moon
Celestial navigattion made simple
At Earth’s North Pole:
P l i is
Polaris
i directly
di l overh
h d
head
At Earth’s
Earth s Equator:
Polaris is due north, on the
t horizon
In Earth’s Northern hem
misphere:
Polaris is due north - height above the horizon
(in degrees) is equal to your
y
latitude (in degrees)
(3) The celestial sphere appears to rotate about
the celestial poles (1 day cycle)
Observation: Stars, Suun, Moon and planets
move in counterclockkwise circles around
north (south) celestial pole.
pole
Objects
j
near the celestiial equator
q
move east to
west when above the horizon (“rising” in
east,
t “setting”
“ tti ” in
i westt)
t).
t
What causes these circu
ular motions?
Explanations of the 1 day cycle:
HYPOTHESIS #1
HYPOTHESIS #2
(P l
(Ptolemy,
2nd century):
)
(Copernicus,
(C
i
16th cent.):
)
Earth is stationary; stars Stars are stationary; Earth
are attached to a sphere
rotates about its axis
per day.
y
that revolves around
once p
the Earth once per day.
RIGHT!
WRONG!
Nicolaus Coperniicus (1473-1543)
Foucault'ss Pendulum
Li ki
Link
kicus
k
(4) The Sun appears to move west to east
relative to starss (1 year cycle)
Today the Sun is “in” Sagittarius, next month
in Capricornus, etc.
Sun’s path on the celesttial sphere = ecliptic
Constellations through which
w
the ecliptic runs
= zodiac
The ecliptic is NOT thee same as the celestial
equator!
Observation: Sun movves west to east relative
to stars (about 1 degrree per day).
What causes this annuaal motion?
Explanations of
o 1 year cycle:
HYPOTHESIS #1
(P l
(Ptolemy):
)
Sun revolves around
Earth at a slightly
slower rate than the
celestial sphere.
WRONG!
HYPOTHESIS #2
(C
(Copernicus):
i )
Earth revolves around the
Sun, once per year.
RIGHT!
(5) The Moon appearss to move west to east
relative to stars (1 month cycle)
Today the Moon is “in” Scorpius
In two weeks: Gemini
In four weeks: Scorpius, again.
a
Observation: Moon movees west to east relative to
stars, taking 27.3 days to
o complete cycle.
What
h causes this
hi monthly
hl motion?
i ?
Explanations off 1 month cycle:
HYPOTHESIS #1
(P l
(Ptolemy):
)
Moon revolves around
Earth at a significantly
slower rate than the
celestial sphere.
WRONG!
HYPOTHESIS #2
(C
(Copernicus):
i )
Moon revolves around
Earth, once per month.
RIGHT!
Few closing
g questions:
1) Can the Sun come closee to the North Celestial
Pole?
2)) Can the Moon come clo
ose to the South Celestial
Pole?
3) Y
You are on th
the E
Earth’s
th’ equator---how
e t
h hi
high
h is
i
Polaris above the horizzon?
4) How often does the Sun
n cross the plane of the
ecliptic?
Few closing quesstions continued:
5) How often does the Moo
on cross the plane of the
ecliptic?
li i ?
6) How often does the Sun cross the Celestial
Equator?
7) How often does the Moo
on cross the Celestial
Equator?
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