Transcript Week 1B

Constellations
Constellations -- groups of stars named by
ancient cultures to honor gods, animals,
legends, etc.
They provide us with ways to recognize and
identify individual stars and groups of stars
(i.e., they are a crude “map” of the sky).
How Many Constellations?
In all, there are 88 constellations;
12 in the zodiac (13 really).
To learn more about constellations see:
The Stars by H.A. Rey
http://StarrySkies.com/
Sky & Telescope and Astronomy magazines
Celestial Sphere
The celestial sphere:
Stars seem to be on the inner
surface of a sphere
surrounding the Earth
They aren’t, but can use twodimensional spherical
coordinates (similar to
latitude and longitude) to
locate sky objects
Eastern Sky
in the
Winter
Orion is a
primary
Winter
constellation
Eastern Sky in the Summer
The Summer Triangle
Constellations, cont
Stars that appear close in the sky may not actually
be close in space:
Big/Little
Dippers
From our
latitude on
the Earth,
Polaris (the
North Star)
and the stars
around it
can be seen
all night
long.
The stars appear to rotate around Polaris.
Constellation Boundaries
A constellation is not just the stars that make up a “picture”, it is
an area of sky that borders the surrounding constellations.
Celestial Sphere
The celestial sphere:
Stars seem to be on the inner
surface of a sphere
surrounding the Earth
They aren’t, but can use twodimensional spherical
coordinates (similar to
latitude and longitude) to
locate sky objects
Right Ascension and Declination
• Declination: degrees north or south of celestial equator
• Right ascension: measured in hours, minutes, and
seconds eastward from position of Sun at vernal equinox
The Celestial Sphere
Equator =>
Celestial Equator
North Pole =>
North Celestial Pole
South Pole =>
South Celestial Pole
Celestial Sphere
is imaginary, not
real. It is used
for convenience.
What happens when you need to know where an
object is more accurately than “the fourth
brightest star in Taurus”?
Define a new coordinate system: the Celestial Sphere
The celestial sphere is essentially a
projection of Earth’s coordinate system of
longitude and latitude out into space.
longitude
Right Ascension (RA)
latitude
Declination (Dec)
So, instead of saying
“The object is just to
the left of Rigel in the
constellation of
Orion”, we have
coordinates much like
longitude and latitude
on the Earth.
SkyGazer Demo
Rotation of the Earth
Earth’s rotation around its axis:
• Causes night and day (Solar day = average
time between consecutive “noontimes”).
• Causes (apparent) motion of the stars
Years
Time for Earth to orbit once around Sun, relative to
fixed stars, is sidereal year
Tropical year follows seasons; sidereal year follows
constellations – in 13,000 years July and August
will still be summer, but Orion will be a summer
constellation
Daily/Nightly Motion of the Stars
The rotation of the Earth causes the stars
in the night sky to rotate.
Depending on where you are on the
Earth, the pattern of rotation is different.
Stars and constellations that never set (as seen
from your location) are called circumpolar
(they rotate around the pole).
Circumpolar
stars rotate
around Polaris (in
the Northern
Hemisphere).
At the North and South Pole, the stars move horizontally.
On the Equator, the stars move vertically.
At midlatitudes, the
stars move
“diagonally”
on an arc
across the
sky.
Daily Motion
Position tutorial page 1-2
Yearly Motion of the Stars
So far we have just talked about the motion of the
stars over the course of a night (due to the Earth’s
rotation).
Over a day, the Sun will not appear to move with
respect to the constellations.
But over the
course of months
or a year, the Sun
appears to move.
Yearly Motion of the Stars
12 constellations Sun moves through during the year
are called the zodiac; path is the ecliptic
Yearly Motion
Ecliptic tutorial page 13-17 (I-III, IV opt)
Seasonal Stars tutorial page 7-9 (take
home if don’t finish), maybe do this after
Horoscopes depending on time.
(Fall 2008 – completely ran out of time!)
Today’s Horoscope
click here
Horoscopes
Read your horoscopes. Which one
sounds most like you?
a) Your sign
b) The previous sign
c) The next sign
d) Six signs later
Precession – Why Your Sign is Wrong
Precession: rotation of Earth’s axis itself; makes
one complete circle in about 26,000 years
Years: Two Types
Time for Earth to orbit once around Sun, relative to
fixed stars, is a sidereal year
A tropical year is the time it takes for the Earth to
be in the same configuration with respect to the
Sun again (as opposed to distant stars).
Tropical year follows seasons; sidereal year follows
constellations – in 13,000 years July and August
will still be summer, but the “Summer Triangle” will
be a winter constellation.
Next Time
Seasons and Lunar Phases
Reading: E2, E3 5.2 (12 pages)
HW #1 Due