Ancient astronomy &
celestial sphere
ASTR 1010
Lecture 3
In this lecture, you will learn
2-1 The importance of astronomy in ancient civilizations around
the world
2-2 That regions of the sky are divided around groups of stars
called constellations
2-3 How the sky changes from night to night
2-4 How astronomers locate objects in the sky
Naked-eye astronomy
Although physical properties of
celestial objects cannot be obtained
from naked-eye astronomy,
questions like
• why are there seasons?
• why the night sky changes over
time?
• why the night sky looks different at
different places on Earth?
can be addressed and studied with
our bare eyes.
Ancient positional astronomy
Positional astronomy : the study of the positions
of objects in the sky.
 Stonehenge – ancient British
 Sun Dagger – ancestral Puebloans
 Machu Picchu
 Egyptian pyramids (3000BC)
 tomb of Shih Huang Ti (~250BC)
Mayan Astronomy : Could measure the period of
Venus (584 days versus 583.92 days)!
Venus being associated with wars, and need to
predict the best dates for attack.
Lead into the divisions of the night sky into
constellations.
Sun Dagger at Chaco Canyon : a band of
light strikes the center of the spiral on the
first day of summer (about 1000AD)
Constellations
 On a clear night, with 20/20 vision, one can see about 3,000 stars at a given time.
 Ancient astronomers traced out “pictures” using groups of these stars. “Group of
Stars” in Latin  constellations
Constellations
 International Astronomy Union (IAU) divided the entire night sky into 88
constellations.
 Helpers to find a way around the sky.
 Connection to the ancient astronomy, and good tool to naming stars (e.g., alpha
Orioni  the brightest star in Orion)
 stars in a constellation only appear to be close in the projected sky. In reality, most
of these stars are not related each other (not even close in 3D).
Diurnal Motion = daily motion of celestial objects
 At any moment, half of
Earth is illuminated by the
Sun  day-side and nightside.
 Seen from North Pole,
Earth rotates from West
to East making one
complete rotation in 24
hours.  for a person on
Earth, the sky appears to
rotate from east to west
 “west to east” rotation is
not a good expression.
Seen from the North Pole,
Earth rotates counterclockwise!
Diurnal Motion (animation)
 Stars do not move!
We are the one
that move!
 This image…
o day in Asia
o America &
Europe = night
 For a person in
California, Cygnus
is directly overhead
Diurnal Motion (animation)
 4 hours later…
 since Earth rotated
60 degrees, now a
different
constellation
(Andromeda) is
directly overhead
now.
Yearly Motion
 As Earth orbits around the Sun, the nighttime
side of the Earth gradually turns toward different
parts of the sky. Hence, the particular stars that
you see in the night sky are different at different
times of the year.
 Winter constellation = Orion
 Spring constellation = ?
 Summer constellation =
 Autumn constellation = ?
North Star and Big Dipper
Winter Triangle
Summer Triangle
Try…
Stellarium
stellarium.org
Free planetarium
software
Celestial Sphere
Ancient people believed that all stars
are at the same distance from Earth
and they are “attached” to the
hollow sphere called “celestial
sphere” and this celestial sphere
rotates once a day.
In truth, the diurnal motion is due to
the Earth rotation.
Visible stars to naked-eye range from
4.2 Ly to ~1,000 Ly.
Celestial sphere is an imaginary sphere
in the sky which has an infinite
radius.
celestial poles
celestial equator
Celestial Sphere
Ancient people believed that all stars
are at the same distance from Earth
and they are “attached” to the
hollow sphere called “celestial
sphere” and this celestial sphere
rotates once a day.
In truth, the diurnal motion is due to
the Earth rotation.
Visible stars to naked-eye range from
4.2 Ly to ~1,000 Ly.
Celestial sphere is an imaginary sphere
in the sky which has an infinite
radius.
celestial poles
celestial equator
zenith
horizon
Motions of the Celestial Sphere
For an observer at the northern
hemisphere…
stars sufficiently close to the the
celestial north pole never set (i.e.,
always stay above the horizon). These
stars are known as circumpolar stars.
Some stars sufficiently close to the
celestial south pole will never rise
above the horizon.
Apparent motion of stars can tell use
about the latitudes of the observer
(and the direction of the observation).
Apparent motion of Stars
Mid-latitude, looking North
Star trails…
Star trails over the Gemini Northern Telescope
dome. The glow from the eruption of the
Kilauea volcano is seen on the left. The trail of a
meteor is visible in this photograph.
mid-latitude looking south
Star trails…
Star tails at middle northern latitudes
Looking West
Looking East
In summary…
Important Concepts
Important Terms
 Ancient positional astronomy
 Constellation
 Celestial Sphere
 Diurnal Motion
 Seasonal stars
 Star trails
 Celestial sphere
 Celestial North Pole
 Celestial South Pole
 Celestial Equator
 Horizon
 Zenith
Chapter/sections covered in this lecture : sections 1.1, 1.2, 1.4, 2.1, 2.3