Post on 23-Dec-2015
transcript
Ancient astronomy & celestial sphere
ASTR 1010
Lecture 3
In this lecture, you will learn2-1 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 night-side.
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 counter-clockwise!
Diurnal Motion (animation) Stars do not move!
We are the one that move!
This image…o day in Asiao 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 planetariumsoftware
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 polescelestial 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 polescelestial equatorzenithhorizon
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).
Motions of the Celestial Sphere
Mid-latitude, looking North
Apparent motion of Stars
mid-latitude looking south
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.
Star trails…
Star tails at middle northern latitudes
Looking West Looking East
In summary…
Important ConceptsAncient positional astronomyConstellationCelestial SphereDiurnal MotionSeasonal starsStar trails
Important TermsCelestial sphereCelestial North PoleCelestial South PoleCelestial EquatorHorizonZenith
Chapter/sections covered in this lecture : sections 1.1, 1.2, 1.4, 2.1, 2.3