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ASTRONOMY 161
Introduction to Solar System Astronomy
Astronomy 161: The Web Page
www.astronomy.ohio-state.edu/~depoy/Astro161/astro161.html
Astronomy Picture of the Day (APOD):
http://antwrp.gsfc.nasa.gov/apod/
The Celestial Sphere
Friday, January 5
The Celestial Sphere: Key Concepts
(1) The sky as seen from Earth is divided into 88
constellations.
(2) It is convenient to pretend the stars are attached to a
celestial sphere.
(3) The celestial sphere appears to rotate about the celestial
poles (1 day).
(4) The Sun appears to move west to east relative to stars
(1 year).
(5) The Moon appears to move west to east relative to stars
(1 month).
(1) The sky is divided into 88 constellations
Mesopotamia, circa 3000 BC:
oldest know constellations
Ptolemy, 2nd century AD:
48 constellations in northern sky
16th to 18th century AD:
unmapped regions of sky filled in
Constellations are largely arbitrary
Other cultures, other constellations:
Example: Ursa Major = bear, dipper, bull's leg,
grain scoop, wagon, plow, etc.
Stars in a constellation usually are not at the same
distance from us.
At a different place in our Galaxy, we would see
different star patterns.
A modern star chart of Ursa Major:
(2) Stars are “attached” to a celestial sphere
Distances to stars are hard to measure.
However, we can pretend all stars are at the
same distance from us, attached to a large
celestial sphere.
Position on the celestial sphere is known even
when the distance in unknown.
Celestial Sphere: A large imaginary sphere centered on Earth
Special locations on the celestial sphere
North Celestial Pole = point directly above Earth’s
North Pole (near the star Polaris)
South Celestial Pole = point directly above Earth’s
South Pole (no nearby bright star)
Celestial Equator = circle directly above Earth’s
Equator
Distances between points on the
celestial sphere are measured in
degrees, arcminutes, and arcseconds
360 degrees in a circle
60 arcminutes in a degree
60 arcseconds in an arcminute
½ degree = angular size of
Sun & Moon
Celestial navigation made simple
At Earth’s North Pole:
Polaris is directly overhead
At Earth’s Equator:
Polaris is due north, on the horizon
In Earth’s Northern hemisphere:
Polaris is due north - height above the horizon
(in degrees) is equal to your latitude (in degrees)
(3) The celestial sphere appears to rotate about
the celestial poles (1 day cycle)
Observation: Stars, Sun, Moon and planets
move in counterclockwise circles around
north (south) celestial pole.
Objects near the celestial equator move east to
west when above the horizon (“rising” in
east, “setting” in west).
What causes these circular motions?
Explanations of the 1 day cycle:
HYPOTHESIS #1
HYPOTHESIS #2
(Ptolemy, 2nd century): (Copernicus, 16th cent.):
Earth is stationary; stars Stars are stationary; Earth
are attached to a sphere
rotates about its axis
that revolves around
once per day.
the Earth once per day.
RIGHT!
WRONG!
Nicolaus Copernicus (1473-1543)
(4) The Sun appears to move west to east
relative to stars (1 year cycle)
Today the Sun is “in” Virgo, next month in
Libra, etc.
Sun’s path on the celestial sphere = ecliptic
Constellations through which the ecliptic runs
= zodiac
The ecliptic is NOT the same as the celestial
equator!
Observation: Sun moves west to east relative
to stars (about 1 degree per day).
What causes this annual motion?
Explanations of 1 year cycle:
HYPOTHESIS #1
(Ptolemy):
Sun revolves around
Earth at a slightly
slower rate than the
celestial sphere.
WRONG!
HYPOTHESIS #2
(Copernicus):
Earth revolves around the
Sun, once per year.
RIGHT!
(5) The Moon appears to move west to east
relative to stars (1 month cycle)
Today the Moon is “in” Virgo
In two weeks: Pisces
In four weeks: Virgo, again.
Observation: Moon moves west to east relative to
stars, taking 27.3 days to complete cycle.
What causes this monthly motion?
Explanations of 1 month cycle:
HYPOTHESIS #1
(Ptolemy):
Moon revolves around
Earth at a significantly
slower rate than the
celestial sphere.
WRONG!
HYPOTHESIS #2
(Copernicus):
Moon revolves around
Earth, once per month.
RIGHT!