ASTRO 101
Principles of Astronomy
Instructor: Jerome A. Orosz
(rhymes with
“boris”)
Contact:
• Telephone: 594-7118
• E-mail: orosz@sciences.sdsu.edu
• WWW:
http://mintaka.sdsu.edu/faculty/orosz/web/
• Office: Physics 241, hours T TH 3:30-5:00
Text:
“Discovering the Essential Universe,
Fifth Edition”
by
Neil F. Comins
Course WWW Page
http://mintaka.sdsu.edu/faculty/orosz/web/ast101_spring2013a.html
Note the underline: … ast101_spring2013a.html …
Also check out Nick Strobel’s Astronomy Notes:
http://www.astronomynotes.com/
• Homework due January 29: Question 15
from Chapter 1 (Why is it warmer in the
summer than in winter?)
• Write down the answer on a sheet of paper
and hand it in before the end of class on
January 29.
Speaking of infinity…
• Which set is larger:
• The set of positive integers (1,2,3,…)
• The set of even integers (2,4,6,…)
Speaking of infinity…
•
•
•
•
Which set is larger:
The set of positive integers (1,2,3,…)
The set of even integers (2,4,6,…)
Both are the same size since there is a oneto-one mapping between the two:
(1, 2, 3, 4, …)
(2, 4, 6, 8, …)
Pop Quiz
• What is the shape of the Earth?
http://www.alaska.net/~clund/e_djublonskopf/Flatearthsociety.htm
http://fixedearth.com
The Earth
• How do we know the Earth is spherical?
The Earth
• How do we know the Earth is spherical?
– Curved shadow of the Earth on Moon during
eclipse
– The manner in which ships at sea disappear
when they sail away
– The fact that as you go north-south certain stars
disappear
The Earth
• How do we know the Earth is spherical?
– Curved shadow of the Earth on Moon during
eclipse
– The manner in which ships at sea disappear
when they sail away
– The fact that as you go north-south certain stars
disappear
Questions for Today
• What causes the seasons? Let’s ask some
Harvard students:
http://www.learner.org/resources/series28.html
• Does the Earth go around the Sun? Let’s ask…
http://www.collegehumor.com/video:1773116
Next:
Discovering the Night Sky
Coming Up:
• Introduction to the Sky
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–
–
–
Constellations
Stellar Brightness
Stellar coordinates and the Celestial Sphere
The “clockwork” of the sky
• Day/night
• Phases of the moon
• The seasons
Looking at the Night Sky
• On any given night, you can see about 3000
stars without a telescope, provided the sky is
dark.
Constellations
• People have long made up stories about
groups of stars that appear close together on
the sky.
• Such groupings are called constellations.
The sky was “officially” divided up into 88
constellations in 1930 so that a star is
associated with only one constellation.
Constellations
• The modern constellations have strictly defined
boundaries by international agreement.
Constellations
• Many constellation
names are derived from
characters in Greek or
Roman mythology.
• Here is Cassiopeia,
with its distinctive
“W”.
• The stars are usually
not physically
associated with each
other.
Constellations
• Here is the “Big
Dipper”, which is not
an “official”
constellation but part of
a larger one.
• Again, the stars are
usually not physically
associated with each
other.
Constellations
• Constellations can help you mark the seasons.
Stellar Brightness: Magnitudes
• Historically (e.g. Hipparcos in the First
Century), the brightness of stars as seen by
the eye have been measured on a magnitude
scale:
– The brightest stars were “first magnitude”.
– The faintest stars were “sixth magnitude”.
• Brighter objects have smaller magnitudes.
Stellar Brightness: Magnitudes
• In modern times, it was discovered that the
human eye has a nonlinear response to light:
if one source of light has twice the light as a
second source, then the first source would
not appear by eye to be twice as bright.
• The response of the eye is logarithmic, so
that differences of magnitudes correspond
to ratios of brightness.
The Magnitude Scale
• The modern of the
magnitude scale is set
up so that a difference
of 5 magnitudes
corresponds to a ratio
of brightnesses of 100.
• Bright objects can
have negative apparent
magnitudes.
The Celestial Sphere
• Imagine the sky as a hollow sphere with the
stars attached to it. This sphere rotates once
every 24 hours. This imaginary sphere is
called the celestial sphere.
• Even though we know it is not the case, it is
useful to imagine the Earth as being
stationary while the celestial sphere rotates
around it.
The Celestial Sphere
• The north celestial pole is directly above
the north pole on the Earth.
• The south celestial pole is directly above
the south pole on the Earth.
• The celestial equator is an extension of the
Earth’s equator on the sky.
• The zenith is the point directly over your
head. The horizon is the circle 90 degrees
from the zenith.
The Celestial Sphere
• The celestial poles and the celestial equator
are the same for everyone.
• The zenith and the horizon depend on where
you stand.
http://www.astronomynotes.com/nakedeye/s4.htm
Stellar Coordinates and Precession
• There are a few ways to specify the location of a
star (or planet) on the sky:
• Altitude/Azimuth:
– The altitude describes how many degrees the star is
above the horizon, the azimuth describes how far the
star is in the east-west direction from north.
– The altitude and azimuth of a star is constantly
changing owing to the motion of the star on the sky!
Stellar Coordinates and Precession
• There are a few ways to specify the location of a
star (or planet) on the sky:
• Equatorial system:
– Lines of longitude on the earth become right
ascension, measured in units of time. The RA
increases in the easterly direction.
– Lines on latitude on the earth become declination,
measured in units of degrees. DEC=90o at the north
celestial pole, 0o at the equator, and -90o at the south
celestial pole.
– http://www.astronomynotes.com/nakedeye/s6.htm
Stellar Coordinates and Precession
• The north celestial pole moves with respect to
the stars very slowly with time, taking 26,000
years to complete one full circle.