Chapter 2
Naming stars:
Brightest stars were known to, and named by,
the ancients (Procyon)
In 1604, stars within a constellation were
ranked in order of brightness and labeled
with Greek letters (Alpha Centauri)
In the early 18th century, stars were
numbered from west to east in a constellation
(61 Cygni)
As more and more stars were discovered,
different naming schemes were developed
(G51-15, Lacaille 8760, S 2398)
Now, new stars are simply labeled by their
celestial coordinates
A pattern or group of
stars in the sky is
called a
constellation.
People of ancient time
saw the constellations as
character or animals in
the sky. They made up
stories to explain how
the object, animal, or
character came into the
night sky.
 Earth rotates on its axis, this makes most constellations appear
to rise in the east and set in the west during the night.
Less formally defined groups of stars.
Part of one or more larger constellations
• As early as 5000 years ago, people
began naming patterns of stars, called
constellations, in the honor of
mythological characters or great heroes.
• Today, 88 constellations are
recognized.
• They divide the sky into disjoint units.
• Every star in the sky is in one of these
constellations.
The Brightness of Stars
- There are approximately 5000 stars viewable with the
unaided eye from the Earth’s surface
- At any one position you could view approximately 300 stars
- HOWEVER – Light and air pollution reduces this number
significantly
- A system of stellar labeling was developed by assigning
numbers based on the relative brightness of stars =>
Magnitude system
1. Magnitude System
- original idea came during 2nd century B.C. from a Greek Scholar
named Hipparchus
- He organized all the visible stars according to their apparent
brightness. The brightest stars were given a magnitude of 1. The
Next brightest a magnitude of 2 and so on till the faintest stars
were given a magnitude of 6.
A. Apparent or Relative BrightnessAmount of light energy striking a surface. (Observer’s eye or
telescope). The brightness is a factor of two things:
1. Luminosity of StarThe amount of Light Energy being given off by a star. IT
DOES NOT DEPEND ON THE LOCATION OF THE OBSERVER
2. Distance to Star
A. Apparent or Relative Brightness-(cont.)
*As distance to star increases brightness decreases (Inverse
Relationship)
* As luminosity of star increases brightness Increases (Direct
Relationship)
B. Apparent Magnitude
A number assigned to a celestial object that is a measure of its
relative brightness. (Based on distance and luminosity)
1. The more positive the number the dimmer the star
2. The more negative the number the brighter the star
B. Apparent Magnitude (cont.)
It turns out that a an average 1st magnitude star appears 100 times brighter than a
6th magnitude star so therefore a change in magnitude of 5 corresponds exactly to
a factor of 100 in brightness.
*** One change in magnitude corresponds to a fifth root of 100 or
2.5 times in brightness
Ex. Magnitude 3 star is 2.5 times brighter than a magnitude 4 star
Magnitude 4 star is 2.5 times brighter than a magnitude 5 star
Question:
1. How much brighter is a magnitude 2 star than a 4 star?
2. How much dimmer is a magnitude 2 star than a –1 star?
Constellations
Celestial Sphere
Our Point of View
Angular Size and Distance
The Earth Rotates
Relevant Geography
Earth’s Rotation
• 23 hours and 56 minutes relative to background
stars (sidereal period)
• Spin axis of Earth defines points on the celestial
sphere (using north and south poles)
– North Celestial Pole
– South Celestial Pole
– Celestial Equator
• Sky appears to rotate east to west about the
celestial poles because Earth rotates west to east.
Rotating Celestial Sphere
The Celestial Sphere
Definitions
 Celestial Sphere: An imaginary sphere where
celestial objects are projected on the basis of
their direction from Earth
 Celestial Poles: The two points where the spin
axis of the Earth’s spin axis intersects the celestial
sphere
 Celestial Equator: The projection of the Earth’s
equator onto the celestial sphere
 Great Circle: A circle on the sphere's surface
whose center is the same as the sphere’s center,
and divides the sphere into two equal
hemispheres
Definitions
 Zenith: The point on the sky that is directly overhead
of the observer.
 Horizon: The great circle on the celestial sphere that
is 90 degrees from the zenith
 Hour circle: The great circle through the position of a
celestial body and the celestial poles
 Meridian: The hour circle that passes through the
zenith and both celestial poles
Directions on the Local Sky
 Altitude: The minimum angular distance between the
position of a celestial body and the horizon
 Azimuth: The angular bearing of an object, measured
from North (0 degrees) through East (90 degrees),
South (180 degrees), West (270 degrees), and back
to North (360 degrees)
 Hour Angle: The angle between the meridian and an
object’s hour circle (west is positive)
Azimuth and Altitude are observer centric.
Celestial Coordinates
• Vernal Equinox: The position of the Sun when it
crosses the celestial equator from south to north
• Declination: The minimum angular distance from the
position of a celestial body and the celestial equator
• Right Ascension: The eastward angle from the vernal
equinox to the intersection of an object’s hour circle
with the equator
• 1 hour of angle = 15 degrees
Right Ascension and Declination
• Right Ascension (RA): Analogous to
longitude, but on the celestial sphere.
– It is the east-west angle between the vernal
equinox and a location on the celestial sphere.
• Declination (dec): Analogous to latitude,
but on the celestial sphere.
– It is the north-south angle between the
celestial equator and a location on the
celestial sphere.
Celestial Coordinates
Units of
R.A.
360o = 24h
 15o/h
Motion Depends on Declination
The Sky at the North Pole
• At the North Pole, the
North Celestial Pole is
at the zenith
• Stars never rise or set
• Planets, Moon, and
Sun do rise and
set…Why?
Stars Rise and Set at the Equator
The Sky at Our Latitude
Circumpolar Constellations
• Circumpolar constellations never set.
• Circumpolar constellations change with
latitude… sky changes with latitude
The Sky at Southern Latitudes
Different sets of constellations are visible in
northern and southern skies.
The Altitude of the celestial pole (Polaris) =
your latitude
Counter-Clockwise
Rotation
Northern Hemisphere
Clockwise
Rotation
Southern Hemisphere
The Altitude of the celestial pole (Polaris) = your latitude
Tilt of Spin Axis
Precession of the Earth’s Axis
North Celestial Pole Changes
Solar vs. Sidereal Day
An Earth Day
• Sidereal Day: 23 hr 56 min 4 sec
Motion relative to background stars
• Mean Solar Day: 24 hours
The average time between meridian crossings
of the Sun
• Apparent Solar Day: varies
The actual time between the meridian
crossings
Rising and Setting
ofofthe
theMoon
Sun
Movement
of Stars
The Seasons
Changing
Constellations
through the sky
The Ecliptic
• The apparent path of the sun in its yearly
motion around the sky.
• The projection of the Earth’s orbit on the sky.
• The plane of the Earth’s orbit.
• These three definitions are equivalent for one
of the most important reference lines on the
sky.
The Ecliptic
Ecliptic on the Celestial Sphere
The Equinoxes and Solstices
• Tilt of the spin axis is related to the seasons
• Effect of tilt on climate is not instantaneous
Precession of the Earth’s Axis
North Celestial Pole Changes
Why do we
have
seasons?
Earth’s rotation
• The Earth rotates on its axis
(imaginary vertical line around
which Earth spins) every 23
hours & 56 minutes.
• One day on Earth is one
rotation of the Earth.
• Day on Earth is when our side
of the Earth faces the sun.
• Night on Earth is when the
side of Earth we are on faces
away from the sun.
Earth’s revolution
• It takes the Earth 365
days (or rotations) to
travel or revolve around
the Earth once.
• This is called a year.
Why do we have seasons?
• The Earth’s orbit around the
sun is NOT a perfect circle.
It is an ellipse.
• Seasons are not caused by
how close the Earth is to
the sun.
• In fact, the Earth is closest
to the sun around January 3
and farthest away from the
sun around July 4.
Ellipse
Why do we have seasons?
• Seasons are the result of the
tilt of the Earth's axis.
• Earth’s axis is tilted 23.5°.
• This tilting is why we have
SEASONS like fall, winter,
spring, summer.
• The number of daylight
hours is greater for the
hemisphere, or half of Earth,
that is tilted toward the Sun.
Why do we have seasons?
• Summer is warmer than winter (in each hemisphere)
because the Sun's rays hit the Earth at a more direct
angle during summer than during winter
Why do we have seasons?
• Also the days are much longer than the nights
during the summer.
• During the winter, the Sun's rays hit the Earth at
an extreme angle, and the days are very short.
These effects are due to the tilt of the Earth's
axis.
Seasons…in a nut shell
Solstices
• Solstices occur twice a year, when the tilt of the Earth's axis is
oriented directly towards or away from the Sun, causing the
Sun to appear to reach its northernmost and southernmost
extremes.
• Winter solstice is the shortest day of the year. In the Northern
Hemisphere. It occurs on December 21 and marks the
beginning of winter.
• The Summer Solstice is the longest day of the year. It occurs
on June 21 and marks the beginning of summer.
Tyrrhenian Sea and Solstice Sky
Credit & Copyright: Danilo Pivato
SOLSTICE
• During the winter the Northern
Hemisphere day lasts fewer than
12 hours and the Southern
Hemisphere day lasts more than
12 hours.
• During the winter solstice, the
North Pole has a 24-hour night
and the South Pole has a 24-hour
day.
• Sunlight strikes the earth most
directly at the Tropic of
Capricorn.
http://k12.ocs.ou.edu/teachers/refer
ence/solstice.gif
Equinoxes
• A day lasts 12 hours and a
night lasts 12 hours at all
latitudes.
• Equinox literally means
"equal night".
• Sunlight strikes the earth
most directly at the equator.
• This occurs twice a year.
http://k12.ocs.ou.edu/teachers/reference/equ
inox.gif
Equinox
• The vernal (spring)
equinox occurs March
21.
• The autumnal (fall)
equinox occurs
September 21.
The Earth's seasons are not caused by the
differences in the distance from the Sun
throughout the year.
The seasons are the result of the tilt
of the Earth's axis.
I know this is a repeat, but it is important that
you understand this idea. Many Americans,
including Harvard graduates, do not know what
causes seasons!
http://www.nmm.ac.uk/uploads/gif/seasons-full.gif
How the Sun’s location affects the seasons:
•The angle of the sun’s rays:
•In the summer it passes closer to overhead and therefore shines more directly
on the summer hemisphere
•The time the Sun is up
•In the summer it spends more than 12 hours above the horizon.
•The seasons are NOT due to the slightly elliptical shape of the Earth’s orbit and the
fact that it is slightly closer to the Sun during part of the year.
•Test of that hypothesis: If the distance were the cause, then when it was summer
in the northern hemisphere, what season would it be in the southern hemisphere?
SUN
(From our Text: Horizons, by Seeds)
67
Special Locations on the Earth
•How close to the North Pole do we need to go before the Summer Solstice sun
becomes a “circumpolar star” and is above the horizon all day?
•Within 23.5o of the pole: THE ARCTIC CIRCLE
•How close to the equator do we need to get before the Summer Solstice sun passes
directly overhead rather than somewhat to the south:
•Within 23.5o of the equator: The TROPICS
(From our Text: Horizons, by Seeds)
68
Why are the planets found near the ecliptic?
•The ecliptic is defined by the plane of the Earth’s orbit around the Sun.
•If the other planets are always found near the ecliptic, they must always be
located near the plane of the Earth’s orbit – at most slightly above or below it.
•The planes of their orbits around the sun must almost match the Earth’s.
•Their slight motions above and below the ecliptic means the match isn’t
exact. (Their orbits are slightly tilted relative to ours.)
From our text: Horizons, by Seeds
69
Superior vs. Inferior Planets
•Superior planets (Mars, Jupiter, Saturn, Uranus, Neptune, Pluto)
have orbits larger than the earth and can appear opposite the sun in
the sky. They can be up at midnight. Never show phases.
•Inferior planets (Mercury, Venus) have orbits smaller than earth
and can never appear far from the Sun. They form “morning stars”
or “evening stars” visible a little before sunrise or after sunset. Show
phases.
From our text: Horizons, by Seeds
70
Apparent Motion of Inferior Planets
•Inferior planets (Mercury, Venus)
have orbits smaller than earth and
can never appear far from the Sun.
They form “morning stars” or
“evening stars” visible a little
before sunrise or after sunset.
•If the inferior planet sets before
the Sun it won’t be visible in the
evening sky. Look for it instead in
the morning sky and vice versa.
From our text: Horizons, by Seeds
71
Ecliptic Constellations & Zodiac Signs
• A 16º - 18º band of 12 constellations around
the sky entered on the ecliptic (apparent path
of the sun on the earth as the earth revolves
around it).
• Aries, Leo, Sagittarius, Taurus, Virgo,
Capricorn, Gemini, Libra, Aquarius, Cancer,
Scorpio, and Pisces.
Ecliptic Constellations
Astronomical Factors that Influence
Climate
• Eccentricity
• Obliquity
• Precession
Eccentricity
Eccentricity =
(distance from focus to
center) / (length of
semimajor axis)
Eccentricity of Earth’s
orbit varies from 0 to
0.05, with 100-kyr, 400kyr and 2 Myr
periodicities.
Fall 2007
Obliquity
Obliquity (i.e., tilt) of Earth’s axis varies from 22° to
24.5°, with a 41-kyr periodicity.
Fall 2007
Precession
The Earth’s axis precesses, or wobbles, with
periodicities of 19 kyr and 23 kyr.
Fall 2007