ASTR 1230 (O'Connell) Lecture Notes
1. INTRODUCTION TO THE NIGHT SKY
Astronomy is primarily an observational science
. It is driven
more by new observational discoveries than by interpretive insights.
Few important astronomical discoveries were predicted, and many were
. The human imagination has never been a match
for the universe.
Astronomical discovery began with the simplest of observations: people
looking at the night sky and trying to understand what they were
seeing -- sometimes in awe and wonder, sometimes in fear of the
unknown powers at work in the heavens.
In the past, most people were well acquainted with the basic features
of the night sky. We are unfamiliar with the sky in modern times
mainly because of the advent
, which makes it difficult to see the night sky in
urban areas (and also unnecessary to know the sky as a
This lecture introduces you to the basic features of the night sky which
are visible to the unaided eye and prepares you for the Constellation
A. NAKED-EYE ASTRONOMY
observations (i.e. without optical aid from lenses or
mirrors) were the only kind
possible for most of human
history! Telescopes were not invented until 1608 AD.
Nonetheless, great accomplishments were possible without telescopes,
The human eye
- Determination of the size and shape of the Earth and Moon; origin
of eclipses (Greeks,
500 BC - 200 AD)
- Discovery of Earth's motion around the Sun (Copernicus, 1513)
- Kepler's Laws of Planetary Motion (1609, based on Tycho's
observations), which led to Newton's Laws of Motion and the
modern scientific revolution
is a remarkably capable
astronomical instrument. Follow this link for background information
on its function and the important observing considerations of
and averted vision
B. MOTIVATIONS TO OBSERVE THE SKY
The concerted study of the sky started long before modern science
arose ca. 1500 AD. How long? We don't really know---probably at
least 8000 years before. Almost every human society, pre-literate or
literate, whose culture we have been able to sample in detail shows
some awareness of celestial phenomena --- if not in the form of
written records then in other ways, such as the alignment of buildings
or other structures to cardinal directions.
- Curiosity---the most enduring motivation for trying to understand nature
- Fear/Religious Belief:
- One example:
Astrology. This is
the idea that the motions of the Sun, Moon, and planets against the
stellar background can be used to predict the future and can influence
human personalities. It derived from the ancient belief that these
objects are living gods, who betray their intentions by their
movements. This was obviously a powerful motivation for observing the
sky. In the carving shown at the right the Egyptian Pharaoh Akhenaton
(ca. 1350 BC) and his family are communing with the Sun god Aten, the
source of Akhenaton's power.
As our scientific understanding grew, astrology lost its interest for
most people. We realized that the Sun, Moon, and planets are
inanimate objects, moving in highly regular and predictable
patterns in response to the well understood force of gravity.
Constellations and the Zodiac were recognized to lack physical
significance (see below). New planetary bodies were discovered that
astrologers had somehow failed to detect. Statistical tests showed no
correlation between "sun signs" and personality or personal history.
There is no evidence, theoretical or empirical, for astrology.
Astrology lingers only as a form of pseudo-science and popular
entertainment. But it, and related ideas, did play an important
historical role in encouraging the systematic observations of the sky
that ultimately led to the scientific interpretation of the solar
Study of the sky quickly reveals the existence of regular cycles in
of the Sun, Moon, & planets. These became the central
concern of early astronomers because of their immense practical
- Navigation: on land & sea
- Time Keeping
- Calendar Keeping: tracking the date & seasons
For some early societies (e.g. the Polynesians, who sailed
thousands of miles across the uncharted ocean), these could
be critical survival technologies.
C. NAKED EYE MEASUREMENTS
Only a few kinds of measurements
are possible with the naked eye:
1. Angular Separations
Studying the geometry of the sky by measuring angles is the most basic
form of astronomy. Apart from time tracking, this was the only
accurate quantitative measurement possible before the
advent of modern instrumentation.
Measured angles can be
all-celestial ("sky", e.g. star-to-star) or celestial-terrestrial.
They can be between different celestial objects, between a
celestial object and a reference point on Earth, or across a
Modern Units: Degrees, minutes, seconds of arc
Full circle = 360 degrees of arc;
1 degree = 60 minutes of arc;
1 arcmin = 60 seconds of arc
Don't confuse these angular units with units of time! Always use
the "arc" terminology for clarity.
Angles subtended by a quarter at distance D:
[Note: the symbol ~ means "approximately"]
The bowl of the "Big
Dipper" is ~ 10 degrees long.
- 1 degree @ D = 56 in
- 1 arcmin @ D = 270 feet
- 1 arcsec @ D = 3 miles
Angular scales of "pan" of Big Dipper
The human eye has 1-2 arcmin resolution---i.e. it
cannot distinguish two stars separated by less than 1-2 arcmin.
The Hubble Space Telescope, for
better than 0.1 arcsec resolution; i.e. it can resolve a quarter at a
distance of 30 miles
"Hand-y" measuring scale (see illustration):
1 degree = width of index finger @ arm's length
10 degrees = width of closed fist @ arm's length
20 degrees = distance thumb to little finger on outstretched
hand @ arm's length
Example: the angular diameter of the Sun or Moon is about 0.5
degree. You can block out either with your index finger held @
arm's length. Try it!
The "Hand-y" scale is useful to remember as you orient yourself
to the night sky and compare objects you see there to star charts.
Astronomers quote star brightnesses on
the magnitude scale. This scale has roots in
star catalogs made by the ancient Greeks (ca. 150 BC). It was based
originally on simply ranking the stars by their apparent
brightness as seen with the unaided eye. Without instruments, this
kind of ranking is about the best observers can do.
Today, the scale has been quantified in terms of the light power
deposited by an individual star per unit area at the Earth and tied
to telescopic measurements made with electronic detectors.
The magnitude scale is logarithmic, open-ended, and runs
"backwards" (like a sports ranking scale):
Brighter objects have smaller magnitudes.
The brightest stars are about 0 magnitude; the faintest visible to the
naked eye are about 5-6 magnitude. The brighter planets and most
familiar stars have magnitudes in the range -4 to +2.
There are only 11 stars brighter than magnitude 1 visible from
Charlottesville but there are 1630 stars brighter than magnitude 5.
The faintest objects yet detected (by the Hubble Space Telescope)
are 30th magnitude, or over 1 billion times fainter than visible to
The human eye can make only rough measures of magnitudes; accuracy
was only possible after the invention of photography and
electronics. Magnitudes are discussed further in the notes on Stellar Astronomy.
3. Colors, Shapes
(in some cases)
Even crude measures of angles and brightnesses, if made
systematically over days, months, or years, immediately reveal the
presence of repeating time cycles in the motions of
the Sun, Moon, and planets. As mentioned above, these were important
for their practical value. But they also showed there was order in
the universe, even if the origin of the motions was mysterious. They
provided strong intellectual stimulus for investigations of the structure of
D. EASILY VISIBLE PHENOMENA
- STARS: form the backdrop or
"reference frame" against which other objects' motions are
measured. About 2000-5000 are visible to the eye (depending on
eyesight) over the whole sky. About 1000 are visible on a dark, clear
night from a given location. The brighter stars form conspicuous
patterns which seem unchanging (to the eye).
- Motion: stars move continuously in lockstep across sky
from East to West. Patterns come back to the same location in the sky
after slightly less than 24 hours. Star locations in the sky
at a given time of night
change systematically throughout the year.
- Can you see all the stars that exist? NO! There are vast
numbers of stars that are invisible to the eye. With our
8-in telescopes, we can see stars about 800 times fainter than the
naked eye limit---and there are 5 million of these over the whole
sky. But our star system (the Milky Way Galaxy) contains about
100 billion stars, and there are billions of galaxies!
- SUN: The most obvious astronomical
object (and most important for us!). Steady brightness. Has a slow
eastward motion relative to the stars (about 1o per
day; we have to infer the Sun's position since the stars are invisible
in daylight). Returns to the same place against the star background in
one year (365.25 days).
- MOON: Very bright, but much fainter than
the Sun. More rapid eastward motion relative to the stars
(about 13o per day).
Shows a drastic change in brightness and phase
(bright part as a fraction of a full circle) during each cycle, from
totally dark to fully illuminated. Takes 29.5 days to return to the
same phase (e.g. "full"). This is the cycle we have formalized in our
calendar as the month ("moonth"). There are 12 lunar cycles
Note: Moonlight was of enormous practical value before electrical
lighting was invented, so the phases of the Moon were closely followed
in earlier times.
- PLANETS: Less obvious. 5
bright, starlike objects; these have a slow, complex
predominantly eastward motion relative to the stars. In order of
speed of motions (fastest to slowest): Mercury, Venus, Mars, Jupiter,
Saturn. Two of these (Mercury, Venus) are always found relatively
near the Sun; others can be up to 180 degrees away. Although all the
planets have perceptible disks in telescopes, the naked eye cannot
Other, less conspicuous, features visible to the eye (with the modern
Interference: sky brightness
- METEORS: sudden streaks of light
in sky; typical rate 5-10 per hour. These are small pieces of ice or
rock, burning up in Earth's atmosphere. Often called, but are
definitely not, "falling stars." Concentrations of debris (from
comets) produce "meteor showers,"
e.g. the Leonids, lasting
up to several days.
- COMETS: occasional (once every
few years) flamelike objects moving slowly through the sky over
several weeks. Comets are large (~ 1 km) chunks of ice, usually on
very elongated orbits, that begin to release gas and dust when they
become heated by the Sun on entering the inner Solar System.
E.g. Hale-Bopp (1997), which is visible
in the night sky panorama at the top of this page.
- STAR CLUSTERS: compact groups of
stars, all formed together. E.g. the Pleiades and Hyades. About a
dozen are visible to the naked eye, but scores are visible in small
telescopes. The largest are the "globular clusters"
(e.g. M13, shown at the right),
containing up to 100,000 stars. Since all stars in a given cluster
have the same age, clusters became the keys to
understanding stellar evolution.
- DIFFUSE NEBULAE: clouds of
interstellar gas lit up by hot stars. A handful are visible without
telescopes. E.g. the Great Nebula in
Orion. These mark the birthplaces of young stars. Smaller
"planetary nebulae" mark the death of old stars.
- MILKY WAY: appears as a faint,
diffuse band of light arcing across the sky. It is the combined light
of millions of distant stars in the plane of our
own Galaxy (a massive, flattened star
system), seen edge-on.
- EXTERNAL GALAXIES: other large star
systems like our Galaxy. 4 are visible to the eye, 2 of these are in
the northern hemisphere. E.g. the Andromeda Galaxy---the most
distant object you can see without a telescope. It is 2 million light
years from us, meaning that the light you see from it tonight started
its journey 2 million years ago. For more discussion of
this lookback effect, see
this ASTR 1210 page. Hundreds of galaxies are detectable in
: your view of
the sky is strongly affected by background sky light
natural and man-made. During the day, sunlight
scattered by molecules
in the Earth's atmosphere produces the "blue sky" that completely
obscures almost all other cosmic objects from our eyesight (though the
Moon is often easy to see in daylight, and you can detect Venus if you
know where to look). Likewise, near full moon, only the brightest
objects are visible in the night sky because of atmospheric scattering
. City lights
create enough local "light pollution"
rival or exceed the effects of the full moon.
E. ORIENTATION IN THE NIGHT SKY
It is important, but difficult, to try to visualize your situation
when you look into the sky at night. You are standing on a
spherical, spinning, moving planet. What you can see in the
sky is determined by the Earth's orientation and position in its orbit
around the Sun. Your view of the sky is always made in your "local reference
The local "horizon plane" is the (ideal, imaginary) plane
"tangent" to (just touching) the Earth at your location; you can see
objects above the plane but not below. The horizon plane sweeps
across the sky as Earth spins; it determines the rising and setting of objects.
The horizon plane is different at each location on Earth.
Sphere (CS) is a geometric construct used to
vizualize the positions of astronomical objects.
The celestial sphere is an imaginary hollow sphere centered on Earth.
It is shown shaded in the illustration above. Directions to key
orienting positions and to each astronomical object are imagined to be
marked on the surface of sphere.
Exactly one-half of the celestial sphere (one hemisphere) is always
above your local horizon.
The zenith is the point directly overhead on the CS. It
is equidistant from all points on the horizon.
The north and south celestial poles are the points on the CS
where the projection of Earth's rotation axis pierces the CS. These
are fixed points.
The celestial equator is the outward projection of Earth's equator to
the CS. It is a "great circle" (i.e. a circle drawn on the celestial sphere
with its center coincident with the center of the sphere).
Your meridian is the great circle on the CS that passes
through both celestial poles AND your zenith. It runs from due
North to due South, splitting the celestial hemisphere in half.
The meridian is not marked on the drawing above; instead, see this figure.
motion: The daily spin
of the Earth on
its axis produces an apparent counter-rotation
of the CS and its
"attached" stars across your local sky. One complete rotation around
its axis with respect to the stars takes 23h56m (note--not quite 24
hours). The Earth rotates eastward
, so the sky
appears to rotate continuously westward
. Objects "rise" in the
east or "set" in the west when they cross your local horizon plane.
See the figure above.
An astronomical object is said to transit when it crosses the
meridian. At this time it is farthest from the horizon and
highest in the sky.
F. NIGHT SKY SIMULATIONS
In class, we will use the "Starry
Night" planetarium software
to simulate the appearance and motions
of the night sky. This is an excellent app that is very useful to
purchase if you're more serious about observing.
The stars are not uniformly distributed on the sky. Many of the
brighter stars form conspicuous patterns
. To the eye, the
patterns seem unchanging: the stars appear "fixed"
one another. Historically, the patterns were very useful for
orientation, navigation, and determining the time of night or the date
and so were given names
(The human brain is wired for this kind of pattern
recognition: people who could recognize the tiger lurking in the
forest shadows survived better than those who could not.)
Each named pattern is called a constellation
Constellations are associated with mythological figures, animals, instruments,
and other features from the natural, human, or religious worlds.
An example of the stick-figure pattern associated with "Orion the
hunter" is shown at
- It was natural for people to seek deeper meaning in these remote,
silent, but majestic figures at the limit of the visible world. So,
the constellations often were given important mythological or
religious associations. These were, however, strongly
culture-dependent, and the same patterns can have very different
interpretations in different cultures.
- Some associations we recognize today are very ancient, going back
to around 2000 BC. For example, we can see Leo the lion and Scorpio
the scorpion carved on the Mesopotamian "boundary stone" from about
1100 BC shown at the right (click for enlargement).
Some are new since 1600 AD (e.g. Microscopium).
Few resemble their namesake closely.
- The Greeks (500 BC - 200
AD) added rich mythological associations and fanciful stories
(e.g. for Taurus, Perseus, Andromeda). Aratus' epic poem
Phaenomena (ca. 270 BC) describes the constellations as a
memory aid to navigators. Ptolemy's Almagest (135 AD) listed
positions of 1022 stars in 48 constellations. His catalogue was used
for the next 1400 years(!)
- The Romans inherited the Greek myths. The names of most
constellations are Latin.
- Arab astronomers were active 700-1600 AD, and many modern star
names have Arabic roots: Algol ("the demon star"), Aldebaran ("the
follower"), Betelgeuse ("armpit of the Central One").
- 1600+: new constellations were added to fill in blanks, mostly
in the Southern Hemisphere. E.g. Telescopium, Pyxis (compass). Elaborate &
beautiful printed atlases of classical associations appear (an
illustration of the north polar constellations from a 1660 atlas by
Cellerius is shown below).
- In 1930 the International Astronomical Union established formal
boundaries for 88 constellations that cover the entire celestial
sphere. 74 are visible from Charlottesville. Click here for an
illustration of the boundaries for Orion.
Significance of the constellations:
- Constellations have no physical significance. The
associations are arbitrary & man-made. Constellations are not natural
groups of stars. The fainter stars in a constellation
don't participate in the pattern
(as illustrated here
in the case of the Orion region.) Stars in a given constellation lie near
the same line of sight from Earth but are not necessarily close
to one another in space. (Click here for an
illustration in the case of Orion.) Shapes are specific to the Earth's
location in 3-D space (a fact not recognized when ancient astrological
systems, which attached significance to the shapes, were
Click here to see a
modern, deep telescopic image of Orion that reveals the many beautiful
fainter features lying in this direction but mostly invisible to
the naked eye.
- The stars are all moving with respect to one another, even
though the changes would not be apparent to the eye except over
thousands of years. Therefore, constellation patterns are
transitory. The changing appearance of the "Big Dipper" (part of
Ursa Major) now and 100,000 years from now is shown below. Here is an
animation of the motion of the Big Dipper stars over 200,000 years.
- Modern astronomers use constellations only as a convenient
"address" to roughly locate objects in the sky.
- The zodiac
("circle of animals") is the set of constellations through which the
Sun passes in the course of a year. The Sun's path is called the
ecliptic, and the Moon and bright planets also stay near this
path. Given the modern boundaries of the constellations, there are 13
ecliptic constellations. But in classical astronomy (and current-day
astrology) there are only 12---one for each month. The ecliptic, and
hence zodiac, is determined by the accidental orientation of the plane
of Earth's orbit. Most zodiacal constellations are faint and
uninteresting (e.g. Libra, Capricorn, Aquarius).
- Constellation names: Latin, often translated from Greek
- Star names: the brighter stars have "common" names derived
from a mix of Greek, Latin, & Arabic. Most stars brighter than 20th
magnitude have catalog numbers (most recently recorded by
space observatory). Fainter stars---i.e. most stars---are
Bright stars have many synonyms since they appear in many catalogs
E.g: the star at the mouth of Canis
Major (the large dog); brightest star in the sky
Bayer's Uranometria (1603)
assigned Greek letters:
alpha, beta, gamma, etc. to stars, usually in order of brightness, in
each constellation; about 1300 stars have Bayer designations. But
because of errors (or, in some cases, actual changes in brightness),
the order is not necessarily correct today. E.g. Alpha Ori
(Betelgeuse) is fainter than Beta Ori (Rigel).
Flamsteed (1712): numbered stars in each constellation in "Right
Ascension" (west to east) order: e.g. 61 Cygni, 40 Eri, etc.; used
today for brighter stars without Bayer designations.
SAO catalog numbers are needed to locate stars in the
database stored in your Celestron telescope computers.
Modern digital catalogs contain up to one billion stars (but this is still
only a small fraction of all stars in our galaxy). Most list objects
in Right Ascension order.
- Sirius ("the scorched one" in Greek) --- common name
- = Alpha Canis Majoris --- Bayer listing
- = 9 Canis Majoris --- Flamsteed listing
- = HD 48915 --- Henry Draper Catalog listing
- = BD -16 1591 --- Bonner Durchmusterung listing
- = SAO 151881 --- Smithsonian Astrophysical Observatory (SAO) listing
- = 0645-16 --- Right Ascension/Declination coordinate listing
- There are also many catalogues of non-stellar objects,
such as nebulae, star clusters, and galaxies. The three you will most
frequently encounter are the New General Catalogue ("NGC"), the Messier Catalogue
("M"), and the The Caldwell
Catalog of Deep-Sky Objects (an updated version of the Messier
H. COMPLETING LABORATORY 1 AND THE CONSTELLATION QUIZ
After you have had time to learn the constellations, you will be
examined individually by a TA on your knowledge of the sky. You will
be expected to be able to identify 20 constellations, bright stars, or
other features of the sky.
- Read the Lab 1 chapter in the Manual.
- The Lab will be given on the first two usable nights on or
after the night of this lecture.
You can come to either night. Check the Observatory status via the
recorded message (924-7238) after 6:30 PM.
- You will work in groups. Each group will be assigned a set of
stars and constellations to learn from the required list in the
Lab 1 description. After a group has mastered these, they
will teach the other groups.
- Advice on learning constellations:
- Review and disregard the misconceptions about the sky that are
listed in Ten Things to Forget
- Because not all objects on the Constellation Quiz are identified by
name on your Sky Wheel, consult the
following sky charts, which contain labeled objects for the Quiz:
- Find the "North Star," Polaris (in Ursa Minor = "the Little
Dipper"). Polaris is near to, but not exactly coincident with, the
North Celestial Pole. It is not a very bright star. Easiest
method is to use the two "pointers" at the end of the bowl of Ursa
Major (the Big Dipper). See the sketch below. Alternatively, if Ursa
Major is low in the sky, you can use the stars in Cassiopeia as
pointers. See this
- Then orient yourself N/S/E/W. When you face Polaris, your right
arm is toward the East and your left is toward the West.
- Find your zenith and your meridian.
- Orient your sky wheel to match the time of night by
aligning the date and time tickmarks. (Note: time marked is always
Standard---not Daylight Savings--- time.) Using the wheel and pattern
recognition, "hop" to other bright stars and groups. Use brighter,
more conspicuous constellations to locate others. Practice locating
all the constellations, stars, and other features on the Manual list
for this semester.
- Difficulties with using charts:
- The scale of the real sky is very different from a chart
- Brightnesses can't be represented well on paper. Relative
brightnesses of real stars look very different.
- Poor sky transparency, city lights, or moonlight reduce visibility.
- Practice using averted vision to find fainter
objects: rather than looking directly at the target, stare about 15
degrees away, but concentrate on the target's location. (This puts the
image on a more sensitive part of your retina.) You can find more
about the capabilities of the naked
- The Constellation Lab (Lab I) will take place on the next two
usable lab nights, starting Monday, 9/5. You must attend
one of the two sessions. Whether the Observatory will be open will be
announced on the recorded message (924-7238) by 6:30 PM. We will
also send an email alert to the class.
It would be a good idea to become familiar with the various
forecasts on the ASTR 1230 Weather Page
in planning for this and later labs.
- To prepare for the Constellation Lab: read Lab 1 description
(Secs. 1.1 to 1.8); consult constellation descriptions (Sec. 1.9) as
- Download, print, & read lecture notes for Lec 1
- Take Review Quiz--Week 2 on the Collab site.
April 2021 by rwo
Text copyright © 1998-2021 Robert W. O'Connell. All rights
reserved. Opening fisheye lens picture of comet Hale-Bopp and night
sky from Ujue, Spain, April 1997, copyright © J. C. Casado. Orion
at horizon picture by B. Tafreshi. Illustrations of the celestial
sphere copyright © by Nick
Strobel. Image of M13 copyright © by J. Ware. These notes
are intended for the private, noncommercial use of students enrolled
in Astronomy 1230 at the University of Virginia.