ASTRONOMY
UNIT 1
DISCOVERING THE NIGHT SKY (CHAPTER 1)
I.
Learning Targets
1. What is in the universe?
2. How are constellations used to locate stars and other celestial
objects?
3. Why are different stars present in the night sky on different
nights?
4. How is the motion of the Earth related to the apparent or real
motion of the Sun, moon and stars?
5. Why are there lunar phases?
6. How and when do eclipses occur?
7. Explain the importance of distance measurements in astronomy.
8. Describe the nature and use of constellations.
9. Define the elements of the equatorial coordinate system on the
celestial sphere.
10. Define two solstices and two equinoxes.
11. Explain the orientation of the ecliptic on the celestial sphere and
how it produces seasons on the Earth.
12. Describe the daily and yearly motions of the Earth.
13. Describe what precession is, what effect it has on our
observations of stars, and why it occurs.
14. Explain how lunar phases are controlled by the relative positions
of the Sun and the Moon.
15. Explain when and why solar and lunar eclipses occur and
why there are not such eclipses every month.
II.
DISCOVERING THE NIGHT SKY CORE PRINCIPLES
1. The universe is composed of exceedingly large and small objects
separated by exceedingly large and small distances.
2. We see different stars and constellations because we face different
parts of the universe as the nights pass and the seasons change. This is
caused by the motion of the Earth.
III.
KEY TERMS
Angular diameter
Celestial equator
Celestial sphere
Modern Constellation
Declination
Eclipse path
Lunar eclipse
Lunar phase
Penumbra
Revolution
Sidereal day
Solar day
Summer solstice
Time zone
Umbra
Winter solstice
Zodiac
Asterism
Autumnal equinox
Celestial pole
Circumpolar star
Ancient Constellation
Diurnal motion
Ecliptic
Line of nodes
Partial eclipse
Precession
Rotation
Sidereal month
Solar eclipse
Synodic month (lunar month)
Total eclipse
Vernal equinox
Zenith
Horizon
Astronomy
The Night Sky
Chapter 1 Topic Outline
1. Scale of the universe
a. powers of ten
b. astronomical units (AU)
c. light year (ly)
d. Parsec
e. four fundamental forces and matter
2. Constellations and asterisms and some prominent stars
a. definition of asterism
b. historical and modern definition of constellation
c. 13 constellations of the zodiac
d. the location of the sun, earth and zodiac constellations
e. circumpolar stars
3. Celestial Sphere
a. model based upon geocentric view of the universe
b. used to locate stars, constellations and asterisms
c. ecliptic, celestial equator and celestial poles
d. determining location using latitude-longitude, altitudeazimuth, and declination-right ascension coordinates
e. zenith and local meridian
4. The Earth’s motion and Time
a. rotation, revolution, precession and nutation
b. sidereal period
c. Sidereal day vs. mean solar day
d. Sidereal year vs. tropical year
e. Time zones
f. The seasons
g. The tropics, polar circles and equator
5. The Moon
a. the orbit of the moon
b. duration of sidereal vs. synodic lunar month
c. dark side vs. far side of the moon
d. eight phases of the moon
e. line of nodes and the orbital plane of the moon
f. lunar vs. solar eclipses
g. umbra and penumbra
h. annular eclipse
Astronomy Review Questions 1-29
1.
What do an AU and a light year measure? What is the difference
between an AU and a light year?
AU measures the average distance from the Earth to the Sun
Light year measures the distance light travels in one year.
An AU is a much smaller distance than a light year
2.
What is the difference between an asterism and a constellation?
An asterism is a prominent pattern of stars with a common historical name
yet is not part of a constellation. These may fall entirely within a single
constellation or may extend into two or more constellations.
A constellation is a region or a group of stars. A constellation cannot
extend beyond its region boundary, but an asterism can.
3.
What are the historical and modern definitions of a constellation?
Historical—a figure suggested by the orientation of the stars that comprise
the constellation
Modern—88 separate but conterminous regions, each with contains one
of the constellations.
4.
Define the ecliptic, celestial equator and celestial poles.
The ecliptic is the apparent path of the Sun around the Earth or the Earth’s
orbital plane around the Sun.
The celestial equator is an imaginary extension of the equator which
intersects the ecliptic at 23.5 degrees.
The celestial poles are imaginary extensions of the Earth’s north-south
rotational axis
5.
Along which plane associated with the celestial sphere can the
zodiac constellations be observed?
The ecliptic plane
6.
Define these pairs: altitude and azimuth and declination and right
ascension. What function does each pair provide to the astronomer?
Altitude: the angle above the horizon
Azimuth: the compass direction (360 degrees) related to the equator
 Declination is related to altitude and latitude
 Right ascension is related to longitude and azimuth
7.
What are the zenith and the local meridian? What is the
significance of each?
Zenith is a point directly overhead on Earth and the local meridian is an
imaginary longitude line that bisects the northern hemisphere from south
to north
8.
What does it mean for the sun to be “in” a zodiac constellation?
It means that the sun is in front of the zodiac constellation with respect to
the Earth, which means the zodiac is not visible.
9.
What is a circumpolar star? Where on earth are the least number of
circumpolar stars observed? The most?
A star that never sets below the horizon during the night. The least are
near the equator, the most near the poles.
10.
Explain the four motions of the earth and the timing for each.
Precession—26,000 yr
Nutation— roughly 18 yr
Revolution—365.25 days
Rotation—24 hours
11.
What is a sidereal period?
The length of any cycle as measured with respect to the background
stars.
12. Explain the difference between the sidereal day and the solar day.
Sidereal day is one complete rotation of the Earth as viewed from the
background stars. (sidereal day is 23 hours 56 minutes)
Solar day is the rotation of the Earth from solar noon to solar noon.
(mean solar day is 24 hours)
Why is a solar day sometimes referred to as a mean solar day?
One solar day is not always exactly 24 hours, so we use the mean of
the Earth’s diurnal motion.
13. What is diurnal motion? Describe what is observed in the sky during
the Earth’s diurnal motion.
diurnal means daily… the daily rotational motion of the Earth: day, then
night.
14. Why does the night sky appear to be different each night when
observing at the same time?
The stars rise approximately 4 minutes earlier each night.
Why does the night sky look the same when you observe it after one
full sidereal day?
This is because the Earth only takes approximately 23 hours and 56
minutes to return to the same view as the night before, which is the
same as one sidereal day.
15. If you want to find a particular star in the exact location you
observed it on the previous night, when should you look for it?
Approximately 4 minutes earlier that the previous day.
16. What led to the development of time zones?
Transportation and travel required time to be standardized.
How were the time zones determined?
The rotation of the Earth after 1 hour--fifteen degrees x 24 hours = 360
degrees in 24 hours
17. Approximately how many degrees longitude separate each time
zone?
15 degrees.
19. What is the cause of the seasons?
The seasons are caused by the 23.5 degree tilt of the Earth on its
rotational axis. As the Earth revolves around the Sun, some parts of the
Earth receive more direct sunlight and other parts receive less.
20. How does the winter turn into summer?
As the Earth makes one-half revolution around the Sun, the angle of the
Sun in the sky will rise above the lowest angle possible, increase past
equinox until it is at the highest angle possible.
21. Explain the difference between an equinox and a solstice. How
many of these occur per year and when?
Equinoxes are points along the Earth revolution where all points of the
Earth receive 12 hours of daylight and 12 hours of night (solar noon to
solar midnight). There are two equinoxes per year (vernal and
autumnal).Solstices occur when the angle of the Sun is either at the
highest (summer solstice) or lowest (winter solstice) as it revolves.
22. Explain the significance of the Tropics of Cancer and Capricorn, the
Arctic and Antarctic circles, and the equator to the path of the Sun.
The tropics represent the northernmost or southernmost latitude at
where the Sun will reach zenith at least once per year.
The polar circles mark the latitude furthest (equatorward) from the
poles where the Sun will neither set nor rise at least once per year.
The equator marks the point on the Earth where the Sun will be at zenith
twice per year.
23. Does the Sun always rise directly east and set directly west along
the horizon? Explain.
No. The Sun will only rise and set due east and due west (90 degrees to
270 degree azimuth) two time per year at the equinoxes.
24. Does the Sun always rise to the same altitude along the local
meridian during each day? Explain.
No. The angle of the Sun at the local meridian changes each day as
the Earth revolves around the Sun.
25. What is an analemma? What causes an analemma?
An analemma is a figure 8 pattern that is traced out by the daily
changes in the altitude of the Sun throughout the year. It is not a
straight line because the Earth does not orbit at the same speed, due
to its slightly elliptical orbit.
26.
What is the minimum and maximum altitude of the Sun during the
passage of a year in Fort Thomas? What does the midway point between
the minimum and maximum altitude represent?
27.
What is a leap year and why is necessary?
A leap year is a year that has 366 days. A year technically is 365.25 days
long, so every four years, one day is added to the calendar (Feb 29) to
compensate for this.
28.
What is meant by the precession of the equinoxes? How does this
affect the direction of the Earth’s north axis (or north celestial pole).
Precession of the equinoxes means an advancement of the two
intersection points along the celestial equator and the ecliptic due to the
Earth’s precessional motion. As the Earth precesses, its rotational axis
points to different areas of the night sky.
29.
Will Polaris always be the north star? Explain.
No. Polaris will not always be the north star. Precession causes the
eventual movement of the Earth’s rotational axis away from Polaris.
Thuban and Vega are also “north stars.”
Constellations and Asterisms
Two ways to define a constellation:
 Historical Definition—A group of stars that outlines a
familiar object or pattern originally recognized by the
ancient “astronomers.”
An asterism is a prominent pattern of stars with a common
historical name yet is not part of a constellation. These may fall
entirely within a single constellation or may extend into two or
more constellations.
 Modern Definition—An entire region in the sky and all the
objects within that region. The celestial sphere is divided
into 88 unequal regions or constellations. (the atlas of the
celestial sphere)
Stars/Constellations/Asterisms to know:
Stars
Polaris, Arcturus, Spica, Procyon, Sirius, Betelgeuse, Deneb,
Altair, Vega, Rigel, Aldebaran, Antares, Pollux, Castor, and
Proxima Centari.
Asterisms
Winter Triangle (Procyon, Sirius and Betelgeuse)
Summer Triangle (Deneb, Altair, and Vega)
Diamond of Virgo
Big Dipper
Little Dipper
Constellations
Orion
Bootes
Zodiac “Signs”
Draco
Cassiopeia
Celestial Sphere, the ecliptic and the sky coordinates
The Celestial Sphere is an imaginary hollow sphere that
surrounds the Earth and is divided into 88 unequal regions
called constellations upon which the stars are attached. It is a
remnant from the geocentric representation of the universe,
yet it remains a useful atlas for navigating the heavens.
The stars appear fixed, but that is merely a function of their
remoteness. Their remoteness from Earth and the shortness of
human lifespan does not make it possible to see their motion
nor perceive their relative distances.
Finding your coordinate location on Earth requires use of
latitude (east-west) and longitude (north-south) lines.
To determine the location of any star on the celestial sphere,
we use declination and right ascension. Declination is
equivalent to latitude and is measured 0 to 90 degrees north or
south of the Celestial Equator. Right ascension is equivalent to
longitude and is measured 0 to 24 hours around the Celestial
Equator.
The zero and 12th hours of right ascension are located at two
places where the Sun’s annual path (or ecliptic plane)
intersects the celestial equator, which are the vernal and
autumnal equinoxes. Zero hour is at the vernal equinox.
The ecliptic is the annual path of the Sun traced upon the
celestial sphere. The ecliptic plane is oriented at 23.5 degrees
to the celestial equator. The ecliptic can also be defined as
the Earth’s path around the Sun.
To observe this relationship, load Starry Night and at the tool
bar open up Favorites—Companion Book—2.2 Co-ordinates—
Intersection. Rotate the celestial equator and observe where
the ecliptic intersects this imaginary plane.
The stars and other astronomical objects are theoretically
imbedded in the celestial sphere; therefore their coordinates
do not appear to change (at least over human lifespan).
When the celestial sphere rotates, so do right ascension and
declination (the celestial meridian indicates the location of RA
= 0 hours and intersects the vernal equinox). The stars appear
to travel in sync with the motion of the celestial sphere each
night (in truth, it is the rotation of the Earth that causes this).
The horizon co-ordinate system uses the altitude-azimuth
system. Altitude is measured from the horizon (at zero degrees)
and ends at 90 degrees at zenith. Zenith represents the point
directly overhead. Local meridian is an imaginary line that runs
north-south through the zenith. The horizon is located in all
directions. Azimuth is measured along the bearing of the
horizon, starting with 0 degrees at the north, followed by 90
degrees (east), 180 degrees (south) and 270 degrees (west).
This is a local system because your coordinates depend upon
you personal location.
The Sun and the Seasons
Astronomy
Unit 1
Discovering the Night Sky
Interesting Statements and Thoughts
For how the sky moves: As the night passes and as the seasons change,
we face different parts of the universe and see different stars and
constellations. These changes happen because of the motions of Earth as
it spins on its axis and orbits the Sun. (Starry Night Notes)
It is helpful to remind the students that the observational (passive) nature
of astronomical investigations means that all direct information about the
physical conditions existing on celestial objects and about their past and
future conditions must be extracted from an understanding of the nature
of atoms and the constituent parts of atoms, the smallest entities of the
universe. Thus, astronomers must frequently deal with very small numbers
as well as very large ones. (Instructor’s Manual: Chapter 1)
Until the past few years, during which satellite navigation has become
common, transoceanic navigation was possible only on the basis of an
understanding of the sky positions of celestial objects and the variation of
those positions with time. (Instructor’s Manual: Chapter 1)
In solar eclipses, the Sun disappears. In lunar eclipses, the Moon
disappears. It might be interesting to note that, as a result of the angular
size of the Sun and its apparent rate of motion along the ecliptic, at least
one eclipse of each type must occur during each eclipse season. If only
one eclipse occurs, it is likely to be a central eclipse (total or annular solar
or total lunar). If more than one eclipse occurs during an eclipse season, it
will probably be noncentral (partial solar or partial or penumbral lunar).
(Instructor’s Manual: Chapter 1)