Study Guide for Test #1

Dr. J. R. Webb

Solar System Astronomy


Chapter 1 Our place in the Universe.


Astronomy is an Observational Science- we cannot usually interact with our subjects.


The scientific method:  1.  Observe some phenomena

                          2.Propose a hypothesis

                          3. Make a prediction based on the hypothesis

                          4. test the prediction


    Some basic theories:  Newtonian mechanics, Theory of Relativity, Quantum Theory

                                       Theories are ALWAYS subject to revision based on new data.


    Basic math:  Angular measure: 360 degrees (o) in a circle

                                                         60 arc minutes (‘) is a degree 60’=1o

                                                         60 arc seconds (“) in a degree 60” = 1’

                         The Moon and Sun both subtend ~ 1/2o (=30’) in the sky.


    Small angle Formula: Relates the apparent angular size to the physical size and the distant of an object.  If you know any two, you can calculate the third.


D = Qd/206265

     Where D is the physical size, Q is the angular size you measure, and d is the distance to the source.  IF you measure  d in meter, D will also be in meters.  Q is always in arc seconds.


1-6      Powers of Ten Notation (Scientific Notation).    

A method of writing and manipulating large and small numbers.


     100 = 1                                       100=1

     101 = 10                                     10-1=1/10=0.1

     102= 10x10=100                        10-2=1/(10x10)=1/100=0.01

     103=10x10x10=1000                 10-3=1/(10x10x10)=1/1000=0.001

     2,000,000 = 2 x 106                   0.000002 = 2 x 10-6


To multiply: multiply the leading numbers (numbers in front); add the exponents:

   (2 x1012) x (2 x 106) = (2x2) x 1012+6 = 4 x 1018


To divide: divide the leading numbers and subtract the exponents

   (2 x 1012)   /  (2 x106) = (2/2) x 10 12-6 = 1 x106




Basic units:  Distance – meters, inches, miles, etc

                     Time – seconds, minutes, days, years

                     Mass – Kg, grams

compound  units:  speed or velocity – km/sec, miles/hr

                              density – kg/meter3, grams/cm3

                              acceleration – m/sec2


units peculiar to astronomy: 


Astronomical Unit   (AU) the average distance between the Earth and the Sun, usually used in discussing distances within the Solar system. 1A.U. = 1.5x108 Km or 93x106 miles.


Light Year   (1LY) the distance light travels in one year in a vacuum.  Used for distance to stars and galaxies. 1Ly = 9.5x1012 Km = 63,240 AU.  If a star is one light year away, its light which is just now arriving, left one year ago.  In essence, we see it as it was 1 year ago, not as it is now!


Parsec (pc) the distance at which 1 AU subtends an angle of 1 arc second.  Used for distances to stars and galaxies. 1 pc = 3.1 x 1013 km = 3.26 ly ß Know this conversion!  1 megasparsec (mpc) = 106 pc, 1 kiloparsec  (kpc)= 103 pc.         


Chapter 2.     


Constellations – A collection of stars that people see patterns in. Different cultures throughout history have seen different patterns in the stars, so the International Astronomical Union divided the sky up in to 88 “official” constellations to standardize them.  Examples of constellations you might have seen:  Orion, Sagittarius, the big Dipper.


Celestial Sphere – An imaginary crystal sphere around the Earth onto which the stars are attached.

                Celestial equator – The intersection of the plane of the earths equator and

                                               the celestial equator.

                Celestial Poles – The extension of the Earths rotational axis to the celestial


Ecliptic – The apparent path of the Sun along the celestial sphere or the plane of

                    the Earth’s orbit around the Sun.


Seasons- Changing weather patterns caused by the tilt of the Earth’s rotational axis to its orbital plane (the ecliptic).



Coordinate Systems-

       Altitude-azimuth personal coordinate system important definitions- 

Altitude      Astronomical Horizon

                Azimuth     Zenith


       Right Ascension – Declination coordinate system

                Right ascension (like longitude except on celestial sphere)

                Declination (like latitude)

                Vernal equinox – starting point of right ascension and the point where the

                                            ecliptic crosses the celestial equator going from south to



Precession – The “wobble” of the Earth’s rotational axis.  The period is 26,000 years!  It is the direction the axis points, not the angle 26.5o , that changes.


Time.  Mean Solar Day- exactly 24 hours by definition.

            Apparent Solar Day- The time between two consecutive noon’s.   Time          

                                              measured with respect to the Sun.

            Sidereal Day- Time measured with respect to (WRT) the “fixed” Stars.

                                    23h 56m long.

            Sidereal Year.  The time it takes the Earth to make a complete orbit around the

                                    Sun.  365.2524 solar days long.


             Tropical (seasonal) Year- The time it takes the Sun to come back to the vernal



Chapter 3.


    Phases of the Moon.

       Lunar phases depend on the Earth-Moon-Sun position.

                        New moon               :moon is 0ofrom the Sun in the sky

                        1st Quarter                :moon is 90o east of the Sun in the sky

                        full Moon                 :moon is 180o from the Sun in the sky

                        3rd quarter                 :moon is 90o west of the Sun in the sky


 additional phases: called waning gibbous, waxing gibbous, waning crescent and waxing crescent.


Moon’s orbit:  Moons orbit is elliptical, and is at a 5o angle with respect to the cliptic plane.  Moon orbits in an eastward direction, and moves 13o further east every day.


Synchronous rotation.- Moon’s rotational period is equal to its orbital period so we only see one side of the Moon. 


Sidereal month – one orbit of the Moon around the Earth WRT the fixed stars.

Synodic Month – one orbit of the Moon WRT the Earth Sun system (i.e. one complete set of phases)


Eclipses:  Solar eclipse- The Moon moves directly between the Earth and Sun.

1.      phase must be New

2.      line of nodes must point toward the Sun

                 Lunar Eclipse- The Moon moves through Earth’s shadow.

1.      phase must be full

2.      line of nodes must point toward the Sun


Total eclipse- observer is in the Umbra, or darkest part of the shadow.

Partial eclipse- observer is in the pneumbra, or lightest part of the shadow.

Annular eclipse- The angular diameter of the moon is small (Moon is at apogee) and cannot completely cover sun in spite of precise alignment.


Chapter 4: Gravitation


Cosmology:  The theory of the creation and evolution of the Universe.

Retrograde motion: The apparent westward motion of the planets among the stars.


Geocentric cosmology – “Earth-centered” model – Ptolemy (~100 AD) – Had the Earth in the center, celestial sphere around the Earth, and planets orbited the Earth.


Heliocentric cosmology- “Sun-centered” model - Copernicus (1543) – The Sun was the center of the Universe.  Copernicus determined: The order of the planets from the Sun, the sidereal periods, and semi-major axes in AU.


Tycho Brahe-  (1547-1601) A great astronomer (not so great swordsman) who made detailed (non-telescope) observations of the planets, the Sun, the moon and meteors.  Was the first to prove meteors were actually no “Shooting stars” but space debris in our atmosphere and to understand supernova are exploding stars.


Johannes Kepler (1571-1630) Used Tycho’s data to deduce 3 laws of planetary motion.

1.      The orbit of a planet around the Sun is an ellipse with the Sun at one focus.

2.      The planets sweep out equal areas in equal times.

3.      P2 = a3 (sidereal period (yrs) squared = the semimajor axis (in AU) cubed.


Galileo Galilei (1564-1642). First person to use a telescope to observe astronomical things.

·        Showed Venus orbited the Sun (had phases)

·        Discovered 4 largest Moons of Jupiter

·        Saw craters and mountains on the Moon

·        Showed the sun had “Sunspots” and rotated.

·        Started branch of Physics known as Mechanics (the study of motion).


Definitions:  Speed, velocity, acceleration, momentum


Vector quantities:  quantities that have both direction and magnitude “How much”

Scaler quantities:  quantities that have only magnitude “how much”.


Isaac Newton (1642-1727).