Use these pages as a study guide-- i.e., a list of terms, facts, concepts, and relationships that you will want to understand for the exams. This review is NOT intended as a synopis of the class notes or textbook. Rather, it mixes some factual information with lists of items that students should review. Sample questions, some given with answers, some without, are included. These are indicated with a preceding "Q?" symbol.
This guide is not guaranteed to be comprehensive - tests cover the text and notes !

As in any discipline, there are many terms and expressions whose definitions are vital for an understanding of astronomy. Make use of the end-of-chapter reviews in the text, where many terms are highlighted. Also be aware of those terms specifically described in your class notes.

Distance Measures:     Be aware of how distances are defined, and the actual values of the most important distance, e.g., the Earth-Sun distance.

• An Astronomical Unit (AU) = the Earth-Sun distance.
• 1 AU = 150 x 10⁶ kilometers.
• Light (and radio waves) travels at 300,000 kilometers per second.
• A light year (LY) = distance light travels in a year
• 1 LY = 6 x 10¹² miles = 3 x 10¹³ kilometers.
• The Moon is 1.3 light seconds from Earth.
• Light takes about 8 minutes to travel from the Sun to the Earth.
• The Solar System is about 11 light hours in diameter.
• The nearest star is 4.3 light years distant.

• Constellations mark specific locations on the sky, all parts of the sky are located in one of 88 constallations.
• Polaris is the "North Star", and marks the position on the sky of the Earth's rotation axis.
• There is no "South Star".
• The Big Dipper is located in Ursa Major.
• The "Summer Triangle" is made up of the stars Vega, Deneb and Altair.
• Orion is a prominent constellation in the winter sky.
• Sirius is the brightest star in the sky.

• North Celestial Pole, celestial equator, the meridian
• Altitude, azimuth, horizon, zenith
• Ecliptic, zodiac, zodical constellations, equinox and solstice points
• Right ascension and declination

• Q? What are circumpolar constellations ?
• Q? How does your latitude determine the "amount" of the celestial sphere visible from any specified location on Earth ?
• Q? How does the celestial sphere change during the course of a night ?

• Define new and full Moon, first quarter and last quarter phases, waxing and waning gibbous phases.
• Know where the Moon is located in its orbit around the Sun at each phase.
• Be aware of the location of the Moon on the sky at each phase (new, full, 1st quarter, 3rd quarter) with respect to sunset and sunrise. When does a full Moon rise ? A new Moon ?
• What is the length of the lunar sidereal and synodic month ? What are they ?

• Understand the differences between total, annular and partial solar eclipses.
• Visualize the shadow pattern cast by the Earth & Moon: the umbra and penumbra.
• Understand what is meant by a total and partial lunar eclipse.
• Review when solar and lunar eclipses occur. What is the appropriate lunar phases during which a solar or lunar eclipse occurs ?

• retrograde motion
• epicycles and deferents
• the geocentric universe model
• the heliocentric universe model

• #1 - the inertial law
• #2 - the force law - F = m x a
• #3 - the reaction law
• the law of gravity - the gravitational force between 2 objects is related to the product of the masses of the objects divided by the square of the distance between the objects. The law of gravity is an example of an inverse square law.

Earth's daily rotation :

• is responsible for the day/night cycle.
• is in the direction from WEST to EAST, which causes objects to rise in the EAST and set in the WEST.
• be able to visualize/describe the daily motions of the stars, sun, and moon across the sky, as seen at different latitudes on earth.
• Q? Where on earth do the stars travel parallel to the horizon during the day/night? Perpendicular to the horizon? How do stars move across the sky in Denton ?

• causes us to see different parts of the sky during the year. (e.g., the "Summer triangle" is not overhead at night during the winter months).
• the sun to appear to travel on the imaginary circle called the ecliptic.
• Q? If the earth did not rotate, would the sun still travel along the ecliptic?
• Q? If the Earth rotated once per revolution about the Sun, what would an observer see ?
• Be able to visualize the celestial equator and ecliptic on the sky.

• the seasons
• the sun's rising/setting position to "move" along the horizon during the year.
• Be able to describe how the sun appears in the sky during the year, at any particular place on the earth.

• causes the monthly lunar phase cycle.
• results in an angular speed against the background stars of 0.5° per hour, or about 13° per day.
• occurs in an eastward direction around the Earth (counterclockwise when viewed from "above").

• Understand why and how the different phases occur (be able to draw a diagram to explain it).
• Be aware of the rising and setting times of various lunar phases.
• Q? Does a waxing crescent moon rise earlier or later than the new moon? When does the new moon rise?   When is the full moon on the meridian?

• The Moon rotates once per orbit about the Earth.
• causes Earth-bound observers to always see same side of the Moon.

• Understand their basic cause.
  
• Be able to sketch or label a diagram of an eclipse.
• Understand why everybody on the night side of the Earth can witness a lunar eclipse, but only a very small fraction of Earth's surface can see a solar eclipse.
• Why are total solar eclipses rare for any given location on Earth ?
• Q? Why doesn't a solar eclipse occur every time there is a new moon (in other words, every month) ?
• Q? What types of eclipses would occur if the Moon was located a few times further from the Earth than its current distance ? A few times closer ?

• All planets orbit eastward around the Sun, as viewed from above.
• The further from the sun, the slower each planet's angular speed across the sky.
• Understand what causes the retrograde motion of a planet against the background stars.
• Understand the basic differences between the geocentric and heliocentric models for the solar system, especially how retrograde motion is explained in each model.
• Be able to explain how/why some models predicted stellar parallax, and others didn't. In general, if the earth moves, then parallax is predicted; if the earth is stationary, no parallax.

• Aristotle
• Erathosenes
• Aristarchus
• Ptolemy
• Copernicus
• Galileo
• Newton
• Herschell
• Hubble

• Q? What was wrong with Copernicus' basic model ?
• Q? What are the elements of the Copernican Principle ?
• Review how advances in technology help foment the Copernican Revolution in the 1600s and the "second" revolution in the 1920s. ?
• Q?   What specific discoveries of Galileo provided support for the heliocentric model ?
• Be aware of the what is meant by the "First" Revolution in our world view, and the "Second" revolution.
• Q?   What are the spiral nebulae and how questions about their nature help ignite the second revolution ?

• Understand what is meant by each of Newton's laws of motion.
• Understand what would happen to a moving object if an applied gravitational force is magically turned-off.
• Understand why the acceleration of a pebble and a boulder falling from the same height above the Earth are identical, yet the gravitational force that the Earth exerts on each of them is different.
• Understand why the force exerted on the above pebble BY the earth is exactly equal (but opposite in direction) to the force exerted ON the earth BY the pebble.
• Understand why the Moon does not collide with Earth, even though the Earth's gravitational force tends to pull the moon directly in towards the Earth.
• Understand why the Moon orbits the Earth, rather than the Earth orbiting the Moon. (But WHY is this the case, if the Moon pulls just as hard on the Earth as the earth pulls on the moon?)
• Understand why planets have longer orbital periods, the further they are from the Sun. (This fact is expressed by Kepler's 3rd law, and explained by the law of gravity.)

This section reviews specific quantitative (meaning mathematical) relations, and how to interpret those relationships.

Some formulae:

(1)	Small Angle formula:	true size = distance x angular size		(2)	Centripedal force:
(3)	Gravitational Force:			(4)	Newton's 2nd Law:

Scientific Notation & Orders-of-Magnitude:
Scientific notation is a set of rules for expressing very large and very small numbers. Use the handout on this webpage to review this notation and be able to translate a number into its "English" eqivalent. Example: 1 billion = 10⁹. An "order-of-magnitude" refers to one unit in the exponent.

Angular size:   Understand the meaning of angular size and the system of units used to express these quantities.

• Angular measure is used to define a distance along an arc.
• There are 360°, or 2 p radians, in the circumference of a circle.
• 1° = 60 arcminutes; 1 arcminute = 60 arcseconds.
• The circumference of a circle = p x diameter or 2 x p x radius.
• The area of a circle is: p x radius ²
• The volume of a sphere is: 4/3 x p x radius ³ .

Be cognizant of the angular sizes of typical astronomical objects, and how to estimate the angular size of an object in the sky.

• The Moon and Sun extend about 0.5°.
• A typical constellation is several degrees across
• Planets typically are observed to extend a few arcseconds up to about 50 arcseconds on the sky. Q? Can you see this with your eye ?

Angular Size and True Size:   Know that the true or physical size of an object (Moon, Sun, building, tree, etc) is directly proportional to its angular size and the distance to the object.

        true size   =   distance × (angular size in degrees / 57.3).

This relation is known as the small angle formula, and allows the direct calculation of the size of an object if the distance is known, or conversely, one can determine the distance to an object if its true size is known. The small angle formula implies that the further away that an object is located, the smaller its angular size will be. If distance increases, then the angular size must decrease.

Ellipses:   Ellipses are geometric figures which can be described as "flattened" circles. Planetary orbits trace ellipses. Know what the eccentricity of an ellipse measures (for a circle, e = 0). Be able to draw an ellipse with a planet and the Sun properly oriented.
Q?   The more "squashed" (flattened) an ellipse, the closer the value of eccentricity is to _______ ?

Understand how to describe forces:

• Any and all forces can be described by Newton's Second Law:
  F = m × a
  where F = the force exterted ON an object, m = mass of an object, a = acceleration of an object, due to application of a force.

• The force due to gravity is expressed:
  
  Know what the symbols "m" and "r" mean in different situations.
  Q? How does the force of gravity change if your distance from a massive object doubles ? triples ?

• A force that causes an object to execute circular motion (i.e., force of gravity, which causes a planet to move within a circular orbit):
  
  where F = the force causing the circular motion, m = the mass of the object that is in circular motion, r = the radius of circular motion, v = the speed of an object along its circular path.

Escape velocity:   Defined as the velocity needed to escape the gravitational pull of a planet or other massive object.
Q? How does the escape velocity change if the mass of an object is increased ?
Q? How does the escape velocity change if the radius of an object is increased while the mass remains constant ?