1. Introduction to Astronomy

2.  Observations lead to theories and laws  Laws are concise statements that summaries a large number of observations.  Theories are explanations for observed phenomena.  Theories are more powerful than laws because of there predictive power.

3.  Newton’s Law of Gravity: Mathematically describes the gravitational force between two objects.  Keplers 3 rd Law: Mathematically describe the relationship between the orbital period of a planet and its distance from the sun.  The Big Bang Theory: Explains how the universe as come to look the way it does today.  Evolutionary Theory: Explains the why life varies in form and distribution.

4.  Earth is average and has no special place in the universe  The earth revolves around the sun with 8 planets and many smaller bodies.  The Sun revolve around the center of the Milky Way Galaxy.  The Milky Way is one of billions of galaxies in the Universe.  The Universe is the totality of all space, time, matter, and energy.

6.  There are about 3000 stars visible to the naked eye on any clear night. Twice that many if you stay up all night.

7.  Constellations are groups of stars that have meaning for the observer.  Stars in a constellation are not actually close together.

8.  There are 88 named constellations in the sky.  12 constellations form the Zodiac.  Constellations that are always visible form a specific location are called circumpolar.  Circumpolar constellations circle a point.

9.  Stars appear to be located on a rotating sphere surrounding the earth.  The location of constellations are fixed on the sphere.

10.  The projection of the earth’s equator onto the sphere is called the celestial equator.  A circular plane marked by the celestial equator is called the equatorial plane.  The projection of the north & south poles are called the north & south celestial poles.  The star that marks the point in the sky closest to the celestial pole is called the pole star. Currently the northern pole star is Polaris.

11.  The suns path along the celestial sphere is the ecliptic.  the vernal and autumnal equinoxes are the points where the ecliptic intersects the celestial equator.

12.  The 12 constellations that lie along the ecliptic are called the zodiac.

13.  The location of stars on the celestial sphere is measured in angles.  360 degrees of arc in a circle  60 minutes of arc per degree  60 seconds of arc per minute of arc.  One fist held at arms length is approximately 10 degrees.  1 finger at arms length is about 1 degrees.

15. Keeping the seasons and calendar aligned

16.  A solar day is the time from solar noon on one day to solar noon the next day.  The length of a solar day is the result of the earth’s rotation on its axis and its revolving around the sun. The mean solar day is 24 hours.  A day measured by the stars is a sidereal day.  A sidereal day (23 h 56 m ) is about 3.9 minutes shorter than a mean solar day.  The length of a sidereal day depends only on the rotational motion of the earth.

18.  The meridian is a line on the celestial sphere that passes through the north and south celestial poles and though your zenith.  Solar noon occurs when the sun is on your local meridian.  Each one hour the sun moves 15 o to the west.  Time zones are placed every15 o to create 24, one-hour time zones.

20.  Year doesn’t quite have a whole number of solar days in it—leap years take care of this.  Add extra day every 4 years  Omit years that are multiples of 100 but not of 400  Omit years that are multiples of 1000 but not of 4000  This will work for 20,000 years.

21.  1 year is the time it takes the earth to complete one revolution around the sun.  The earth is tilted 23.5 o relative to the plane of the orbit.  It’s the tilt of the earth not how close the earth is to the sun that causes the seasons.

23.  Precession is change in the direction of the earth’s rotational axis.  Precession is a 26000 year cycle.  As the axis moves the pole star

25.  Because of precession the constellations change relative to the seasons. In 13000 years Orion will be a summer constellation

27.  Moon takes about 29.5 days to go through whole cycle of phases—synodic month  Phases are due to different amounts of sunlit portion being visible from Earth  Time to make full 360° around Earth, sidereal month, is about 2 days shorter

28.  Waxing Crescent  1st Quarter  Waxing Gibbous  Full  Waning Gibbous  3rd Quarter  Waning Crescent  New

29.  A lunar eclipse occurs when the moon passes into the shadow of the earth.  Lunar eclipses only occur during a full moon.  A solar eclipse occurs when the moon passes between the earth and sun.  Solar eclipses only occur during a new moon.

30.  Partial when only part of Moon is in shadow  Total when it’s all in shadow

32.  Partial when only part of Sun is blocked (penumbra shadow)  Total when it all is blocked (umbra shadow)  Annular when Moon is too far from Earth for total

34.  The earth rotates on an axis that runs from the north pole to the south pole.  Midway between the poles, an imaginary line called the equator divides the earth into equal halves.  Latitude is an angle measured north or south from the equator.  Lines of latitude are also called parallels.

35. Latitude measure your angular distance from the equator.

36.  The Tropic of Cancer is the circle of latitude on the earth that marks the northernmost path of the sun (23.5 o N).  The Tropic of Capricorn is the circle of latitude on the earth that marks the southernmost path of the sun — the apparent "equator" (23.5 o S).  The Arctic Circle marks the point where north of the circle experiences a continuous 24 hours of daylight at least once each year (66.5 o N).  The Antarctic Circle marks the point where south of the circle experiences a continuous 24 hours of daylight at least once each year (66.5 o S).

37.  Longitude is the angle measured east or west around the equator.  The zero point for longitude is a point just south of Greenwich, England.  The line of longitude passing through Greenwich is called the prime meridian or the Greenwich line.

41. Two Systems: Alt-Az and Equitorial

42.  The celestial sphere is an imaginary sphere surrounding the earth to which the stars are attached.  The celestial sphere is divided into to two halves by the celestial equation which is the projection of the earth’s equator onto the sphere.  The top and bottom of the celestial sphere is marked by the celestial poles which are the projections of the earth’s north and south poles.

44.  Also know as the local system  Zenith is the point directly overhead.  The celestial horizon is the great circle 90 o from the zenith.  Altitude is the angular distance above the celestial horizon.  Azimuth is the angular distance measured east from north.  The point opposite the zenith is called the Nardir.  The altitude of the north celestial pole is the same as the observer's latitude.  Example: 45 o altitude and 270 o azimuth would be 45 o above the horizon and due west.

45.  Azimuth is the angle measured from north to east along the horizon.

46. EARTH

47.  Declination measures the angle above and below the celestial equator.  Right ascension measures east from the vernal equinox.  Right ascension is usually measure in hours, minutes & seconds.

48.  The horizon system (alt/az) is easier to use and understand.  The equatorial system gives the same coordinates regardless of your location.  Modern astronomers use software to transform the equatorial coordinate to local coordinates.

49. Chapter 1