1. Chapter 1
The Quest Ahead

2. What is Astronomy? Science: The quest for understanding. Astronomy:
The oldest science.
The study of the universe from the smallest such as atomic reactions in stars to the largest such as galaxies and the evolution of the entire universe.

3. 1-1 The View from Earth Besides stars one can see: Nebulae
Our Galaxy (the Milky Way)
Galaxies
Planets
Comets
Sun
Moon
With telescopes many more objects can be seen.
Courtesy of Dave Palmer

4. 1-2 The Celestial Sphere
Celestial sphere: An imaginary sphere around the Earth on which all heavenly objects appear to be located.
It appears to rotate around us.
Extensions of Earth's poles and equator:
North celestial pole
South celestial pole
Celestial equator

5. Constellations
Constellations are areas of the sky with stars in a particular pattern.
They are accidental patterns; stars are at different distances and not really related to each other.
Most patterns have been in use for 1000s of years and usually based on myths.
Courtesy of Akira Fujii/NASA.

6. Today constellations are a useful way to divide the sky into regions.
There are 88 constellations recognized by international agreement.

7. Measuring the Positions of Celestial Objects
Angular separation: The angle between lines originating from the observer toward two objects.
60 arcminutes = 1 degree
60 arcseconds = 1 arcminute
At arm's length, roughly:
Fist = 10o across
Little finger = 1o across
Sun and Moon both = 0.5o wide

8. Celestial Coordinates
Locations of objects in sky are given by two coordinates:
Declination: Angle north or south of celestial equator.
Right ascension: Angle eastward around the celestial sphere.

9. 1-3 The Sun's Motion Across the Sky
Rises in the east and sets in the west just like stars.
Over time slowly drifts eastward relative to the stars.
Takes about days to return to the same location in the sky. This equals one year.
(A leap day occurs because that extra 0.25 days accumulates to one day after every four years.)

10. The Ecliptic
Ecliptic: The path of the Sun in the sky. It is not the same as the celestial equator.
Zodiac: The constellations that the Sun passes through.

11. The Sun and the Seasons
In the Northern Hemisphere the Sun rises and sets further north in the summer than in the winter.
The Sun is in the sky longer each day in the summer than in the winter. Longer days mean more solar energy hits the Northern Hemisphere.

12. When the Sun is higher in the sky in summer the solar energy is more concentrated.
Summer is hotter than winter because:
Days are longer = more solar heating.
Sun is higher in the sky = more direct solar energy.
In the Southern Hemisphere the seasons are reversed.

13. Altitude: The height of a celestial object measured at an angle above the horizon. The Sun has a higher altitude in summer than in winter in the Northern Hemisphere.
Solstice: Points on the celestial sphere where the Sun reaches its northernmost or southernmost points.
Northernmost = summer solstice around Jun 21st.
Southernmost = winter solstice around Dec 21st.
Equinox: Point on the celestial sphere where the Sun crosses the celestial equator.
Vernal equinox is around Mar 21st.
Autumnal equinox is around Sep 22nd.

14. A Scientific Model
Scientific model: An idea, a logical framework, that accounts for a set of observations and/or allows us to create explanations of how we think a part of nature works.
It is not necessarily real, but more like an analogy.
Examples:
The celestial sphere. The stars just appear to be on a great sphere.
The ecliptic. The Sun doesn't actually move on a track in the sky.

15. 1-4 The Moon's Phases
Rotation: the spinning of an object about an axis that passes through it.
Revolution: the orbiting of one object about another.
The Moon's period of rotation equals its period of revolution around the Earth.

16. The phases of the Moon depend on the relative positions of the Earth, Moon, and Sun.

17. Order of phases: A. Waxing crescent B. First quarter C. Waxing gibbous
D. Full
E. Waning gibbous
F. Third quarter
G. Waxing crescent
Phases depend on elongation, which is the angle of an object from the Sun in the sky.
© J. Gatherum/ShutterStock, Inc.

18. Sidereal period: The amount of time required for one revolution (or rotation) of an object with respect to the distant stars. For the Moon this is about 27.3 days.
Synodic period: The time interval between successive similar alignments of an object with respect to the Sun.
Lunar month: The Moon’s synodic period, or the time
between successive similar phases (about 29.5 days).

19. This shows the difference between the Moon's sidereal and synodic periods.

20. 1-5 Lunar Eclipses
Lunar eclipse: The Moon passes into the Earth's shadow.
Umbra: Portion of a shadow that receives no direct light from the light source, as at point A.
Penumbra: Portion of a shadow that receives direct light from only part of the light source, as at point B.

21. Eclipse seasons: A time of year during which a solar or lunar eclipse is possible.
Lunar eclipses don't occur at every full moon.
The Moon's orbit is tilted 5o with respect to the Earth's orbit. Eclipses can only occur if the Sun, Earth, and Moon are in a direct line.

22. Types of Lunar Eclipses
Penumbral: Moon passes through the penumbra but not the umbra. (A)
Total: Moon is completely in the umbra. (B)
Partial: Only part of the Moon passes through the umbra. (C)
The Moon will not ever be totally dark because sunlight is refracted toward it by the Earth's atmosphere.

23. 1-6 Solar Eclipses
Solar eclipse: The Sun‘s light is blocked by the Moon.
Total: When light from the normally visible part of the Sun is completely blocked by the Moon. It occurs when the Moon's umbral shadow to touches the Earth.
Corona: The Sun's outer atmosphere that is only visible during an eclipse.
Courtesy of Alex York.

24. The Partial Solar Eclipse
Partial eclipse: One in which only part of the Sun's disk is covered by the Moon. This is the view from location Y in the diagram. Location X sees a total eclipse.

25. The Annular Eclipse
Annular eclipse: When the Moon is too far from the Earth for it to cover the Sun completely. An outer ring of the Sun is seen.
Location A: annular eclipse.
Location P: partial eclipse.
Courtesy of Alex York.

26. 1-7 Observations of Planetary Motion
Without a telescope you can see 5 planets: Mercury, Venus, Mars, Jupiter, Saturn.
They generally move eastward like the Sun and Moon.
They remain within a few degrees of the ecliptic.
Mercury and Venus never appear too far from the Sun. They are seen just after the Sun sets or just before it rises. Their elongations are limited.

27. Retrograde motion: The planets mostly move eastward, but at times some can change direction and move westward.
Courtesy of NASA/JPL-Caltech.

28. 1-8 Rotations
Solar day: The amount of time that elapses between successive passages of the Sun across the meridian.
Meridian: An imaginary line that runs north to south, passing through the observer's zenith.
Sidereal day: The amount of time that passes between successive passages of a given star across the meridian.

29. Earth's solar day is about 4 minutes longer than its sidereal day.

30. 1-9 Units of Distance in Astronomy
Astronomical unit (AU): Equal to the average distance between the Earth and the Sun, which is about 150 million km. Used to measure distances within the solar system.
Light-year: The distance that light travels in one year, which is about 9.6 trillion km. Used for longer distances.

31. 1-10 The Scale of the Universe
The tremendous differences of sizes in the universe can be represented with scientific notation.

32. We can represent the relative sizes with a scale model:
Sun = a basketball 9.4 inches in diameter.
Earth = the head of a pin that is 84 feet away.
Moon = size of a period that is 2.6 inches from the Earth.
Jupiter = a grape that is 146 yards away.
Pluto = a grain of sand 6 tenths of a mile away.
The nearest star would be 4460 miles away.
The Galaxy would be 164 million miles across.

33. Simplicity and the Unity of Nature
The more we learn about the universe the more patterns we see and the more order we find.
Physics is used very much in astronomy that the term astrophysics is often used instead. It means physics applied to extraterrestrial objects.
Astronomy incorporates other sciences such as chemistry, biology, and geology as well.

34. 1-11 Astronomy Today
In the past few decades we have sent robotic vehicles to land on some planets and fly by others. Humans have walked on the Moon.
Modern telescopes on the ground and in space are discovering new objects and addressing basic questions such as the origin of the universe.
Astronomy is about basic science and not technological applications. Curiousity is a basic human trait.