ASTRO 101

1
ASTRO 101

• Principles of Astronomy

2
Instructor Jerome A. Orosz
(rhymes with boris)Contact

• Telephone [phone]
• E-mail orosz_at_sciences.sdsu.edu
• WWW http//mintaka.sdsu.edu/faculty/orosz/web/
• Office Physics 241, hours T TH 330-500

3
Text Perspectives on Astronomy First
Editionby Michael A. Seeds Dana Milbank.
4
Course WWW Page

• http//mintaka.sdsu.edu/faculty/orosz/web/ast101_f
  all2009a.html
• Note the underline ast101_fall2009a.html
• Also check out Nick Strobels Astronomy Notes
• http//www.astronomynotes.com/

5
Astronomy Help Room Hours

• Monday 1200-1300, 1700-1800
• Tuesday 1700-1800
• Wednesday 1200-1400, 1700-1800
• Thursday 1400-1800, 1700-1800
• Friday 900-1000, 1200-1400
• Help room is located in PA 215

6

• Planetarium Shows in PA 209
• Thursday, September 10 10 -- 11 AM and 4 -- 5
  PM
• Friday, September 11 2 -- 3 PM
• Monday, September 14 10 -- 11 AM and 3 -- 4
  PM
• Tuesday, September 15 11 AM -- 12 PM
• Wednesday, September 16 11 AM -- 12 PM
• Thursday, September 17 2 -- 3 PM
• Friday, September 18 4 -- 5 PM
• Can be used in place of homework 2 (just turn in
  a sheet with your name and time)

7
Homework

• Assigned question due September 10 Question 13,
  Chapter 2 (Why are the seasons reversed in the
  southern hemisphere relative to the northern
  hemisphere?)

8
Coming Up

• Introduction to the Sky
• Constellations
• Stellar Brightness
• Stellar coordinates
• The clockwork of the sky
• Day/night
• Phases of the moon
• The seasons
• Solar and Lunar Eclipses

9
Questions from Before

• What is the shape of the Earth, and is it
  stationary?
• We all know the answers to the above, but can you
  give evidence for the correct conclusion without
  using modern technology?

10
Questions from Before

• What is the shape of the Earth, and is it
  stationary?
• We all know the answers to the above, but can you
  give evidence for the correct conclusion without
  using modern technology?
• The stars change as you go north-south
• The shape of the Earths shadow during lunar
  eclipse
• The manner in which ships disappear.

11
Questions for Today

• What causes the seasons? Lets ask some Harvard
  students
• Does the Earth go around the Sun? Lets ask
• http//www.collegehumor.com/moogaloop/moogaloop.sw
  f?clip_id1773116

12
Stellar Brightness Magnitudes

• Historically (e.g. Hipparcos in the First
  Century), the brightness of stars as seen by the
  eye have been measured on a magnitude scale
• The brightest stars were first magnitude.
• The faintest stars were sixth magnitude.
• Brighter objects have smaller magnitudes.

13
Stellar Brightness Magnitudes

• In modern times, it was discovered that the human
  eye has a nonlinear response to light if one
  source of light has twice the light as a second
  source, then the first source would not appear by
  eye to be twice as bright.
• The response of the eye is logarithmic, so that
  differences of magnitudes correspond to ratios of
  brightness.

14
The Magnitude Scale

• The modern of the magnitude scale is set up so
  that a difference of 5 magnitudes corresponds to
  a ratio of brightnesses of 100.
• Bright objects can have negative apparent
  magnitudes.

15
The Celestial Sphere

• Imagine the sky as a hollow sphere with the stars
  attached to it. This sphere rotates once every
  24 hours. This imaginary sphere is called the
  celestial sphere.
• Even though we know it is not the case, it is
  useful to imagine the Earth as being stationary
  while the celestial sphere rotates around it.

16
The Celestial Sphere

• The north celestial pole is directly above the
  north pole on the Earth.
• The south celestial pole is directly above the
  south pole on the Earth.
• The celestial equator is an extension of the
  Earths equator on the sky.
• The zenith is the point directly over your head.
  The horizon is the circle 90 degrees from the
  zenith.

17
The Celestial Sphere

• The celestial poles and the celestial equator are
  the same for everyone.
• The zenith and the horizon depend on where you
  stand.
• http//www.astronomynotes.com/nakedeye/s4.htm

18
Stellar Coordinates and Precession

• There are a few ways to specify the location of a
  star (or planet) on the sky
• Altitude/Azimuth
• The altitude describes how many degrees the star
  is above the horizon, the azimuth describes how
  far the star is in the east-west direction from
  north.
• The altitude and azimuth of a star is constantly
  changing owing to the motion of the star on the
  sky!

19
Stellar Coordinates and Precession

• There are a few ways to specify the location of a
  star (or planet) on the sky
• Equatorial system
• Lines of longitude on the earth become right
  ascension, measured in units of time. The RA
  increases in the easterly direction.
• Lines on latitude on the earth become
  declination, measured in units of degrees.
  DEC90o at the north celestial pole, 0o at the
  equator, and -90o at the south celestial pole.
• http//www.astronomynotes.com/nakedeye/s6.htm

20
Stellar Coordinates and Precession

• The north celestial pole moves with respect to
  the stars very slowly with time, taking 26,000
  years to complete one full circle.

21
The Clockwork of the Universe
22
The Clockwork of the Universe

• There are many familiar astronomical cycles

23
The Clockwork of the Universe

• There are many familiar astronomical cycles
• The Day/Night cycle.

24
The Clockwork of the Universe

• There are many familiar astronomical cycles
• The Day/Night cycle.
• The seasons of the year.

25
The Clockwork of the Universe

• There are many familiar astronomical cycles
• The Day/Night cycle.
• The seasons of the year.
• The phases of the Moon (the lunar cycle).

26
The Clockwork of the Universe

• There are many familiar astronomical cycles
• The Day/Night cycle.
• The seasons of the year.
• The phases of the Moon (the lunar cycle).
• The seven day week???

27
The Day/Night Cycle

• Every day, the Sun rises in the east, and sets in
  the west.
• At night, the stars move in fixed patterns. The
  specific paths depend on where you look

28
The Day/Night Cycle

• The stars rotate about a fixed point in the
  northern sky (for observers in the northern
  hemisphere).

29
The Day/Night Cycle

• The positions of the stars relative to each other
  stays fixed.
• This triangle of bright stars is seen in the
  summer.

30
The Day/Night Cycle

• These bright stars are seen in the winter.

31
Day/Night Cycle

• We have the Sun rising and setting, and the stars
  moving in fixed patterns about a point in the
  sky.

32
Day/Night Cycle

• We have the Sun rising and setting, and the stars
  moving in fixed patterns about a point in the
  sky.
• Two possible reasons

33
Day/Night Cycle

• We have the Sun rising and setting, and the stars
  moving in fixed patterns about a point in the
  sky.
• Two possible reasons
• The Earth is fixed in space, and the Sun and the
  celestial sphere move around it.

34
Day/Night Cycle

• We have the Sun rising and setting, and the stars
  moving in fixed patterns about a point in the
  sky.
• Two possible reasons
• The Earth is fixed in space, and the Sun and the
  celestial sphere move around it.
• The Earth is spinning about its axis.

35
Day/Night Cycle

• We have the Sun rising and setting, and the stars
  moving in fixed patterns about a point in the
  sky.
• Two possible reasons
• The Earth is fixed in space, and the Sun and the
  celestial sphere move around it.
• The Earth is spinning about its axis.

36
Day/Night Cycle

• The Earth is spinning on its axis, while it and
  the other planets revolve around the Sun.
• The apparent motion of the Sun defines the day.
• The rotation of the Earth affects weather
  patterns, ocean currents, the paths of missiles
  etc.

37
Day/Night Cycle

• In what direction does the Earth rotate (as seen
  from above the North Pole)?
• Clockwise
• Counterclockwise

38
Day/Night Cycle

• In what direction does the Earth rotate (as seen
  from above the North Pole)?
• Clockwise
• Counterclockwise, since the Sun rises in the east
  and sets in the west.

39
How Fast does the Earth Rotate?
40
How Fast does the Earth Rotate?

• Pretty fast the circumference of the Earth is
  about 24,000 miles at the equator, so a point on
  the equator moves 1000 miles per hour (24,000
  miles in 24 hours).

41
How Fast does the Earth Rotate?

• Pretty fast the circumference of the Earth is
  about 24,000 miles at the equator, so a point on
  the equator moves 1000 miles per hour (24,000
  miles in 24 hours).
• Pretty slow just watch the hour hand on the
  clock for a while. That hand moves twice as fast
  as the earth!

42
How Long is a Day?
43
How Long is a Day?

• It depends

44
How Long is a Day?

• It depends
• If you measure the length of time between
  successive noons (the time when the Sun is at
  its highest point in the sky), the day is on
  average 24 hours. This is a mean solar day.

45
How Long is a Day?

• It depends
• If you measure the length of time between
  successive noons (the time when the Sun is at
  its highest point in the sky), the day is on
  average 24 hours. This is a mean solar day.
• If you do the same thing, but with a star rather
  than with the Sun, the day is about 23 hours and
  56 minutes. This is a sidereal day.

46
Solar vs. Sidereal

• Since the Earth is moving around the Sun, it has
  to rotate an extra bit each day so that the Sun
  appears in the same point in the sky.

Image from Nick Strobels Astronomy notes
(http//www.astronomynotes.com/))
47
Solar vs. Sidereal

• Your favorite star rises 4 minutes earlier each
  successive night.
• Eventually, it will rise when the Sun is still
  out. Hence, with a few exceptions, a given
  constellation is visible at night only during
  certain times of the year.

48
What Time is it?

• A nice map of time zones
• http//www.worldtimezone.com/datetime.htm

49
The Cycle of the Sun

• The Sun would be in different constellations
  during certain times of the year, if you could
  see the stars in the day. Where the Sun is
  depends on the season.

50
Seasons in Detail

• If we do some careful observations, we find

51
In Detail

• If we do some careful observations, we find
• The length of the daylight hours at a given spot
  varies throughout the year the Sun is out a
  longer time when it is warmer (i.e. summer), and
  out a shorter time when it is colder.

52
In Detail

• If we do some careful observations, we find
• The length of the daylight hours at a given spot
  varies throughout the year the Sun is out a
  longer time when it is warmer (i.e. summer), and
  out a shorter time when it is colder.
• On a given day, the length of the daylight hours
  depends on where you are on Earth, in particular
  it depends on your latitude e.g. in the summer,
  the Sun is out longer and longer the further
  north you go.

53
In Detail

• Near the North Pole, the Sun never sets in the
  middle of the summer (late June).
• Likewise, the Sun never rises in the middle of
  the winter (late December).

54
In Detail

• In most places on Earth, the weather patterns go
  through distinct cycles
• Cold weather winter
• Getting warmer spring
• Warm weather summer
• Cooling off fall

55
In Detail

• In most places on Earth, the weather patterns go
  through distinct cycles
• Cold weather winter
• Getting warmer spring
• Warm weather summer
• Cooling off fall
• These seasons are associated with the changing
  day/night lengths.

56
In Detail

• In most places on Earth, the weather patterns go
  through distinct cycles
• Cold weather winter, shorter daytime
• Getting warmer spring
• Warm weather summer
• Cooling off fall
• These seasons are associated with the changing
  day/night lengths.

57
In Detail

• In most places on Earth, the weather patterns go
  through distinct cycles
• Cold weather winter, shorter daytime
• Getting warmer spring, equal daytime/nighttime
• Warm weather summer
• Cooling off fall
• These seasons are associated with the changing
  day/night lengths.

58
In Detail

• In most places on Earth, the weather patterns go
  through distinct cycles
• Cold weather winter, shorter daytime
• Getting warmer spring, equal daytime/nighttime
• Warm weather summer, longer daytime
• Cooling off fall
• These seasons are associated with the changing
  day/night lengths.

59
In Detail

• In most places on Earth, the weather patterns go
  through distinct cycles
• Cold weather winter, shorter daytime
• Getting warmer spring, equal daytime/nighttime
• Warm weather summer, longer daytime
• Cooling off fall, equal daytime/nighttime
• These seasons are associated with the changing
  day/night lengths.

60
In Detail

• When it is summer in the northern hemisphere, it
  is winter in the southern hemisphere, and the
  other way around.

61
What Causes the Seasons?
62
What Causes the Seasons?

• Is the Earth closer to the Sun during summer,
  and further away during winter?

63
What Causes the Seasons?

• Is the Earth closer to the Sun during summer,
  and further away during winter? (This was the
  most commonly given answer during a poll taken at
  a recent Harvard graduation).

64
What Causes the Seasons?

• Is the Earth closer to the Sun during summer,
  and further away during winter? (This was the
  most commonly given answer during a poll taken at
  a recent Harvard graduation).
• No! Otherwise the seasons would not be opposite
  in the northern and southern hemispheres.

65
What Causes the Seasons?

• The Earth moves around the Sun. A year is
  defined as the time it takes to do this, about
  365.25 solar days.

66
What Causes the Seasons?

• The Earth moves around the Sun. A year is
  defined as the time it takes to do this, about
  365.25 solar days.
• This motion takes place in a plane in space,
  called the ecliptic.

67
What Causes the Seasons?

• The Earth moves around the Sun. A year is
  defined as the time it takes to do this, about
  365.25 solar days.
• This motion takes place in a plane in space,
  called the ecliptic.
• The axis of the Earths rotation is inclined from
  this plane by about 23.5 degrees from the normal.

68
What Causes the Seasons?

• The axis of the Earths rotation points to the
  same point in space (roughly the location of the
  North Star).

69
What Causes the Seasons?

• The axis of the Earths rotation points to the
  same point in space (roughly the location of the
  North Star).
• The result is the illumination pattern of the Sun
  changes throughout the year.

70
What Causes the Seasons?

• Here is an edge-on view, from the plane of the
  Earths orbit.

71
What Causes the Seasons?

• Here is a slide from NASA and NOAA.

72
What Causes the Seasons?

• A slide from Nick Strobel.

73
What Causes the Seasons?

• Because of the tilt of the Earths axis, the
  altitude the Sun reaches changes during the year
  It gets higher above the horizon during the
  summer than it does during the winter.

74
What Causes the Seasons?

• Because of the tilt of the Earths axis, the
  altitude the Sun reaches changes during the year
  It gets higher above the horizon during the
  summer than it does during the winter.
• Also, the length of the daytime hours changes
  during the year the daylight hours are longer
  in the summer and shorter in winter.

75
What Causes the Seasons?

• The altitude of the Sun matters when the Sun is
  near the horizon, it does not heat as efficiently
  as it does when it is high above the horizon.
• Image from Nick Strobels Astronomy Notes
  (http//www.astronomynotes.com/).

76
What Causes the Seasons?

• Winter The combination of a short daytime and a
  Sun that is relatively low above the horizon
  leads to much less heating in the day, plus a
  longer period of cooling at night. Overall, it
  is colder.

77
What Causes the Seasons?

• Summer The combination of a long daytime and a
  Sun that is relatively high above the horizon
  leads to much more heating in the day, plus a
  shorter period of cooling at night. Overall, it
  is warmer.

78
What Causes the Seasons?

• Spring and Fall The number of hour of daylight
  is about equal to the number of nighttime hours,
  leading to roughly equal times of heating and
  cooling.

79
Next

• The Moon