6202009

1
Course AnnouncementsFriday, August 27

• Student pictures Monday, Wed,last 5-10 min of
  lecture
• Clear Sky Observng reequiremnent
• starts next week (Mon-Thurs 9pm - 11pm)
• Go to east elevator, 7th floor. Door to roof is
  next to elevator
• No PRS quiz next Wednesday.
• Email send 3-4 course goals to me.

2
A brief tour of the solar system
Solar system one star (the Sun), at least eight
planets, nearly 100 moons, countless asteroids
and comets.
3
Mercury

• No Atmosphere
• Mercurys day is 57 Earth days, year is 88
  Earth days (32 tidal resonance)
• Surface remarkably similar to Earths Moon
• Possible water inside polar craters (recent
  radar evidence)
• Not visited with spacecraft since late 1970s

4
Venus

• Thick atmosphere (x100 Earth), CO2 and sulphuric
  acid
• Venus day is 264 Earth days, Sun rises is the
  East! (retrograde rotation)
• Surface is invisible because of perpetual clouds
• Radar maps show craters, volcanoes
• Surface temperature 750?K because of greenhouse
  effect (CO2 in atmosphere)

5
Mars

• Thin atmosphere (1 Earth), mostly CO2
• Mars day is nearly same as Earth (24h 10m)
• Surface has many craters, volcanoes, polar ice
  (CO2)
• All volcanoes dormant
• Average surface temperature 218?K (-67 ?F)
• Martian Seasons 2x Earth

6
Jupiter

• Gaseous planet, mostly H, He
• Rapidly Rotating (10 hrs)
• Bands of gas, storms (red spot visible for
  300 yrs)
• Mass all other planets combined
• Average surface temperature 115?K (-252 ?F)
• Many satellits (20) including 4 Galilean (large)

7
Saturn

• Gaseous planet, mostly H, He
• Rapidly Rotating
• Rings are bands of 1m size rocks (not solid)
• Bands of gas, storms
• Average surface temperature 95?K (-290 ?F)
• Many satellites (18 named) including Titan (has
  atmosphere, largest in SS)

8
Uranus

• Gaseous planet, mostly H, He
• Discovered 1781 (Herschel)
• Rotation axis tilted almost in ecliptic plane!
• Thin rings (discovered 1980)
• Banded surface structure
• Many satellites (20 named), largest Umbriel,
  Titania (also discovered by Herschel)

9
Neptune

• Gaseous planet, mostly H, He
• Discovered 1846 (Adams and LeVerrier, predicted
  from Neptune orbital variations)
• Rapid Rotation 16hrs
• Thin darkrings (discovered 1990)
• Banded surface structure, stroms
• Many satellites (20 named), largest Triton

10
Pluto

• No longer considered 9th planet largest KBO
  (Kuiper Belt Object
• Discovered 1931 (Tombaugh)
• Only a little larger than Earths Moon (2274 km
  vs. 1731 km radius)
• Surface temperauter about 60K (-350 F)
• One satellite (Charon)

11
Planets can be classified into three groups
terrestrial, Jovian, Kuiper belt objects (KBOs
are now considered minor bodies, not planets)
Sun Most of Mass
Terrestrial Planets Rocky, dense, small
Kuiper belt Objects (Pluto largest)
Jovian Planets Gaseous, large, density of water
12
Test your knowledge

• The only planet with a retrograde rotation is
• Mercury
• Venus
• Mars
• Neptune
• Pluto
• Jupiters composition is mostly
• Silicon and Oxygen
• Hydrogen and helium
• Nitrogen and carbon dioxide
• Methane and ammonia
• Argon and corbon monoxide
• Which planet(s) has rings?
• Jupiter
• Saturn
• Uranus
• Neptune

13
Observation The Sun, a massive, luminous ball of
gas. Problem What is structure and history of
the Sun (and all stars)? Observation Explore
properties of Sun and stars
14
By studying stars and nebulae, astronomers
discover how stars are born, grow old, and die
(stellar evolution).
Crab supernova (1054 AD)
Orion Star formation region (106 stars)
15
Astronomers use angles to denote the positions
and apparent sizes of objects in the sky.
16
Angular Measure

• Basic unit of angular measure is the degree º
• Full circle measures 360º
• Right angle measures 90º
• Angular distance is the number of degrees across
  the sky between two points.
• Angular diameter or angular size is the number of
  degrees from one side of an object to the other
  side.
• The angular size of the Moon is 0.5º (1/2º )
• The angular size of the Sun is also 0.5º (1/2º )

17
Big Dipper (Ursa Major)
18
(No Transcript)
19
Try this What is the angular size of this sphere
in degrees as you see it? (Answers will vary from
front to back of room)
20
Angular Measure for Small Angles

• 1º 60 arcminutes 60'
• 1' 60 arcseconds 60''
• So 1º 60x60 3,600 arcseconds 3,600''

Test question What is the angular size of the
Moon ( 0.5) expressed in arcseconds? Answer
1,800''
21
(No Transcript)
22
Example On November 28, 2000, the planet Jupiter
was 609 million kilometers from Earth and had an
angular diameter of 48.6''. Using the
small-angle formula, determine Jupiters actual
diameter in kilometers (km).

• D 48.6'' x 609,000,000 km / 206265 143,000 km

23
Powers-of-ten notation is a useful shorthand
system of writing very large numbers.
100 1 101 10 102 100 103 1,000 104
10,000 106 1,000,000 109 1,000,000,000
One Ten (deca-) Hundred (centa-) Thousand
(kilo-) Ten thousand Million (mega-) Billion
(giga-)
24
Powers-of-ten notation is a useful shorthand
system of writing very small numbers.
100 1 10-1 0.10 10-2 0.01 10-3 0.001 10-4
0.0001 10-6 0.000001 10-9 0.000000001
One One-tenth (deci-) One-hundredth
(centi-) One-thousandth (milli-) One-ten-thousandt
h One-millionth (micro-) One-billionth (nano-)
25
Test your knowledge

• 10-6 109 ?
• 100
• 0.01
• 1,000
• 0.001
• 10

• 2106 /4109 ?
• 510-4
• 8104
• 210-3
• 4103
• 5104

26
(No Transcript)
27
Astronomical distances are often measured in
astronomical units, parsecs, or light years.
Astronomical Unit (AU) One AU is the average
distance between Earth and the Sun (1.496 X 108
km or 92.96 million miles). Light Year (ly) One
ly is the distance light can travel in one year
at a speed of about 3 x 105 km/s or 186,000
miles/s (9.46 X 1012 km or 63,240 AU). Parsec
(pc) One pc is the distance from which Earth
would appear to be one arcsecond from the Sun.
(3.24 LY, or 3.1 x 1013 km).
28
Eighty-eight constellations cover the entire sky.
29
Eighty-eight constellations cover the entire sky.
30
Constellations

• About 6,000 stars are visible to unaided eye
  (about half above the horizon 3,000).
• 88 semi-rectangular groups of stars called
  constellations entire sky is covered
• Hence, all stars are members of a constellation
• Some stars in the constellations are (relatively)
  close while others are very far away.
• Hence, There is no physical connection between
  stars in a given constellation

31
Constellation names are derived from the myths
and legends of antiquity.
Sword of Orion
32
(No Transcript)
33
Winter constellations
34
As Earth orbits our Sun, different constellations
are visible at different times of the year.
The circumpolar constellations are always the
same because they are visible no matter where
Earth is in its orbit.
35
Note that the constellations at sunset
change about 1 constellation/month (i.e. 12
forms) e.g. Virgo, Libra, Scorpius, etc
ZODIAC
36
The Zodiac
37
The Zodiac is the set of constellations through
which the Sun appears to pass during one year
(because of the Earths motion around the Sun,
though!)
38
Ecliptic the path of the Sun (and planets) in
the sky complicated by the fact that Earth is
tilted on its own axis by 23.5 degrees
Note Constellations of the zodiac lie on the
path of the ecliptic, since they are behind the
Suns path.
39
As the Earth moves in its orbit, The path of the
Sun (and planets) changes because of the tilt of
the rotation axis Note Since rotation axis is
fixed with respect to the stars, star paths do
NOT change!
40
Northern Hemisphere Winter
Northern Hemisphere Summer
41
What dates correspond to each figure?
4
1
3
2
42
Cause of seasons
43
(No Transcript)
44
Azimuth of sunrise changes with season
East
Northeast
Southeast
Dawn
45
Stonehenge Ancient astronomical site aligned to
solar azimuth
46
Seasons Summary

• Due to the Earths 23.5º tilt on its axis

N. Summer Northern Hemisphere tilted toward
Sun N. Winter Southern Hemisphere tilted toward
Sun

• 4 important dates
• 1. Summer solstice (June 21) sun most northerly
  on ecliptic
• 2. Autumnal equinox (Sep 21) sun crosses
  celestial equator
• 3. Winter solstice (Dec 21) sun most southerly
  on ecliptic
• 4. Vernal (spring) equinox (Mar 21) sun crosses
  c. equator

• Myth The Earth is closer to Sun in summer

Wrong! The suns rays reach (N. hemisphere) of
the Earth more DIRECTLY in (Northern H.) summer,
less DIRECTLY in winter (doesnt have to do with
sun-earth distance)
47
PrecessionThe Sun and Moon cause precession, a
slow, conical motion of Earths axis of rotation.
The precession period is 26,000 yrs.
48
12,000 years from now, the bright star Vega will
be the new North Star because of precession.
Current position
49
Precession and the mystery of the Egyptian
pyramids

• The sides of the great (Middle Kingdom, c.2550
  BCE) pyramids are very accurately aligned to true
  north, but there was no bright star within 2 of
  the North Celestial Pole in 2550BCE because of
  precession. How did the ancient Egyptians do this?

Sides are aligned within 4 arcmin (0.07 deg) of
true north!
50
Solution?

• In 2000, Kate Spence (Cambridge Univ.)
  suggested the Egyptians used the bright stars
  Kochab and Mizar in the Big dipper.
• A plumb line was used, and when the 2 stars
  aligned with the plumb line, that was the
  direction of true north.
• This would only work for a few decades near
  2,480 BCE, dating the pyramids very accurately.

51
Positional astronomy plays an important role in
keeping track of time.

• Key Question When is the Sun on the meridian
  (directly in the south)?

• Apparent solar day the interval between two
  successive meridian transits of the Sun (varies
  around 24 hrs as Earth orbits the Sun at varying
  speeds Earths orbit is elliptical).
• Mean solar day the interval between two
  successive meridian transits of the Sun IF it
  moved at a constant rate (exactly 24 hrs).
• Sidereal time the interval of time between two
  successive meridian transits of a star (23 hrs 56
  min).

52
(No Transcript)
53
A (Very) Brief History of the Calendar

• The Earth takes 365.24220 days to orbit around
  the Sun once (one year). This period is called a
  tropical year. Note that it is not an even number
  of days!
• 0.24220 fractional days is 5 hours, 48 minutes,
  and 46 seconds a fraction that has caused
  endless headaches for calendar makers who would
  rather the year was exactly 365 days long!
• If all years were 365 days, after 4 years the
  calendar would be in error (not in accord with
  the Suns position) by
• 4 x 5.81hr 23¼ h 1 day.
• In 45 BCE Julius Caesar decreed that years are
  365 days long with one extra day added in
  February, every four years (accurate to one day
  in 128 years). This is the Julian calendar, and
  was used in Europe from 45 BCE to 1582 AD.
• This worked OK for centuries, but it meant that
  by 1580 AD the calendar was off by about 10
  days. In other words, the Sun no longer was at
  the Vernal Equinox on March 21, but rather about
  March 11. This interfered with agricultural
  planting times, religious feast days, etc.
• Aside The synodic period of moon (time from new
  to new moon) is 29.52 days, so 12 lunar months
  are 29.5212 354.24 days. This is one day short
  of a tropical year, so lunar calendars (e.g.
  Islamic) slowly migrate w.r.t solar (i.e. Julian,
  Gregorian) calendars.

54
Gregorian Calendar

• In 1582 AD, Pope Gregory XIII introduced the
  currently used Gregorian calendar.
• To fix the Julian calendar, Gregory decreed
• There would be no Mar 10-20, 1582. (skipped 10
  days)
• Do not allow leap years in Centuries unless the
  year is evenly divisible by 400 (good to one day
  in 3300 years).
• This means 1900, 2100 were not leap years, but
  2000 was a leap year.
• Although the U.S. does not have a legal
  calendar, it unofficially has adopted the
  Gregorian calendar, based on Act of Parliament of
  the United Kingdom in 1751, which specified use
  of the Gregorian calendar in England and its
  colonies.

55
Other calendars currently in use

• Hebrew calendar (official calendar of Israel)
  12 or 13 months, each month 29 or 30 days, era
  mundi starts at 3760 BCE (i.e. this is year
  5,763). A year is 50 weeks plus 3, 4, or 5 or
  days leap year has 54 weeks.
• Islamic calendar (first described in Koran)
  Strictly lunar. Months start at first sighting of
  lunar crescent. Calendar starts from Era of the
  Hijra, commemorating the migration of the Prophet
  and his followers from Mecca to Medina in 622 AD
  1 A.H. (Anno Higerae). There are 11 leap years
  in 30 year cycle. Ramadan (month 9) is month of
  fasting, starts at lunar crescent sighting. Since
  this is a lunar calendar, the (Gregorian) dates
  of Ramadan vary.
• Chinese calendar. 12 months(29 or 30 days),
  cycles of 60 years with 12-yr periods for Earthly
  cycles (year of dragon, snake, ox, etc).
  Occasional a 13th intercalary month is added.
  No specific year 0, but the calendar is at least
  2,500 yrs old. Chinese government and businesses
  use Gregorian calendar.

56
Some interesting calendar factoids

• What date is Easter? Why does it change every
  year?
• Part of Pope Gregory XIIIs calendar reform
  stated Easter Day is the first Sunday after the
  first full moon that occurs after the vernal
  equinox.
• For Example next spring (2005), the first full
  Moon after March 21 is Friday March 25, so Easter
  2005 is Sunday, March 27, 2005.
• What are leap-seconds? When and why are they
  used?
• The Earth does not rotate exactly with a fixed
  period (recall sidereal period is 23h 56m 4s
  appx).
• It is slowing, largely because of tides.
• The length of the mean solar day has increased by
  roughly 2 milliseconds since it was exactly
  86,400 seconds of atomic time about 184 years ago
  (i.e. the 184 year difference between 2004 and
  1820).  
• That is, the length of the mean solar day is at
  present about 86,400.002 seconds instead of
  exactly 86,400 seconds. (The second is defined by
  an atomic clock).
• They are inserted as needed, on Jan 1 and/or July
  1). There have been 25 leap seconds inserted
  since 1972.

57
Review Quiz

• The Sun is viewed at noon at the zenith on June
  21. What is the observers latitude?
• 0 (equator)
• -23.5
• 23.5
• 90
• The star Sirius rises tonight at 930pm. One week
  from tonight, Sirius will rise at
• 930pm
• 902pm
• 926pm
• 958pm
• Suppose the Earths rotation axis were
  perpendicular (90) to the ecliptic plane. How
  would this change affect the seasons?
• No effect (seasons are caused by changing
  Earth-Sun distance)
• The length of the seasons would double in
  duration
• The length of the seasons would be noticeably
  shorter
• There would be no seasons

58

• At the summer solstice, an observer at the North
  pole sees the Sun at midnight at what altitude?
• Below the horizon
• On the horizon
• Above the horizon at 23.5 altitude
• At the zenith
• When the Sun is on the celestial equator, what
  day is it?
• June 21
• Mar 21
• Sep 21
• Could be either Mar 21 or Sep 21
• The constellations of the zodiac are all located
  on
• The celestial equator
• The ecliptic
• The central meridian
• The vernal equinox

59

• Approximately how many stars can be seen in a
  dark location using only the naked eye?
• 3,000
• 300
• 30,000
• At least 100,000
• Why couldnt the ancient Egyptians have used the
  North star (Polaris) to align the pyramids along
  the north-south direction?
• The north star is not visible from Egypt at that
  time.
• The north star sets at night in Africa and cannot
  be seen
• The Egyptians religion worshiped the Sun, so
  stars could not be used.
• The direction of north was not same direction as
  Polaris because of precession of the Earths
  axis.
• Why are leap years needed in our calendar?
• Because the sidereal day and solar day are
  slightly different
• Because the Earths orbital period around the Sun
  isnt exactly 365 days
• Because the Earths axis is tilted 23.5 to the
  ecliptic plane.
• Because the Earths orbit is elliptical, not
  circular.

60

• How long does the Earth take to rotate once on
  its own axis?
• 23h 56m 3s
• 24h 0m 0s
• 24h 3m 56s
• Varies throughout the year, longest in winter.