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Announcements Friday Jan 27

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Note: Constellations of the zodiac lie on the path of the ecliptic, since they ... Chinese calendar. ... Chinese government and businesses use Gregorian ... – PowerPoint PPT presentation

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Title: Announcements Friday Jan 27


1
AnnouncementsFriday Jan 27
  • Homework 2 due date Sunday _at_ midnight
  • Todays lecture
  • Constellations, Zodiac (12)
  • Seasons
  • Equinox, solstice dates
  • Precession
  • Calendars
  • Review quiz (doesnt count)

2
Observing the night skyPolaris has two
companions !
Orbital period 30 yrs
3
Winter constellations
Winter Triangle
4
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.
5
Note that the constellations at sunset
change about 1 constellation/month (i.e. 12
forms) e.g. Virgo, Libra, Scorpius, etc
ZODIAC
6
The Zodiac
7
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!)
8
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.
9
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!
10
Northern Hemisphere Winter
Northern Hemisphere Summer
11
What dates correspond to each figure?
4
1
3
2
12
Cause of seasons
13
(No Transcript)
14
Azimuth of sunrise changes with season
East
Northeast
Southeast
Dawn
15
Stonehenge Ancient astronomical site aligned to
solar azimuth
16
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)
17
PrecessionThe Sun and Moon cause precession, a
slow, conical motion of Earths axis of rotation.
The precession period is 26,000 yrs.
18
12,000 years from now, the bright star Vega will
be the new North Star because of precession.
Current position
19
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 2550 BCE because of
    precession. How did the ancient Egyptians do this?

Sides are aligned within 4 arcmin (0.07 deg) of
true north!
20
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.

21
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.

22
A (Very) Brief History of the Calendar(continued)
  • 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.

23
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 leap years, but 2000
    was not.
  • Although the U.S. does not have a legal
    calendar, it unofficially has adopted the
    Gregorian calendar, based on Act of Parliament,
    1751.

24
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.

25
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.

26
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

27
  • 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

28
  • Approximately how many stars can be seen in a
    dark location near Iowa City 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.

29
  • 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.
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