Cosmic Perspective II

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Cosmic Perspective II
Lecture 2
James J Marie, Astronomy 2005
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Solar Time
Solar Time Time measured relative to the
position of the Sun (a solar day is an
example. Solar time can be measured
with a sundial (during
daylight).
Solar day - Unit of time measured from noon to
noon. Alternatively, it is the
time it takes the sun to cross the meridean
(directly overhead) to when
the sun crosses the meridean again. Solar time
varies slightly 24 h
25 s depending on the time of year.
Note A solar day is not the Earths true
rotation period.
James J Marie, Astronomy 2005
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Sidereal Day

• Sidereal Day - Time it takes the
• Earth to make 1 complete rotation
• (360?) relative to the fixed star(s).
• A sidereal day is about 4 minutes
• shorter than a solar day.
• Avg. sidereal day 23h 56m 4.09s
• (more precise than a solar day)

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Lunar Phases
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Lunar Phases
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Measuring a Month
A month is based on the phases of the moon. The
Moons phases repeat every 29 ½ days.
Synodic Month - 29 ½ day period for the lunar
cycle.

• The synodic month is not the Moons true
  orbital period
• (time for one complete orbit).
• Earths motion implies that the Moon completes
  more
• than 1 full orbit from one new moon to the
  next new
• moon.

Sidereal Month - Orbital period of the Moon
measured relative to the fixed
stars (true orbital period)
1 sidereal month ? 27 1/3 days
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Synodic vs. Sidereal Month
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Eclipses
Eclipses - The shadow of one celestial body is
cast upon another.
Umbra - darker, central part of the shadow
Penumbra - lighter, outer part of the shadow
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Eclipse Geometry
Types of Solar Eclipses
Lunar Eclipse
?
Earths shadow is on the Moon.
?
Moons shadow is on the Earth.
Why doesnt an eclipse happen with every new moon
and every full moon?
Path of Solar Eclipse
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Inclination of Moons Orbit

• The Moons orbit is inclined (tilted) to the
  plane of the ecliptic by 5?.
• The Moon crosses thru the ecliptic only twice
  during each orbit.
• The 2 points where the Moons orbit intersects
  the ecliptic are called nodes.

Ascending node - Moon passes thru the ecliptic
on the way up.
Descending node - Moon passes thru the ecliptic
on the way down.
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Inclination of Moons Orbit
Ascending node - Moon is going up
Descending node - Moon is going down
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Ecliptic Plane
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Conditions For an Eclipse
Line of Nodes - An imaginary line drawn from
the ascending node to the
descending node.

• The orientation of the line of nodes is
  maintained
• throughout the Earths orbit.

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Line of Nodes

• The orientation of the line of nodes is
  maintained
• throughout the Earths orbit.

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Lunar Eclipse Types

• Penumbral Lunar Eclipse
• Total Lunar Eclipse
• Partial Lunar Eclipse

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Shadows
Total lunar eclipse - Hawaii
Total solar eclipse
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Eclipsed Earth
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Seasons
The seasons are caused by the tilt of the Earths
axis by 23 1/2? with respect to the ecliptic.
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Seasons
Note The Earth is actually slightly closer to
the sun during the winter than during
the summer.
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Origin of the Calendar

• Egyptian Calendar (1000 BC) First solar
  calendar. Had 12 months
• 1 month
  30 days. Began numbering years when
• the star
  Sirius rose at the same place as the Sun.
• Problem
  Gradually gets out of sync with lunar
• 
  phases.
• Metonic Calendar - (440 BC) Based on 29 ½
  day lunar cycle. Has 12
• months,
  but only has 354 or 355 days.
• Problem
  Has 11 fewer days than a solar based
• 
  calendar. Gradually gets out of sync with
  
• 
  the seasons! (summer in January!?!)

James J Marie, Astronomy 2005
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Metonic Cycle
The Greek astronomer, Meton, recognizes that
19 years 235 lunar months.
This means that he can synchronize the Metonic
calendar with the seasons by adding a 13th month
to 7 out of every 19 years.
So a new year comes approximately the same date
every 19th year.
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More Calendars
Jewish Calendar Follows the Metonic cycle.
Adds a 13th month In the 3rd, 6th, 8th, 11,
14th, 17th, 19th years of each cycle.
Roman Calendar Had only 10 months (304 days).
This calendar was a mess!
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Julian Calendar
The Greek Sosigenes convinced Julius Caesar to
reform the Roman calendar. Consists of cycles of
3 365-day years followed by a 366-day leap year.
Vastly improved but still not quite in
synchronization with the seasons.
Leap Year An extra day added to the calendar to
synchronize it with the seasons.
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Gregorian Calendar
The English scholar, Bade (673-735) noticed that
the vernal equinox slipped 3 days earlier than
March 21. The German astronomer, Christoph
Clavius (1537-1612) pushed for reforming the
Julian Calendar. He devised new rules for leap
years. This reform was adopted by Pope Gregory
XIII who decreed the day after October 4, 1582
would be October 15,1582. Hence, the Gregorian
Calendar which we still use today.
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Leap Years

• Every year divisible by 4 is a leap year.
• But every year divisible by 100 is NOT a leap
  year.
• Unless the year is also divisible by 400, then it
  is still a leap year.

This means that year 1800, 1900, 2100, 2200, 2300
and 2500 are not leap years, while year 2000 and
2400 are leap years. Year 2000 is special, as it
is the first time the third rule has been used in
many parts of the world. A leap year has 29
days in February instead of 28 days (February 29
gets added).
James J Marie, Astronomy 2005
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Precession
The axis of any spinning object (like a top) may
rotate.
The earth is just like a spinning top and its
axis rotates (precesses).
The precession is caused by gravitational pulls
from the sun and moon which try to straighten
the earths axis.
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Precession Path
In 13,000 years Vega will be the new north star
because of precession.
James J Marie, Astronomy 2005