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Ch 1--Discovering the night sky
31 Aug 2000
ASTR103, GMU, Dr. Correll
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Outline
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Patterns in the stars
Earthly cycles
Phases of the Moon
Eclipses
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Patterns of Stars
• Only about 6000 stars are visible to the eye
– other than the milky way, to be discusses later
• Skywatchers have always imagined groups of
stars depicting mythical, natural, or heroic
beings
• These constellations, or star groupings, are
convenient ways to talk about areas of the
sky
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Constellations
The Constellation Orion
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Navigating the sky
• To talk about where stars are in the night sky
we need to define a reference system
– How can we do this?
– Let’s consider the natural motions of the earth!
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The Celestial Sphere
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Navigating the sky
• North and South Celestial Poles--defined by
the Earth’s axis of rotation
• Celestial Equator--a flat plane through the
middle of the Earth and at right angles to the
rotation axis
• Right Ascension--angle along the equator
measured from the Vernal Equinox
• Declination--angle measured above or below
the equator
31 Aug 2000
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Outline
•
•
•
•
31 Aug 2000
Patterns in the stars
Earthly cycles
Phases of the Moon
Eclipses
ASTR103, GMU, Dr. Correll
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Motion of the night sky
• Diurnal motion--daily
motion of the stars
– some rise and set
– some always visible
near the celestial pole
(circumpolar)
– some never seen near
the opposite celestial
pole
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Daily Motion of the Sun
• The sun rises and sets with a period of 24 hours
– That is, the daily cycle of the sun in the sky was divided
into 24 equal time periods.
– This motion defines a measure of time call the “day”
• Since everyone wants the noon to be the middle of
the day, we’ve developed time zones
• We’ve also marked a grid of Latitude and
Longitude to describe positions on the Earth
– similar to Right Ascension/Declination, but fixed to the
Earth (Prime Meridian in Greenwich England) rather than
the distant stars
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Daily Motion of the Sun
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Sun/Earth motion
• The year is defined by the annual cycle of the
apparent motion of the Sun in the sky over the
course of the seasons
– At higher northern latitude during summer
– At lower southern latitude during winter
• Approximately 365.25 days per year
– We have a leap year (Feb 29--to give 366 days) every
four years--Julian calendar (Julius Ceasar)
– Modified by Pope Gregory, Oct 5/15, 1582
• This keeps the seasons at the same place in
the calendar--very important for agricultural
planning and religious festivals!
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Sun/Earth motion
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Daily Cycle Errata
• Siderial day--time for Earth to make one rotation
compared to the fixed stars
– 23 hours 56 minutes
• Synodic day--time for the Earth to complete one
full cycle of phases--that is, relative to the Sun-high noon to high noon
– 24 hours
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Solar and Sidereal Days
Earth’
Orbit
1o
Day 2
1o
Day 1
Sun
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Rotation/Revolution
• As the Earth revolves about the Sun, it
defines a reference plane called the ecliptic!
• The ecliptic and the celestial equator are not
lined up--this is because the axis of Earth’s
rotation is tilted 23.5 deg from it’s axis of
revolution about the Sun!
• This causes the seasons!
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The Four Seasons
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The Four Seasons
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Astrological aside--constellation of the Zodiac
Capricornus
Aquarius
Sagittarius
Scorpius
Ecliptic
Pisces
Libra
Aries
January 1
March 1
Taurus
Virgo
Gemini
Leo
Cancer
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Precession of Earth’s axis
• The tilt of the Earth’s axis precesses with a period
of 26,000 years. Due to the gravitational pull of the
the Sun and Moon on the Earth’s equatorial bulge
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Precession of Earth’s axis
• Location of the
Pole Star changes
over the millenia
• Precession of the
Equinoxes also
results
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Outline
•
•
•
•
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Patterns in the stars
Earthly cycles
Phases of the Moon
Eclipses
ASTR103, GMU, Dr. Correll
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Phases of the moon
Waxing--growing
Waning--diminishing
New--obscured or
small sliver
Cresent
Quarter--a half circle
Gibbous--more than
half
Full
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Lunar Cycle
• Siderial month--time for Moon to make one orbit
about the Earth compared to the fixed stars
– 27.3 days
• Synodic month--time for the Moon to complete one
full cycle of phases--that is, relative to the Sun
– 29 days
• Plane of Moon’s orbit about the Earth is inclined
about 5 deg from the Ecliptic
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Outline
•
•
•
•
31 Aug 2000
Patterns in the stars
Earthly cycles
Phases of the Moon
Eclipses
ASTR103, GMU, Dr. Correll
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Eclipses
• Eclipse--when one body shadows another
• Lunar Eclipse--Moon enters Earth’s shadow (moon
hidden)
• Solar Eclipse--Earth passes under Moon’s shadow
(Sun hidden)
• Umbra--full shadow
• Penumbra--partial shadow
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Eclipses
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Lunar Eclipses
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Lunar Eclipses
• Penumbral--Moon
passes through
Earth’s penumbra
only
• Partial--Only part of
Moon passes through
Earth’s umbra
• Total--Moon passes
entirely through
Earth’s umbra
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Solar Eclipses
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Solar Eclipses
• Sun and Moon about the
same angular size--0.5 deg
– Total Eclipse--Moon completely
obscures the Sun
– Annular Eclipse--Moon at
greatest distance from Earth
and is smaller in angular size
than Sun
• Partial Eclipse--Earth enters
Moon’s penumbra only
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