Transcript March 2006 - Otterbein University

Welcome to
Starry Monday at Otterbein
Astronomy Lecture Series
-every first Monday of the monthMarch 6, 2006
Dr. Uwe Trittmann
Today’s Topics
• Basic Observations in Astronomy
• The Night Sky in March
On the Web
• To learn more about astronomy and physics at
Otterbein, please visit
– http://www.otterbein.edu/dept/PHYS/weitkamp.a
sp (Observatory)
– http://www.otterbein.edu/dept/PHYS/ (Physics
Dept.)
Basic Observations in Astronomy
• Positions of objects (sun, moon, planets, stars …)
• Motion of objects
– with respect to you, the observer
- with respect to other objects in the sky
• Changes (day/night, seasons, etc.)
• Appearance of objects (phases of the moon, etc.)
• Special events (eclipses, transitions, etc.)
 All “in the sky”, i.e. on the Celestial
Sphere
What’s up in the night sky?
The Celestial Sphere
• An imaginary sphere
surrounding the earth,
on which we picture the
stars attached
• Axis through earth’s
north and south pole
goes through celestial
north and south pole
• Earth’s equator
Celestial equator
Celestial Coordinates
Earth: latitude, longitude
Sky:
• declination (dec)
[from equator,+/-90°]
• right ascension (RA)
[from vernal equinox,
0-24h; 6h=90°]
Examples:
• Westerville, OH
40.1°N, 83°W
• Betelgeuse (α Orionis)
dec = 7° 24’
RA = 5h 52m
What’s up for you?
Observer
Coordinates
• Horizon – the
plane you stand on
• Zenith – the point
right above you
• Meridian – the
line from North to
Zenith to south
…depends where you are!
• Your local sky –
your view depends on your location on earth
Look
North in
Westerville
Look
North on
Hawai’i
Daily Rising and Setting
• Due to the rotation of the
Earth around its axis
• Period of rotation:
1 siderial day= 23h56m4.1s
• 1 solar day (Noon to Noon) =24h
• Stars rotate around the
North Star – Polaris
Solar vs Siderial Day
• Earth rotates in 23h56m
• also rotates around sun
 needs 4 min. to “catch up”
• Consequence: stars rise 4
minutes earlier each night
• after 1/2 year completely
different sky at night!
What time is it?
• Depends on where you are on the Earth!
• Time zones ensure that the noon is really
noon, i.e. sun is at highest point
• To avoid confusion, use universal time (UT),
the time at the meridian in Greenwich
UT = EST + 5 hrs
• Daylight savings adds one hour in spring, so
UT = EDT+ 4 hrs
The Time Zones
Established to insure that sun is at highest point
approximately at noon in the middle of the time zone
Another Complication: Axis Tilt!
• The Earth’s rotation axis is tilted 23½ degrees
with respect to the plane of its orbit around
the sun (the ecliptic)
• It is fixed in space  sometimes we look
“down” onto the ecliptic, sometimes “up” to it
Rotation axis
Path around sun
Position of Ecliptic on the Celestial Sphere
•
•
•
Earth axis is tilted w.r.t. ecliptic by 23 ½ degrees
Equivalent: ecliptic is tilted by 23 ½ degrees w.r.t. equator!
 Sun appears to be sometime above (e.g. summer
solstice), sometimes below, and sometimes on the celestial
equator
The Seasons
• Change of seasons
is a result of the tilt
of the Earth’s
rotation axis with
respect to the plane
of the ecliptic
• Sun, moon, planets
run along the
ecliptic
The Zodiac throughout the Year
Example: In Winter sun in Sagittarius, Gemini at night sky;
in summer sun in Gemini, Sagittarius at night sky
Constellations of Stars
• About 5000 stars visible with naked eye
• About 3500 of them from the northern hemisphere
• Stars that appear to be close are grouped together
into constellations since antiquity
• Officially 88 constellations
(with strict boundaries for classification of objects)
• Names range from mythological (Perseus,
Cassiopeia) to technical (Air Pump, Compass)
Constellations of Stars (cont’d)
Orion as seen at night
Orion as imagined by men
Constellations (cont’d)
Orion “from the side”
Stars in a constellation are not connected in any
real way; they aren’t even close together!
Zodiacal signs vs. Constellations
•“Constellation” is a modern,
well-defined term
- Some constellations are big, some
are small on the celestial sphere
•“Zodiacal sign” is the old
way of dividing the year and
the Sun’s path into 12 equal
parts
-
360/12=30, so each zodiacal sign is exactly 30 degrees “long”
0 degrees: Aries, 30 degrees: Taurus, 60 degrees: Gemini, 90
degrees: Cancer, etc.
Example
The vernal equinox
happens when the
sun enters the
zodiacal sign of
Aries, but is actually
located in the
constellation of
Pisces.
Reason: Precession of the Equinoxes
Precession period
about 26,000 years
“The dawning of the age of
Aquarius”
Motion of Sun, Moon and other Planets
•
•
All major bodies in the Solar System move around ecliptic
Slow drift (from W to E) against the background of stars
Reason: All planets move in same
plane!
All planets? - Pluto’s strange Orbit
• Very far out there: 40 A.U.
• Pluto’s year = 248 Earth years
• Orbit inclined 17° w.r.t. ecliptic
• Very eccentric orbit:
• Perihelion: 30 A.U.
(inside the Neptune orbit!)
• Aphelion: 50 A.U.
Is Pluto a planet?
• Orbit too weird
• Too small
 Today Pluto
would
probably not
be classified as
a planet!
Why do all planets move in the
same plane?
• Reason: Formation process
of the Solar System
• Condenses from a rotating
cloud of gas and dust
– Conservation of angular
momentum flattens it
• Dust helps cool the nebula
and acts as seeds for the
clumping of matter
Formation of Planets
• Orbiting dust – planitesimals
• Planitesimals collide
• Different elements form in
different regions due to
temperature
• Asteroids
• Remaining gas
Motion of the Moon
• Moon shines not by its own light but by reflected
light of Sun
 Origin of the phases of the moon
• Moon revolves around the Earth
• period of revolution = 1
month
Phases of the Moon
Phases of the
Moon (cont’d)
• Moon rotates around
earth in one month
• Moon rotates around
itself in the same time
•  always shows us the
same side!
•  “dark side of the moon”
(not dark at all!)
Motion of the Planets
• Along the
ecliptic as Sun
and Moon
• But: exhibit
weird,
“retrograde”
motion at
times
“Strange” motion of the Planets
Planets usually move from W to E relative to the stars,
but sometimes strangely turn around in a loop, the so
called retrograde motion.
The heliocentric Explanation of
retrograde planetary motion
See also: SkyGazer
SkyGazer
• A computer program that simulates the
vision of the sky during day and night
Things to observe:
• Set your position on Earth: observe how view of
sky changes as you move E,W, N,S
• Note the distribution of sunlight on Earth!
• Rotation is around Polaris which is not in zenith
SkyGazer
Things to observe (cont’d):
• Sun, moon, planets, stars rise (E) and set
(W)
• In the southern hemisphere the sun is
highest in the north
• Planets sometimes move backward
• Moon phases
• Planets have phases, too!
Eclipses
• One celestial object hidden by other or in
the shadow of another
• Solar eclipse: sun hidden by the moon
• Lunar eclipse: moon in earth’s shadow (sun
hidden from moon by earth)
• Also: eclipses of Jupiter’s moons, etc.
• Most spectacular because moon and sun
appear to be the same size from earth
Solar Eclipses
•
•
•
•
Umbra – region of total shadow
Penumbra – region of partial shadow
Totality lasts only a few minutes!
Why isn’t there a solar eclipse every month?
Solar Eclipse
Solar Corona
Question
Why isn’t there
an eclipse
every month ?
Answer: because
the Moon’s
orbit is
inclined w.r.t.
the ecliptic
Lunar Eclipses
Moon moves into
earth’s
shadow…
…and out of it
(takes hours!)
Partial Eclipse
Not an
Eclipse !
Towards Totality
Almost total…
Totality
Totality
The Night Sky in March
• Long nights, early observing!
• Winter constellations are up early: Orion, Taurus,
Gemini, Auriga, Canis Major & Minor, the spring
constellations come up: Cancer, Leo, Big Dipper
• Saturn dominates the evening, Jupiter early
morning.
Moon Phases
• Today (First quarter Moon)
• 3/ 14 (Full Moon)
• 3 / 22 (Last Quarter Moon)
• 3 / 29 (New Moon)
Today
at
Noon
Sun at
meridian,
i.e.
exactly
south
10 PM
Typical
observing
hour,
early
March
Saturn
Mars
Moon
SouthWest
Plejades
Mars in
Aries /
Taurus
Zenith
Big Dipper
points to the
north pole
West
High in the
sky:
Perseus and
Auriga
with Plejades and
the Double
Cluster
SouthWest
The Winter
Constellations
–
–
–
–
–
Orion
Taurus
Canis Major
Gemini
Canis Minor
The
Winter
Hexagon
•
•
•
•
•
•
Sirius
Procyon
Pollux
Capella
Aldebaran
Rigel
South
• Saturn
near
Praesepe,
an open
star
cluster
East
• Spring
constellations:
– Leo
– Hydra
Mark your Calendars!
• Next Starry Monday: April 3, 2005, 8 (!!!) pm
(this is a Monday
• Observing at Prairie Oaks Metro Park:
– Friday, May 5, 9:00 pm
• Web pages:
– http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.)
– http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)
)
Mark your Calendars II
•
•
•
•
Physics Coffee is every Wednesday, 3:30 pm
Open to the public, everyone welcome!
Location: across the hall, Science 256
Free coffee, cookies, etc.