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Starry Monday at Otterbein
Astronomy Lecture Series-every first Monday of the month-
March 6, 2006
Dr. Uwe Trittmann
Welcome to
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 CoordinatesEarth: 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 GreenwichUT = 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
Path around sun
Rotation axis
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
- 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.
•“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
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
South-West
Plejades
Mars in Aries / Taurus
Zenith
• Big Dipper points to the north pole
West
High in the sky:
Perseus andAuriga
with Plejades and the Double
Cluster
South-West
• 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.