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.