Basic Observations in Astronomy

Classes without Quizzes-Alumni Weekend at Otterbein-

June 8-10, 2007

Dr. Uwe Trittmann

Welcome to

Basic Astronomy

• Possible topics:– Basic observations – Star Maps– Telescopes– The Night Sky in June– Eclipses

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!

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!

Understanding and using Star

Maps

• The night sky appears to us as the inside of a sphere which rotates

• Problem: find a map of this curved surface onto a plane sheet of paper

• Let’s explore our turning star map!

Fixed and unfixed Stuff

• The stars are “fixed” to the rotating sky globe They move from East to West and also

from near to the horizon to higher up in the sky

• The Solar System bodies (Sun, Moon, Planets, Asteroids, Comets) move with respect to the fixed stars

• SSB’s have complicated paths: their own motion is added to the overall motion of the celestial sphere

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!

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!

Telescopes

• Light collectors

• Two types:– Reflectors

(Mirrors)– Refractors

(Lenses)

Refraction

• Lenses use refraction to focus light to a single spot

Reflection• Light that hits a mirror is

reflected at the same angle it was incident from

• Proper design of a mirror (the shape of a parabola) can focus all rays incident on the mirror to a single place

Newtonian Telescope

• Long tubes (approx. focal length)

• Open at front

• Eyepiece on side

Schmidt-Cassegrain Telescope (CAT)

• Very compact & easy to use

• Closed (Corrector plate)

• Resonably priced

Refractor

• Two lenses -> inverted image

• Long tube (approx. focal length of objective)

• Usually pretty expensive

Binoculars

• Erect image -> good for terrestrial viewing• Prisms needed to produce erect image• Typical specs: 8x60, means magnifies 8x

and objective lens is 60 mm in diameter

A good starting point

• A pair of binoculars and a star map will keep you busy for a long time – anywhere!– constellations – Planets– Moon– Orion nebula– Andromeda Galaxy– star clusters– …

The Night Sky in June

• The sun is at its highest -> shortest nights!

• Summer constellations are coming up: Hercules, Scorpius, Ophiuchus (Snake Bearer), Snake

lots of globular star clusters!

• Center of the Milky Way in Sagittarius

• Jupiter, Saturn & Venus are easily seen

Moon Phases

• 6 / 8 (Last Quarter Moon)

• 6 / 14 (New Moon)

• 6 / 22 (First Quarter Moon)

• 6/ 30 (Full Moon)

Today at

Noon

• Sun at meridian, i.e. exactly south

10 PM

Typical observing hour, early June

• no Moon

• Jupiter• Pluto

(experts only)

Saturn

Venus

South-West

Virgo and

Comawith the

Virgo-Coma galaxy cluster

Virgo-Coma

Cluster

• Lots of galaxies within a few degrees

M87, M88 and M91

Zenith

• Big Dipper points to the north pole

South– Canes Venatici

– Corona Borealis

– Bootes

– Serpens

Globular Star Clusters:• M 3• M 5• M 13

M 5

Galaxies: • M 51• M 101• M 64 (Bl. Eye)

South-East

– Hercules– Ophiuchus– Serpens

Globular Star Clusters:• M 13• M 92• M 12• M 10 …

M 5Summer is Globular Cluster time!

M13: Globular Cluster

South-East

– Ophiuchus– Serpens– Scorpius

Globular Star Clusters:• M 4• M 19• M 62• M 80 …

M 5Summer is Globular Cluster time!

Appendix: Eclipses

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