Download Basic Observations in Astronomy

Welcome to
Basic Observations in Astronomy
Classes without Quizzes
-Alumni Weekend at OtterbeinJune 8-10, 2007
Dr. Uwe Trittmann
Basic Astronomy
• Possible topics:
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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 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!
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!
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!
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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
Venus
Typical
observing
hour, early
June
no Moon
Jupiter
Pluto
(experts
only)
Saturn
SouthWest
Virgo and
Coma
with the VirgoComa
galaxy
cluster
VirgoComa
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:
•M3
•M5
• M 13
Galaxies:
• M 51
• M 101
• M 64 (Bl. Eye)
M5
SouthEast
– Hercules
– Ophiuchus
– Serpens
Globular Star
Clusters:
• M 13
• M 92
• M 12
• M 10 …
Summer is
Globular
Cluster time!
M5
M13: Globular Cluster
SouthEast
– Ophiuchus
– Serpens
– Scorpius
Globular Star
Clusters:
•M4
• M 19
• M 62
• M 80 …
Summer is
Globular
Cluster time!
M5
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
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•
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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