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ASTR 101
The Earth and the Sky
2/3/2017
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Sky and the atmosphere
Twinkling of stars
The celestial sphere
Constellations
Star brightness and the magnitude system
Naming of stars
Scattering of light and color of the sky
Sun light
air particle
more
scattering
less
scattering
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During the daytime we cannot see stars due to the glare in the
atmosphere (from sunlight).
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When sunlight travels through the Earth’s atmosphere some of the light
is scattered (deflected) off air molecules.
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White light is a combination of different colors, blue light is scattered far
more than red light.
Blue Sky during the day
Sun
Earth
atmosphere
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During the day we see this scattered light in the atmosphere.
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Since most of it is blue, a lot of blue light is seen in the atmosphere in
all directions which makes the sky look blue.
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Light from stars are fainter than scattered sunlight in the atmosphere,
so we cannot see them (unless a star happen to be very bright)
In the outer space sky is always dark
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Away from the atmosphere (in space , on moon…) sky is always dark.
It is possible to see stars and the sun at the same time.
The Sun looks red at Sunrise/Sunset
Sun
atmosphere
Earth
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At sunrise and sunset, Sun is close to the horizon. Sunlight has to travel a
longer distance in the atmosphere to reach us.
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By the time sunlight reaches ground most of the blue light has scattered off.
Only red light remains.
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So the Sun looks red at sunrise or sunset.
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If the sky is cloudy, smoggy or there is a lot of dust
particles in air they may reflect the sunlight and
also looks red.
that part of the sky
Twinkling of Stars (stellar scintillation)
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When starlight travels through the atmosphere light refracts slightly (changes
direction) as it passes through air layers of different densities.
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When the atmosphere is turbulent this refraction is not uniform or steady, it
changes from moment to moment, changing the direction of starlight all the
time.
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Since angular size of stars are extremely small (less than 0.01”) even a tiny
change in the direction is noticeable, resulting “twinkling” of stars.
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When a star is low in the sky, light has to travel a longer distance through the
atmosphere.
– Stars closer to the horizon appear to twinkle more than stars that are
overhead
Twinkling
View of a star through a telescope in
poor seeing conditions
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Twinkling is undesirable for astronomy. For
astronomical observations, a steady
atmosphere with good “seeing” is needed.
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Major Astronomical observatories are located in
higher elevations (mountain tops).
Twinkling
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Planets have larger angular sizes.
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So a very small changes in the position of a planet in the sky is not so
noticeable, they twinkle less.
– This is one way to distinguish a planet from a star.
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But in poor seeing conditions planets also twinkle, especially when they
are low in the sky.
The Earth and the Sky
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From a location on ground, the Earth looks flat
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because we see only a tiny part of the vast spherical surface of the Earth
At a given time and location we see half of the sky, the sky directly above
that location.
View of the other half is blocked by earth.
Earth spins on its axis (west to east) and goes around the sun
Celestial objects (planets, stars, galaxies…) are located at different distances
Just as someone in a boat sailing forward sees
unmoving [objects] going backward , on the
earth, sees the unmoving stars going uniformly
westward…
-Aryabhata, ca 500CE
https://en.wikipedia.org/wiki/Aryabhata
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On earth we do not feel its motion or rotation.
Instead it appears that the whole universe, sun, moon, stars… goes around the
Earth east to west.
All celestial objects appears to be located at the same distance, like located
inside a giant sphere centered at Earth.
Just as someone in a boat sailing forward sees
unmoving [objects] going backward , on the
earth, sees the unmoving stars going uniformly
westward…
-Aryabhata, ca 500CE
https://en.wikipedia.org/wiki/Aryabhata
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But on earth we do not feel its motion or rotation.
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Instead it appears that the whole universe, sun, moon, stars… goes around the
Earth east to west.
All celestial objects appear to be located at the same distance, like they are on a
giant sphere centered at Earth.
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The Celestial Sphere
www.phy.olemiss.edu/~perera/animation_cele/celes_sp.gif
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This apparent sphere around us with all celestial objects on it, is called the
Celestial Sphere.
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We know this is an illusion:
– different celestial objects are at different distances from us.
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it is the earth that rotates, not the sky
Nevertheless considering sky as a sphere centered at earth is a useful
concept for observational purposes.
The Earth and the Sky
zenith
N
W
E
S
nadir
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At a given location and time only half of the celestial sphere (sky) is visible, view of
the other half is blocked by the earth.
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On a typical moonless clear night, about 3000 stars are visible from a dark location.
– Zenith: the point directly above in the sky at a given time and location.
– Nadir: Point in the sky directly below. It cannot be seen because it is on the
other side of the earth.
– Celestial Meridian: The imaginary circle passing through the North and South
points on the horizon and through the zenith.
Celestial Poles and Equator
North celestial pole
Celestial equator
South celestial pole
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The Earth’s rotation axis extended to the sky (to the celestial sphere)
meets the celestial sphere at two locations.
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They are called celestial poles.
Celestial equator is the great circle on the celestial sphere in between
celestial poles.
– the imaginary circle in the sky directly above the Earth's equator.
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zenith 90⁰
Sky Coordinates
altitude
N
W 270⁰
Azimuth
E
90⁰
S 180⁰
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Simplest way to give the location of a celestial object in the sky is to give
its apparent position in the sky
– Altitude: vertical angular elevation above the horizon
– Azimuth: direction from North to the object: angle measured along the
horizon towards the east along the horizon.
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But the apparent position of a celestial object changes with time and
location.
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Not the best way to identify an object.
Logitude and Latitude
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Position of a place on the Earth can be given by its longitude and latitude.
– Latitude: angular distance from the equator.
– Longitude: angular separation of the meridian through the location and the
reference meridian trough Greenwich.
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A similar mechanism can be used to give the position of a celestial object
on the celestial sphere (sky).
Right ascension and Declination
N
celestial north pole
Vernal Equinox
(first point of Aries)
celestial equator
declination
right ascension
reference median
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Position of an object on the celestial sphere can be specified similarly.
– When refer to locations in the sky they are called declination and right
ascension.
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Declination: angular distance to an object from the celestial equator.
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Right ascension: angle between the celestial meridian through an object
and a reference median.
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North Celestial pole
Polaris – the North Star
Long exposure image of the north pole region
Star trails over the Kennon Observatory
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Earth’s axis always points to the same point in the sky regardless the rotation or
the orbital movement.
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locations of celestial poles in the sky are fixed
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the whole sky appears to rotate about celestial poles.
Since celestial poles don’t move, so do any objects located at the poles.
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There happen to be a bright star very close to the north pole:
The north star ( Polaris).
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It appears to stay fixed in the sky while other stars move around it.
Stars near the north celestial pole
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There isn’t any bright star close to
the South celestial pole.
So there is no “south star”.
Stars near the
south celestial pole
Magellanic
clouds
Constellations
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Southern sky this evening form a dark location ((8.00PM, Oxford MS).
On a moonless clear night about 3000 stars are visible
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Ancient observers imagined groups of brighter stars as patterns, called
constellations.
– Related to mythological characters, heroes, animals, royalty whatever
important or fascinated them.
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Many ancient cultures saw patterns in the sky and established their own
constellations and star lore based on their culture and mythology.
Most northern constellations dates back to Babylonians times, 4-5 millennia ago,
– Later Greeks adopted them and interpreted according to their mythology
Examples of few constellation figures
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All above are Zodiac constellations
– the narrow band in the sky where sun, moon and planets are visible.
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Their origin dates backs to ancient Sumerians and Babylonian times
Western (Greek)
Ancient Egyptian
Ancient Chinese
Constellations in the same part of the sky according to Greeks ,Egyptians and Chinese
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From the planetarium software : Stellarium
– Available from (free):
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www.stellarium.org
Also demo version of Skygazer planetarium software is available form
http://www.carinasoft.com/downloads.html#VdemoAnchor
Modern Constellations
constellation
boundaries
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16th-19th century astronomers added many new constellations to fill in the gaps
between classical constellations and cover uncharted regions of the southern
sky.
In 1922, the International Astronomical Union (IAU) adopted 88 constellations
as internationally accepted constellations.
– That included the 48 classical constellations, 32 southern constellations
and few other constellations formed since the 16th century.
– 88 constellations divide the sky in to 88 regions according to boundaries defined
by the IAU.
distances to stars in the Big Dipper.
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Constellations are not physical objects:
– Group of stars just happened to be in the same direction in the sky.
– No physical relationship between stars of a constellation
– distances between stars could be hundreds of light years
Antique star maps/atlases
Albrecht Dürer, 1515, Germany
A star map by Ignace Pardies , 1693 CA
http://www.davidrumsey.com/luna/servlet/view/search?q==
%22Jamieson%2C%20Alexander%22
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Ancient star maps were vividly decorated with constellation figures
– Stars were indentured from their locations in the constellations.
Uranographia , Johann Bode (1801)
Alexander Jamieson’s sky atlas, early 19th Century
http://www.davidrumsey.com/blog/2012/8/21/mapping-theheavens-in-1693
• Today constellations are used as mnemonics, an easy way to remember and
identify different regions of the sky.
• They play less important role in professional astronomy.
• Instead star catalogs and databases play a major role.
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One of the largest catalog, Hubble guide star catalog has over 945 million
stars down to magnitude 21.
Asterisms
Sickle (in Leo)
Delphinus
Tea pot (in Sagittarius)
Orion’s belt
W in Cassiopeia
A distinctive pattern of stars, but often not an official
constellation.
examples: Big dipper, Tea pot , belt of Orion, Pleiades, W
in Cassiopeia, Delphinus, southern cross
Star brightness:
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magnitude system
Stars have different apparent brightness.
In astronomy apparent brightness of a star (or other celestial object) is
given by a number called the magnitude.
It is a backward scale, smaller the magnitude number brighter the star and
vice versa.
Star magnitude
7
6
5
4
3
2
1
0
0
50
100
150
200
250
300
Star brightness
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It is a nonlinear scale:
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For each magnitude difference, actual brightness changes by a factor of about 2.5
magnitude 2
2.5 x brighter
2.5 x brighter
magnitude 3
magnitude 4
2.5 x2.5 = 6.25 x brighter
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5 magnitudes difference corresponds to 100 times in actual brightness
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Faintest stars can be seen with the naked eye are about magnitude 6
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When the object is too bright (like Venus, Sun, Moon) its magnitude could be
negative.
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Examples:
– Faintest object (galaxy) in Hubble extreme deep field: magnitude 31
– Faintest object observable with a 6 inch aperture telescope: 13
– Faintest object observable with a typical binocular magnitude: 9
– Faintest stars visible to naked eye: magnitude 6
– North star 2
– Mars these days 1.1,
– faintest stars visible near the physics department: magnitude 1-2
– Betelgeuse: 0.4
– Vega:
brighter
magnitude 0 – reference star
– Sirius (brightest star in the sky): -1.3
– Venus these days: –4.7,
– Full Moon
– Sun -27
-13
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Hipparchus’ magnitude system
The reason for this seemingly awkward system is in its’
history.
Greek astronomer Hipparchus prepared a detailed star catalog
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in 129 BCE.
– It was the most accurate star catalog of the antiquity.
– He wanted to record the brightness of a star as well as its
position in the sky
• He called the brightest stars in the sky 1st class stars
• the next brightest stars 2nd class stars, and so on
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Hipparchus’ star classification became popular with later astronomers and
they kept using his system of 1st to 6th (magnitude) for almost two millennia.
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With a telescope stars fainter than 6th magnitude are visible
– so after the invention of the telescope in 17th century, there was a need to
extend the magnitude scale.
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19th century astronomers refined and quantified the magnitude system to the
modern form, while keeping most of the features of Hipparchus original
‘backward’ system.
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Response of the human eye is not linear, but logarithmic
actual brightness
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Thus the star brightness classification scale which was originally
based on observations by eye has this nonlinear logarithmic nature
Naming of Stars
A page from the star catalog in Ptolemy’s Almagest (2nd century CE)
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Most bright stars have proper names, like Polaris, Sirius, Rigel or Antares.
– Most star names are of Arabic origin few are Greek
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During the middle ages Arabic astronomers adopted Greek constellations.
– Greek astronomers identified stars by their location in constellations.
– Arabic astronomers followed the same in Arabic.
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Naming of Stars
When the Arabic texts were translated to
Latin in late Medieval period, those
descriptions of star positions in Arabic
were directly passed down to the west:
– Often in garbled Arabic with
translation errors and misplaced
meanings.
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and became the star names in use
today.
• Betelgeuse: armpit (of Orion in
Arabic)
• Rigel: left foot (of Orion)
• Deneb: tail of Swan
• Denebola: Lions tail
A page from Johann Bayer’s star atlas, Uranometria.
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Naming of Stars
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In 1603, the German astronomer Johann Bayer produced a star atlas,
Uranometria.
– There he assigned each star in a constellation a Greek letter, approximately in
the order of their apparent brightness.
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Bayer naming scheme is still in use for most of the bright stars
– ex: alpha Centauri: star a in the constellation Centaurus
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If the constellation has more than 24 stars, Greek letters followed by upper and
lower case Latin letters.
– a, b, g... w, A, b, c, … z
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In addition index numbers from many other catalogs are used - HD, SAO… 37
Motion of stars and shape of constellations over time.
A star moving 100 km/s travels 3.2x109km in a year
100 ly=9.5x1014 km
• Stars appear to have fixed positions in the sky while whole
sky rotates with them.
• Constellation patterns have not changed over millennia
• Hence the name “fixed stars” given by ancient astronomers
• Stars do move in space with considerable speeds
(e.g. the Sun moves 120 km/s in the Milky way)
• But distances to them are so large, change in their angular
positions too small for us to see.
• A star moving 100 km/s travels 3.2 billion km in a year
• If it is 100 light years (950 trillion km) away its position in
the sky changes only by 0.7 arc-second.
0.7”
100,000 years ago
now
in 100,000 years
Shape of the Big Dipper over time
Animation: www.phy.olemiss.edu/~perera/animations/um.gif
Change of the shape of Big Dipper due to motion of stars.
• But over long time spans (many thousands of years), there could be
noticeable changes.
Review Questions
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What would be the color of the sky few thousand kilometers above the Earth.
It is often said that stars twinkle but planets won’t. Do you agree?
On hot summer days often you see the view closer to the surface of a paved road
blurry and wavy. Why?
Twinkling is not desirable for astronomical observations since it blurs astronomical
images. What could astronomers do to overcome that?
Why giving the location of altitude and azimuth is not the best way to specify the
location of an object in the sky.
What is the declination of a celestial object if it is located
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(a) on the celestial equator
(b) on the North celestial pole
(c) on the South celestial pole
(d) if sometimes it is directly overhead (at the Zenith) from Oxford MS.
What is the location on Earth you would be able to see the whole sky.
Why does the North Star appear to be fixed in the sky while all other object
move?
What would be the view of the night sky from the North pole?
Why can't we see the same constellations in the evening sky throughout the year?
Why haven’t constellation patters changed since they have been established 3-4
thousand years ago.
How many constellation are there according to modern convention?
Name the three brightest celestial objects in the night sky?
Why isn’t the star magnitude scale linear?
Why are most star names Arabic while constellation names are Latin (or Greek
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origin) ?