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Stars in the night Sky
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At night the sky appears like a giant dome overhead, or an upside-down bowl set upon
the horizon as if on a table.
The bowl of night is studded with the light of thousands of stars, of varying apparent
magnitude.
o Some stars always retain the same spatial orientation with respect to each other;
these are the fixed stars.
o The fixed stars are distributed among 88 constellations (such as Ursa Major the
Big Bear).
o To the imagination, many star patterns other than the official constellations are
discernible; these patterns are called asterisms (such as the Big Dipper and the
Winter Hexagon).
 The rim of the bowl of night is the horizon, or azimuth circle.
 Azimuth = measured along the horizon, in degrees. How many degrees in a circle?
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By convention, azimuth is measured clockwise from due north.
Direction Azimuth
North
0 degrees
East
90 degrees
South
180 degrees
West
270 degrees
North-East-South-West = Never eat slimy worms.
Find north by using the Big Dipper to locate Polaris, the north star. Polaris is
closer to true north than a magnetic compass.
o Note: In Japan, azimuth is measured clockwise starting from the south.
The point directly overhead is called an observer's zenith. Opposite the zenith is the
nadir, directly beneath one's feet.
o Are zenith and nadir points horizon-dependent? That is, do they differ for
observers at different locations?
o Are zenith and nadir points time-dependent? That is, do they differ for an
observer at the same location but at different times?
o Is it meaningful to speak of the azimuth of a star at the observer's zenith?
A line (arc) from the point due north on the horizon (0 degrees) passing through the
zenith and intersecting the horizon due south (180 degrees) is called the meridian.
o Polaris always lies on or near the meridian. What is the azimuth of Polaris as
seen from Shawnee?
Altitude = measured above the horizon in degrees.
o What is the altitude of Polaris as seen from New Paltz?
o What is the altitude of a star at the observer's zenith?
o Is it meaningful to speak of altitudes greater than 90 degrees?
o The maximum altitude, 90 degrees, is the zenith. (Zenith is a great name for a
TV: Since dust and horizon haze obscure the sky at lower altitudes, when you
look toward the zenith you get a "clearer picture.")
o Note that altitude in this sense is measured in angular degrees, and has nothing to
do with height above the ground or elevation above sea level.
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Altitude-Azimuth coordinates uniquely specify a given point with respect to an
observer's horizon at an indicated time.
o Do any two different locations in the sky have the same pair of altitude-azimuth
coordinates?
o Use a protractor outdoors to estimate the altitude of Polaris, or to measure
degrees of azimuth along the horizon from due north.
o Horizon coordinates vary with locality, but are still useful in sky watching and
are used with many telescope mounts.
Any star or planet that can be located on the meridian is said to be at meridian transit or
culmination.
Definition of "meridian transit"
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The passage of the apparent centre of an object's disk through a meridian. The meridian is usually
the observer's meridian, and the meridian transit is almost coincident with the time when the object
reaches its highest point in the sky. Sometimes, just the word "transit" is used to indicate a meridian
transit, and this is not to be confused with the transit of one body in front of another.
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When should I look for a planet or a constellation?
 How can I tell whether a constellation is ever visible from my location?
 When a planet or star is transiting the meridian it is at its highest in the
sky and therefore it is farthest from dust and horizon haze. The best time
to see it, then, would be around its midnight culmination.
 When a planet or star is transiting the meridian at midnight and it is
opposite the sun--it would be noon on the opposite side of the earth. If it
is high enough, a planet or star at "opposition" may be visible all night
long:
 Rises in the east when the sun sets in the west.
 Transits the meridian at midnight.
 Sets in the west when the sun rises in the east.
 Consider: What is the phase of the moon when it is in
opposition?
 Where should I look for a planet or star after its meridian transit?
 If its altitude on the meridian is high enough, a given
constellation may be visible in the early evening sky for two or
three months after the date of its midnight culmination.
 After the date of its midnight culmination it will transit the
meridian at an ever earlier time each evening, until eventually it
will already be in the western sky when it first appears at sunset.
 Finally, it will set with the sun and enter the daylight sky.
 List of constellation midnight culminations
 Try to identify the constellations listed for each month of the
year! Find one for each month.
 Before midnight, look for each month's constellations rising
higher in the eastern sky.
 After midnight, look for them setting in the western sky.
 Before midnight, look for the previous month's constellations
setting in the western sky.
 The most southerly constellations may only be visible at their
midnight culmination, if they never get very high in the sky.
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Use a planisphere to locate any constellation visible at any time
of night on any day of the year.
What is an asterism? Give two favorite examples.
An asterism is a star-pattern that is not a constellation. Asterisms may be contained within a
single constellation or consist of stars belonging to different constellations. The Big and Little
Dippers are asterisms within the constellations of Ursa Major and Ursa Minor. The Summer
Triangle and Winter Hexagon are asterisms that include stars from several constellations.
Related Note: "Asterism" derives from the Greek word for star, aster. However, in ancient
astronomical texts aster was often used for a combination of stars rather than for single stars.
Aster could also refer to a planet, or to a particular configuration of planets or a conjunction of
planets and stars. This usage might be implicit in Matthew's description of the Star (aster) of
Bethlehem which, if it was not an angelic or supernatural phenomenon, need not have been a
single star.
What is diurnal motion?
How long does it last before repeating?
Is circumpolar motion one form of diurnal motion?
"He whose roof is heaven, who has no other cover, over whom the stars continually rise and set
in one and the same course, makes the beginnings of his affairs and his knowledge of time depend
upon them."
Al-Biruni
Diurnal motion is the daily motion by which the stars appear to revolve around the North Pole
roughly once every 24 hours. It is as though the stars, one might imagine, were bright spots fixed
on the inside surface of a giant celestial sphere which rotates around us.
Stars that are close to the North Pole never set beneath the horizon, and their circular paths are
referred to as circumpolar. Stars lying farther away from the pole dip below the horizon, which
cuts off the circular pattern of their motion. These stars appear to rise in the east, ascend to the
meridian, and then set in the west, roughly once per day.
Daily Motion: the alternation of day and night.
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The Sun, Moon, planets, and stars all move westward across the sky each day (that is,
from east to west). They repeat this general westward motion about a day later. All rise
roughly in the east, ascend in the eastern sky until they reach their maximum height
(when they "transit the meridian"), then descend in the western sky until they set roughly
in the west. This is called their daily or "diurnal" motion.
"Diurnal" derives from the Latin noun "dies" which means "day" (as in "diary"), and
from "diurnalis," which means "daily." The latter, pronouncing the "i" as a "j," is the
etymological source for "journal" as well.
How can one find Polaris?
In which constellation does Polaris lie?
Find Polaris by tracing a line from the pointer stars of the Big Dipper. Polaris lies in the
constellation Ursa Minor, the Little Bear. As explained in Starstruck Tonight:
If you can find the Big Dipper in the sky, you have a skymark to orient yourself both on the Earth
and in the Heavens. The two stars that form the pouring side of the bowl point to Polaris, the
North Star. Polaris is a rather faint star about five times farther away than the distance between
the pointers themselves, and marks the tip of the handle of the Little Dipper. The Big and Little
Dippers pour into each other, just as the Big Bear and the Little Bear ceaselessly turn around and
around the northern sky. The Guard stars of the Little Dipper protect Polaris from the Great Bear,
just in case he might try to catch the North Star for himself.
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Precession: North Pole drift
o Is Polaris always the North Star?
o When will Polaris be closest to true north?
o What was the North Star when the Great Pyramid of Khufu (Cheops) was built?
o What will be the North Star 2000 years from now?
o What is the period of precession?
o Sky chart showing North Pole drift during precession.
What does circumpolar mean?
Circumpolar means to circle around the pole. Circumpolar stars or constellations daily trace
circles around the north celestial pole, without setting or dipping below the horizon. They move
in a counterclockwise direction. On any given night, the constellation is visible during a portion
of the circle; the rest is traced out when the constellation is hidden in the daylight sky.
Important circumpolar constellations:
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Ursa Major, the Big Bear (includes the Big Dipper)
Ursa Minor, the Little Bear
Cassiopeia, the Queen of Ethiopia
Cepheus, the King of Ethiopia
Draco the Dragon
In Summary
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As we face north from New Paltz, the north celestial pole is found 42 degrees above the
northern horizon (latitude of New Paltz = 42 degrees north).
Polaris, the tip of the tail of Ursa Minor, the Little Bear, marks the north celestial pole to
with an error of about one angular degree.
The pointer stars of the Big Dipper point toward Polaris. The pointer closest to Polaris is
Dubhe, the tail of the Big Bear, which lies about 28 angular degrees from Polaris.
Every 24 hours the Big Dipper completes a full circle around Polaris (or the Big Bear
circles the smaller bear). This counterclockwise motion can be used as a star clock to tell
local, universal, or sidereal time.
The Big Dipper is found crossing the meridian at midnight near the end of March.
What is UT?
Universal Time (UT) is the time at the longitude of Greenwich Observatory in England.
Why do I need to know about UT?
By convention, astronomers give the times of planetary events (e.g., eclipses, occultations, etc.) in
UT. Each observer at any longitude around the world is expected to know how to convert a date
and time from UT into his or her own time. For example, the planet table in every issue of Sky
and Telescope lists planetary positions on given dates at 0 hours UT.
How to Convert UT to North American local times
1. Subtract the following number from UT to obtain the indicated North American standard
time:
o East coast: Subtract 5 from the UT to obtain Eastern Standard Time (EST)
o Midwest: Subtract 6 from the UT to obtain Central Standard Time (CST).
o Mountains: Subtract 7 from the UT to obtain Mountain Standard Time (MST)
2. From October to April, add 1 to the Standard Time in order to obtain Daylight Savings
Time (DST; = Central Daylight Time, CDT).
3. If the result is a negative number, add 24 but use the day before the UT date.
The 88 officially-recognized constellations
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ANDROMEDA
ANTLIA
APUS
AQUARIUS
AQUILA
ARA
ARIES
AURIGA
BOOTES
CAELUM
CAMELOPARDUS
CANCER
CANES VENATICI
CANIS MAJOR
CANIS MINOR
CAPRICORNUS
CARINA
CASSIOPEIA
CENTAURUS
CEPHEUS
CETUS
CHAMAELEON
CIRCINUS
COLUMBA
COMA BERENICES
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CORONA AUSTRALIS
CORONA BOREALIS
CORVUS
CRATER
CRUX
CYGNUS
DELPHINUS
DORADO
DRACO
EQUULEUS
ERIDANUS
FORNAX
GEMINI
GRUS
HERCULES
HOROLOGIUM
HYDRA
HYDRUS
INDUS
LACERTA
LEO
LEO MINOR
LEPUS
LIBRA
LUPUS
LYNX
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LYRA
MENSA
MICROSCOPIUM
MONOCEROS
MUSCA
NORMA
OCTANS
OPHIUCHUS
ORION
PAVO
PEGASUS
PERSEUS
PHOENIX
PICTOR
PISCES
PISCIS AUSTRINUS
PUPPIS
PYXIS (OR MALUS)
RETICULUM
SAGITTA
SAGITTARIUS
SCORPIUS
SCULPTOR
SCUTUM
SERPENS
SEXTANS
TAURUS
TELESCOPIUM
TRIANGULUM
TRIANGULUM AUSTRALE
TUCANA
URSA MAJOR
URSA MINOR
VELA
VIRGO
VOLANS
VULPECULA
Month of midnight culmination for constellations at least partially visible from America
Prominent or important constellations for northern hemisphere observers are noted in bold. To
plan a skywatch, look at the web pages for the constellations in bold a month or two on either side
of the current date. For example, in January, plan to look for the constellations listed in bold for
November through March. Also check out the circumpolar constellations visible anytime (below).
Where should I look for a constellation on a date before or after its midnight culmination? What
is midnight culmination?
January
February
March
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CANCER
CANIS MAJOR
CANIS MINOR
GEMINI
LYNX
MONOCEROS
PUPPIS
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CANES VENATICI
COMA
BERENICES
VIRGO
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BOOTES
CORONA BOREALIS
LIBRA
LUPUS
NORMA
CENTAURUS
CORVUS
CRATER
HYDRA
LEO
June
May
April
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ANTLIA
ARGO NAVIS (extinct
constellation)
LEO MINOR
PYXIS (OR MALUS)
SEXTANS
VELA
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CORONA
AUSTRALIS
HERCULES
OPHIUCHUS
SCORPIUS
SERPENS
July
August
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AQUILA
CYGNUS
DELPHINUS
LYRA
SAGITTA
SAGITTARIUS
SCUTUM
TELESCOPIUM
VULPECULA
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October
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ANDROMEDA
ARIES
CETUS
PHOENIX
TRIANGULUM
September
AQUARIUS
CAPRICORNUS
EQUULEUS
GRUS
LACERTA
MICROSCOPIUM
PISCIS AUSTRINUS
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December
November
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PEGASUS
PISCES
SCULPTOR
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ERIDANUS
FORNAX
PERSEUS
TAURUS
AURIGA
CAELUM
COLUMBA
LEPUS
ORION
Constellations visible anytime
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CAMELOPARDUS (December)
CASSIOPEIA (October)
CEPHEUS (September)
DRACO (May)
URSA MAJOR (March)
URSA MINOR (May)
Never visible from most of America
Magnificent southern skies!
These constellations lie far south of the celestial equator. Their declination is more than 55
degrees south, therefore they are never even partially visible from a terrestrial latitude of 35
degrees north.
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ARA
APUS
CARINA
CHAMAELEON
CIRCINUS
CRUX
DORADO
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HOROLOGIUM
HYDRUS
INDUS
MENSA
MUSCA
OCTANS
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PAVO
PICTOR
RETICULUM
TRIANGULUM
AUSTRALE
TUCANA
VOLANS