Transcript CONSTELLATIONS

CONSTELLATIONS
• Constellation: formation of stars
perceived as a figure or design.
– 88 recognized groups named after characters
from classical mythology and various common
animals and objects.
– Example: Ursa Major and Ursa Minor
• Asterism: star patterns that are not
constellations. They may be contained
within single constellations or between
multiple constellations.
• Example: Big Dipper (in Ursa Major)
• Ecliptic: the great circle representing the
apparent annual path of the Sun; the plane
of the Earth's orbit around the Sun; makes
an angle of about 23 degrees with the
equator; "all of the planets rotate the sun
in approximately the same ecliptic"
http://brahms.phy.vanderbilt.edu/~rknop/astromovies/ecliptic.html
Celestial Sphere
• A sphere surrounding the Earth that helps
us designate a location for stars in our
night sky.
• The Celestial Equator is an extension of
our own equator
• http://brahms.phy.vanderbilt.edu/~rknop/as
tromovies/celsphere1.html
• Declination: one of the two coordinates of
the equatorial coordinate system.
Comparable to Latitude. Expressed as an
angle with respect to the celestial equator.
Declination
• Declination is
measured in terms of
the Celestial Equator
• The zenith is the
direction pointing
directly "above" a
particular location
• A celestial object that
passes over zenith has
a declination equal to
the observer's latitude.
What would our Zenith’s declination
be???
Declination
• An object on the
celestial equator has a
declination of 0°.
• An object at the
celestial north pole has
a declination of +90°
(North Star).
• An object at the
celestial south pole has
a declination of −90°
(Southern Cross).
Declination
• A pole star therefore
has the declination
near to +90° or −90°.
• Your latitude on Earth
determines how much
of the Southern or
Northern sky you can
see…
• The Celestial Equator
for us is…90°- 45° =
45° latitude
So how much of the Southern
Celestial Sphere can we see???
• Side view of a Declination Arc for an observer at
45° Latitude.
• Right Ascension: a celestial longitude
measured in the direction of the Earth’s
rotation. The notation adopted for right
ascension is in terms of hours and minutes
with 24 hours representing the full circle.
The “left-right” coordinates.
Right Ascension
• Since the Earth is rotating constantly, the Right
Ascension "value" changes continuously - every minute
of the night. It also changes every day of the year as
Earth goes around the Sun.
We start this numbering system at the Vernal Equinox i.e. the point in the sky where the Sun was at the time of
the Spring Equinox. From that specific point on the
Celestial Equator, they simply numbered the "hours" 0,
1, 2, 3, and so on up to 22, 23, and 24 was made the
same as the 0 line.
These are numbered East (i.e. to the left) to West (Sun
rises in East and sets in West).
• Circumpolar star: a star that as viewed
from a given latitude on Earth, never
disappears below the horizon, due to its
proximity to one of the celestial poles.
Visible for the entire night (and throughout
the day if it wasn’t for the sun’s glare)
every night of the year.
Northern Circumpolar Stars
Northern Circumpolar Stars
Northern Circumpolar Stars
Northern Hemisphere Circumpolar
Constellations
• In the Northern Hemisphere, there are 6 such
constellations (above latitude 40 N).
• The constellations are:
• Ursa Minor (the Little Dipper or the Lesser Bear)
• Ursa Major (the Big Dipper or the Great Bear)
• Cassiopeia (the Lady in the Chair - the constellation
shaped like a "W“)
• Cepheus (the King)
• Draco (the Dragon)
• Camelopardalis
Southern Hemisphere Circumpolar
Constellations
• In the Southern Hemisphere, there is no “South
star”. However, there is the Southern Cross
(about 90°)
• The constellations seen here are:
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Toucan
Octans
Triangulum Australe
Chameleon
Mensa
Hydrus
Southern Cross
Apus
Musca
Volans
Reticulum
Pavo
Traditions
• Ursa Major: Navajo (Native
American) myth
• “The Great Bear”
• Story: A girl accepts a bear as her
husband. Her younger sister tells the
father, who in turn kills the bear. The
older sister changes into a bear to get
revenge.
• The younger sister and seven brothers
tried to flea their sister. The bear turns
back into a girl and chases after her
siblings. She eventually killed six of
the brothers.
• The seven brothers flew up into the
sky and became Ursa Major.
Traditions
• Ursa Major: Greek myth
• Story: Callisto (river goddess) was a
companion of Diana (moon goddess). Diana
warned Callisto about men/gods.
• Jupiter saw Callisto and fell in love with her.
He pretended to be a goddess and made
Callisto his girlfriend. She gave birth to a boy
(Arcas) but Jupiter’s wife (Juno) was angry
and changed Callisto into a bear.
• When Arcas was a young man he went
hunting. Callisto (now a bear) rushed toward
him to hug him. Arcas was afraid and aimed
to shoot the bear with his bow.
• Just before the arrow struck, Jupiter tossed
Callisto and Arcas into the heavens as the
constellation Ursa Major (the Great Bear and
Bootes, the Bear Warden).
• Now Arcas is always next to his mother.
Why do cultures create stories
about the sky?
• Religion – gods, telling stories about the gods
• Greek heroes and legends
• Calendars and Agriculture (Orion indicated
Winter was coming)
• Navigation
• Today, constellations allow us to name stars
systematically (Alpha Tau is brightest star in
Taurus constellation, Beta Ori is second
brightest in Orion constellation, etc)
The North Polar Sky
declination 90° to 50°
• Polaris lies less than 1° from the North
celestial pole
• For observers in the Northern
Hemisphere the stars in declinations
90° to 50° are circumpolar
• The viewer’s latitude will determine
how much of the sky is circumpolar
The South Pole Sky
-50° to -90°
• There is no Southern equivalent of
Polaris
• Many of these stars are circumpolar for
observers in the Southern Hemisphere.
Equatorial Sky
(Chart 1: 21h to 3h contains 0h line of right ascension)
• Observed during evenings in Sept, Oct
and Nov
• Contains the Vernal equinox in Pisces
(the Sun will pass through here in late
March)
Equatorial Sky
(Chart 2: 15h to 21h)
• Observed during evenings in June, July
and August
• Contains the point where the sun
reaches its most southerly declination
in Sagittarius around Dec 21
The Summer Triangle (Asterism)
• Deneb (Cygnus)
• Vega (Lyra)
• Altair (Aquila)
Equatorial Sky
(Chart 3: 9h to 15h)
• Observed on evenings in March, April
and May.
• Contains the Autumn equinox in Virgo
around Sept 21
Equatorial Sky
(Chart 4: 3h to 9h)
• Observed during evenings in
December, January and February
• Contains the point at which the Sun is
furthest North on the border of Taurus
and Gemini around June 21