CONSTELLATIONSCONSTELLATIONS

• 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/astromovies/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:

• Toucan Southern Cross • Octans Apus • Triangulum Australe Musca• Chameleon Volans • Mensa Reticulum • Hydrus 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 SkyThe North Polar Skydeclination 90declination 90°° to 50 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 SkyThe South Pole Sky-50-50°° to -90 to -90°°

• There is no Southern equivalent of Polaris

• Many of these stars are circumpolar for observers in the Southern Hemisphere.

Equatorial SkyEquatorial Sky(Chart 1: 21h to 3h contains 0h line of right ascension)(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 Equatorial Sky (Chart 2: 15h to 21h)(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 SkyEquatorial Sky(Chart 3: 9h to 15h)(Chart 3: 9h to 15h)

• Observed on evenings in March, April and May.

• Contains the Autumn equinox in Virgo around Sept 21

Equatorial Sky Equatorial Sky (Chart 4: 3h to 9h)(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