by tonytran2015 (Melbourne, Australia).
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(Blog No. 168).
#find North, #finding North, #direction, #by stars, #Altair, #Fomalhaut, #September, #unclear sky,
Finding North from unclear sky around September.
Around September there are some bright stars shining the whole night. They include Deneb close to the Celestial North pole and Fomalhaut in the Southern sky. These stars can be used to locate the Celestial poles in the sky and subsequently the terrestrial principal directions.
1. Celestial poles and terrestrial directions.
Figure: The Sun, the Moon and the stars are attached to a Celestial sphere which encloses the Earth like a giant rotating cage.
To an Earth bound observer, the Earth appears to be enclosed by a large rotating spherical shell called the Celestial Sphere with all stars attached to it. This shell rotates around the Earth nearly one revolution every 24 hous. This rotation leaves unmoved only 2 points on the shell. They are called the Northern and Southern Celestial poles of the Celestial Sphere.
If an observer can locate one Celestial pole then the projection to the ground of the line from him to the pole will be along his terrestrial North South direction.
2. Locating the Northern Celestial Pole in Northern hemisphere.
Figure: Polar Inversion map for the Northern Celestial hemisphere. The map should be read with its September marking on its rim pointing towards the ground as illustrated here.
An observer has to align the polar map with marking for September on the rim (at 6 o’clock position) pointing downward. An observer in Northern latitude above 30 degrees will see the rotation of bright stars Vega, Deneb , Cassiopeia constellation, bright star Capella then Big Dipper constellations in that order.
Cassiopeia goes highest around 01am.
The bisector of the M shaped Cassiopeia goes through the Northern Celestial pole. The Northern Celestial pole is almost 30 degree below Cassiopeia.
Figure 2: Polar Inversion map of Northern Celestial 3/4 sphere.
3. Locating the Southern Celestial pole in Southern hemisphere.
Figure: Polar Inversion map for the Southern Celestial hemisphere. The map should be read with its September marking on its rim pointing towards the ground as illustrated here.
Figure: Polar Inversion map of Southern Celestial 3/4 sphere.
An observer has to align the polar map with marking for September on the rim pointing downward as illustrated here. An observer in Southern hemisphere or on the tropical zone would see Achernar  rising highest around 2am. Southern Celestial pole is the midpoint of Achernar and the two Pointers and is about 30 degree from Achernar.
4. Locating the Celestial poles from tropical stars.
A observer in the tropic should already know the two brightest stars Scorpius Antares (at the heart) and Scorpius Shaula (at the stinger end) of the Scorpius . The straight line from Antares to Shaula goes through the bright star Fomalhaut which is of 60 degree distances from both Scorpius Shaula and Altair  which is a star of July and is close to the Celestial equator.
Figure 2: The Mercator map of the sky for inhabitants of Tropical Zone. North direction is on its top. 24hr of R.A. is near the center and R.A. increases towards the left (East) of the map. The map is to be read South side up in the Southern hemisphere.
Figure: Fomalhaut and its nearby stars.
The bisector of the angle Shaula, Altair, Fomalhaut points to the Southern Celestial pole.
Southern Celestial pole is is of 90 degree distance from Altair and of equal 60 degree from both Scorpius Shaula and Fomalhaut.
5. Visibility of the stars.
Scorpius Altairs and Scorpius Shaula are stars of June that set close to midnight when viewed from tropical zone. Altair is a star of July that sets close to 02 am. Fomalhaut is visible for nearly the whole night in September.
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