by tonytran2015 (Melbourne, Australia).
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#find North, #finding North, #direction, #by stars, #Vega, #Deneb, #Altair, #July, #unclear sky
Around July there are some bright stars shining the whole night. They include Vega, Altair and Deneb. These three 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 July marking (at 4 o’clock position) on its rim pointing towards the ground.
An observer has to align the polar map with marking for July on the rim (at 4 o’clock position) pointing downward. An observer in Northern latitude above 30 degrees will see the rotation of three bright stars Vega, Deneb, Capella then Big Dipper constellations in that order.
Vega and Deneb go highest around 24 hr.
The bisector of the line Vega Deneb goes through the Northern Celestial pole. The pole is almost of equal distances of 45 degrees from each of them.
3. Locating the Southern Celestial pole in Southern hemisphere.
Figure: Polar Inversion map for the Northern Celestial hemisphere. The map should be read with its July marking (at 8 o’clock position) on its rim pointing towards the ground.
An observer has to align the polar map with marking for July on the rim (at 8 o’clock position) pointing downward. An observer in Southern hemisphere or on the tropical zone would see the Southern Cross Pointers highest around 18 hr, then Antares around 20 hr. Achernar is seen rising before Sunrise. The midpoint between the Pointers and Achernar is almost the Southern Celestial pole.
4. Locating the Celestial poles from tropical stars.
A observer in the tropic should already know the very bright star Bootes and the bright star Antares in the Scorpius constellation used in April.
Figure 1: Photograph of Spica (near the bottom edge), Bootes Arcturus (near the right edge) and Antares (1/8 of the width from the left edge) forming a triangle. Celestial North is at 01 o’clock position (30 degree clockwise from vertical) in this photo. There is a very bright planet (1/2 from left edge, 1/3 from bottom) traveling on the Ecliptic in this photo.
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.
On the trailing side of Bootes Arcturus and Antares, (click the above Mercator map for details) there is a bright star of equal distances of 60 degrees to both of them. This star is Altair, which is as bright as Antares.
Following the line Antares to Bootes Arcturus, turning anti-clockwise by 90 degrees at Bootes Arcturus and traveling for 60 degrees takes us to Vega, which is as bright as Bootes Arcturus. The line (Antares, Arcturus) is nearly at right angle to the almost straight line (Spica, Arcturus, Vega).
The line (Bootes Arcturus, Antares) is nearly parallel to the line (Vega, Altair) which lie slightly nearer to Bootes Arcturus.
Vega is as bright as Bootes Arcturus while Altair is as bright as Antares.
Figure 3: Polar Inversion map of Northern Celestial 3/4 sphere. The line (Vega, Altair) is 30 degrees long and is nearly parallel to the line (Bootes Arcturus, Antares) which is 60 degrees long.
Rotate the line (Altair, Vega) about Altair by 10 degrees counter-clockwise give the great circle through the two Celestial poles. Northern Celestial pole is nearer to Vega and is 80 degrees from Altair. Southern Celestial pole is nearer to Altair and is 100 degrees from Altair.
Trailing 2 hour behind Vega is the bright star Deneb (see the Mercator star-map). (Vega, Altair, Deneb) is known as the Summer Triangle. The bisector of the angle Vega, Altair, Deneb points to the Northern Celestial pole.
Figure 4: Table of 20 brightest +2 stars in order of appearance.
5. Visibility of the stars.
Altair is visible for nearly the whole night in July. The Summer Triangle is visible for nearly the whole night in July. Click on the Mercator map for details.
6. CAUTION with planets
The Moon and planets travel on the Ecliptic. Observers should take care not to mistake any planet for Antares in the Scorpius constellation.
A planet is always brighter than any star, including Sirius, moves from night to night, and does not twinkle in clear sky.
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