by tonytran2015 (Melbourne, Australia).
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#find North, #finding North, #direction, #by stars, #Spica, #Bootes Arcturus, #Antares, #April, #unclear sky
Around April there are some bright stars shining the whole night. They include Spica, Bootes Arcturus and Antares. 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 1: Stars in the Northern hemisphere rotates anticlockwise around the North pole.
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.
Big Dipper constellation goes highest around 22 hr.
3. Locating the Southern Celestial pole in Southern hemisphere.
Figure 1: Stars in the sothern hemisphere rotates anticlockwise around the North pole.
An observer in Southern hemisphere or on the tropical zone would see the Southern Cross Pointers for the whole night.
4. Locating the Celestial poles from tropical stars.
Figure 1: 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.
An Earth bound observer in Southern hemisphere or on the tropical zone can identify the forward swept broom (or a duck foot (?), a bird foot (?) or a tree with 3 upper branches (?)). formed by the brightest star Sirius and four surrounding bright stars Canopus, Orion-Rigel, Betelgeuse and Procyon. The line Canopus to Sirius make the 35 degrees long broomstick handle and three lines from Sirius to each of the other three stars form three branches of the forward swept broom head (see the star maps). Sirius to Procyon is the trailing branch of the (three branched) broom head.
Doubling the travel from Sirius to Procyon takes us to another bright star named Pollux.
Two thirds of the line from Procyon to Pollux is a point on the Ecliptic (the great circle containing the.Sun and all planets). Turning anticlockwise 100 degrees at this point and traveling by a distance of 40 degrees takes us to a less bright star Leo Regulus. Keeping the direction from that two thirds point to Leo Regulus and travel for another 50 degrees takes us to a brighter star Spica. Spica is 90 degree in distance from the that two thirds point. (Observers from the Southern Hemisphere may also see that the great circle arc going through the long axis of the Southern Cross goes by 50 degrees to get very close to Spica. Draw the line from Southern Cross to Spica and then turns 30 degrees anticlockwise and continue for another 30 degrees to reach Bootes Arcturus.)
Turning clockwise by 90 degrees at Spica to leave the Ecliptic and traveling by 30 degrees takes us to a much brighter unmistakable star Bootes Arcturus.
Instead of turning right toward Bootes Arcturus, traveling along the Ecliptic for another 50 degrees take us to a bright star Antares in the Scorpius (Observers from the Southern Hemisphere may also see that Antares is 45 degrees clockwise and 45 degrees distance from the direction of dim Pointer to bright Pointer.).
Figure: Antares is the bright star in the Scorpius constellation which has the shape of a declawed scorpion. Two bright objects on the third top of the photo are planets traveling on the Ecliptic. Northern Celestial pole is from the top left (11 o’clock) direction of the photo.
The stars Spica, Bootes Arcturus, Antares form an arrow-head pointing North.
The midpoint of the great circle arc from Spica to Bootes Arcturus is almost on the Celestial equator. Rotating this arc clockwise by 30 degrees makes its extension goes through both Celestial poles. Northern Celestial pole is 90 degrees from the midpoint and on the side of Bootes Arcturus. Southern Celestial pole is 90 degrees from the midpoint and on the side of Spica.
The internal bisector of the angle formed by (Spica, Bootes Arcturus, Antares) points to the Southern Celestial pole while its rearward extension points to the Northern Celestial pole.
Figure: 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: Table of 20 brightest +2 stars in order of appearance.
5. Visibility of the stars.
Orion constellation, Sirius and its surrounding stars are visible after Sunset. Spica, Bootes Arcturus and Antares are all visible for nearly the whole night in April.
Figure 1: Azimuth and elevation angles of stars for equatorial observers.
Figure 2: Azimuth and elevation angles of stars for observers on 30 degrees North latitude.
Figure 3: Azimuth and elevation angles of stars for observers on 30 degrees South latitude.
6. CAUTION with planets
The Moon and planets travel on the Ecliptic. Observers should take care not to mistake any planet for a navigational bright star.
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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