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Equatorial Coordinate Measures
How can we measure the position of a body in the globe?
What in heaven?
Before studying the systems used in astronomy to fix the position of the objects on the celestial sphere ...
It is necessary to remember how to do this on the map and three-dimensional space
Objects of various sizes surround us everywhere occupying different positions in space
We know intuitively what is the closest to us
But how can we know mathematically what purpose is closer or farther from a point?
Or, What is the biggest? ...
To do this we use the comparison
But what do we compare? Two objects? ... If we superimpose two objects, we obtain a finding...
Some are bigger than others ... but also that some are closer than others.
The comparison can be done with many objects, but it is better to do it with an object that is taken as reference for all measures: the rule.
The rule is a measuring instrument which includes a graduated scale divided into units of length
eg cm or inches
Its divisions are based on the pattern length
The rule is based on what is known as a real line
What is the geometric representation of all real numbers
It has its origin at zero and extends to the right to infinity
And left the negative range, also to infinity
To put a point in a plane we use the so-called "Cartesian diagram"
Since it was invented by French mathematician Rene Descartes
Consisting of two real straight lines perpendicular to each other and intersecting at one point.
The horizontal axis is called x-axis (abscissa)
The vertical axis is the y-axis (ordinate)
The point where they intersect is the "origin of coordinates"
To find the coordinates of a point, we place it between axes and projected it on these
The place where it intersect will show us its coordinates
Each point has two coordinates
The first is "x" and the second "y"
Point 1 is located at (3,4), for example ...
How can we measure the position of a point on the sphere?
What must we take references?
To start, it is necessary to give some definitions
If we cut a sphere into two equal hemispheres, we obtain a great circle
The great circle will be called "EQUATOR"
The line perpendicular to this plane, which passes through the center of the sphere, is the LINE OF POLES
Poles are the points of the sphere cut by this line.
The line of the poles is called the "axis of the world"
And the Earth revolves around it
There are also other smaller circles, parallels to Equator that we calls "parallels"
And there are other greats circles: THE MERIDIANS
They are all the circles that pass through the line of poles and cut perpendicularly the Equator.
The ecliptic is the line which the sun appears to move over the year, same the planets
The Ecliptic down a level that is inclined 23° 27' respect the Equator.
Both planes intersect at one point: the VERNAL POINT
VERNAL POINT is the position of the Sun when it crosses the Equator (when its declination is 0°)
And this change from - to + or from + to -
This occurs on March 21th (Spring) and September 23th (Fall)
To put a point on the celestial sphere we need TWO PLANES OF REFERENCE
From which the distances are measured
We also need a starting point to start counting.
The plans depend on the system we used
The coordinates measured from EQUATOR are called "Equatorial Coordinates"
The foreground is the EQUATOR and the starting point is the VERNAL POINT
The second level is the meridian of the place we want to measure
How can we measure the absolute equatorial coordinates of a P dot located on the celestial sphere?
We do it through two arcs
One of them is called Right Ascension (R.A.)
And it is the arc that starts in the Vernal point and ending point on the meridian of P
Measured in the forward direction (counter clockwise)
Gives us the distance from the Vernal point. Can be measured both in degrees and in hours, minutes and seconds.
Before it was called "AR" and today is represented by "A"
The other is called DECLINATION: it is the arc from the Equator to the point P.
It gives us the height from Equator
It is measured in degrees and minutes and is represented by "d"
If the decline is measured in the southern hemisphere, "d" will be negative.
Thus, the equatorial coordinates are defined in following intervals:
"AR" varies between 0º and 360º (or between 0h and 24h)
"D" ranges from -90º to +90º
To know the equatorial coordinates of a star
We will measure the arches of the AR and the declination of the same
To measure the position of an object on the surface of Earth, we take two levels of reference:
Equator and the Greenwich Meridian
This meridian is used as 0 Meridian
Based on it for measuring lengths
LONGITUDE is measured on the Equator and at the most can be 360º
Latitude is measured on the meridian and may be +90° (North) or -90° (South)
LATITUDE (globe) is equivalent to declination (celestial sphere)
Another set of coordinates are the ecliptic coordinates. The difference in the equatorial is used to measure the plane of the ecliptic
It is also used the Vernal Point as the origin point and the same forward direction
But this is the subject of another documentary ...