Field Astronomy Survey terminology notes

Home

Here are some expected FAQ related to Field Astronomy in SSC, PSC and other competitive exams.

Celestial sphere  

It is an imaginary sphere on which the stars appear to lie or to be studded.

Figure shows Celestial Sphere

Zenith (z)  

It is the point in the upper portion of the celestial sphere marked by plumb line above the observer. It is the point on the celestial sphere immediately above the observer’s station.

Nadir (Z’, or, N) 

It is the point on the lower portion of the celestial sphere marked by plumb line below the observer. It is the point on the celestial sphere vertically below the observer’s station.

Celestial Horizon or true or Rational horizon or geocentric horizon 

It is the great circle traced upon the celestial sphere by that plane which is perpendicular to the zenith –Nadir line and which passes through the centre of the earth.

(One should not confuse Horizon with equator. Equator is perpendicular to the axis of rotation of earth while Horizon is perpendicular to Zenith -Nadir which is dependent on observors position)

Sensible horizon
It is a circle in which a plane passing through the point of observation and tangential to the earth’s surface intersects with celestial sphere. The line of sight of an accurately leveled telescope lies in this plane.

Visible horizon
It is a circle of contact, with the earth, of the cone of visual rays passing through the point of observation.

Figure shows Zenith,Nadir, Sensible horizon, Visible horizon and Celestial horizon

Terrestrial poles and equator 
Terrestrial poles are the two points in which the earth’s axis of rotation meets the earth’s sphere. 

Terrestrial equator is the great circle of the earth, the plane of which is at right angles to the axis of rotation. The two poles are equidistant from the terrestrial equator.

The celestial poles and equator 
If the earth’s axis of rotation is produced indefinitely, it will meet the celestial sphere in two points called the North & South celestial poles (P and P’).
The celestial equator is the great circle of the celestial sphere in which it is intersected by the plane or terrestrial equator.

Figure shows Terrestrial poles, Terrestrial equator ,Cellestial poles and Celestial Equator

Vertical circle 
A vertical circle of the celestial sphere is great circle passing through the zenith and nadir. They all cut the celestial horizon at right angles.

Prime vertical

It is the particular vertical circle which is at right angles to the observers meridian that passes through the east & west points of horizon.

Figure shows Vertical circle, Prime Vertical and Observer's Meridian

Meridian is a great circle on the surface of earth pasing through the poles or circle of constant longitude or It is the great circle passing through zenith nadir and poles.

Observers Meridian or Reference Meridian
Observers meridian is a special meridian which passes through the zenith and nadir of the point as well as through the Celestial poles.

Latitude (θ)

It is the angular distance of any place on the earth’s surface measured towards north or south of the equator. It is also defined as the angle between the Zenith and the Celestial equator or Nadir and Celistial equator. Range 0°to 90°

Co-latitude

The co-latitude of a place is the angular distance from the Zenith or Nadir to the pole. It is the complement of the latitude and equal to (90°- Latitude).

The longitude (Ф)

The longitude of a place is the angle between a fixed reference meridian called the prime or first meridian and the meridian of the place. Range  0°to 180°

Figure shows Azimuth 'A' and Altitude 'α'

Azimuth (A) It is the horizontal angle between the observer’s meridian and the vertical circle passing through the heavenly body(declination circle).

The altitude (α)

The altitude of celestial or heavenly body (i.e., a sun or star) is its angular distance above or below the horizon, measured on the vertical circle passing through the body.

The co-altitude or zenith distance (z)
It is the angular distance of heavenly body from the zenith. It is the complement of the altitude.

. 

Declination 
The declination of a celestial body is angular distance from the plane of the equator, measured along the star’s meridian.Declination varies from 0° to 90°, and is marked + or – according as the body is north or south of the equator. Great circle of celestial sphere passing through Celestial body as well as celistial poles.

Figure Shows Declination circle or Hour circle

Hour circle 

Hour circles are great circles passing through the north and south celestial poles. The declination circle of a heavenly body is thus its hour circle.

Figure shows Hour angle

Hour angle (H)
The hour angle of a heavenly body is the angle between the observer’s meridian and the declination circle passing through the body. The hour angle is always measured westwards.

Ecliptic

Apparent path of sun on celestial sphere with respect to earth.

Hope it was helpful.

Home

 
Some Basic Definitions from KMBR 2019    Comparing NBC 2016 and KMBR 2019 Building Groups
Colour Schemes in KMBR 2019    Well and Rainwater harvesting    Coverage and Floor space index    Staircases and fire escape staircases and Ramps    Astronomical Field survey  Terminology    Building laws NBC 2016+ KMBR 2019    Major Highway developments in India -Shotnotes    Why is clay impermeable? Introduction to Survey    Chain Survey      Strength of Materials - Stress Strain    Static and Kinematic Indeterminacy    Methords of Structural Analysis    Arches

AutoCAD    Building Rules Kmbr 2019 + NBC2016     Surveying    Estimation and Valuation     Structural Analysis     Strength of Material    RCC    Geotechnical Engineering    Transportation Engineering