Equatorial System

Observer/Local’s Horizon: Always perpendicular to the line joining observer's zenith and nadir.

Observer/ local meridian: A Great Circle which passes through N.C.P., Zenith(Z), S.C.P. and Nadir(Z’).

👉visit for Nadir and Zenith here

East and West Point: where celestial equator and observer’s horizon are meet.
East is in the clockwise direction of rotation from north.

Diurnal motion: We all know that earth moves from west to east. Then celestial’s objects will appear to move in east to west direction. This type of motion is called diurnal motion. And when we assume that we are at rest. Then celestial objects move from east to west and again come the same position. They are move along circular path. Like, our sun, we can assume that our sun move from east to west and come again same position at the after 24h. then we can say that sun along a circular path. For celestial objects this circular path is called Diurnal circle.

Celestial objects have completed one revolution in 24h. Because earth complete one revolution in 24h.
When star touch upper side of the Horizon. Then we call it Upper transit.
When it touch lower side of the Horizon then we call it Lower transit.

Hour Circle: always perpendicular to Celestial equator and joining the N.C.P. to star to S.C.P..
Hour Angle(H): arc length RJ along Celestial equator(0 to 24h) from Observer’s meridian(N.C.P. + Z+ S.C.P + Z’).
R= intersection of observer’s meridian (N.C.P. + Z+ S.C.P + Z’) and celestial equator above the horizon.
J= intersection of hour circle(N.CP. to STAR to S.C.P.) and celestial equator.
Declination(δ): arc length JX along the star’s hour circle(0 to 90). It is tells us position of star.
If δ= +ve , means objects above the equator.
If δ= -ve , means objects below the equator.

When star move along the diurnal circle then JX will not change. Because Celestial equator and diurnal circle are parallel.
Earth moves from west to east. if we consider star is fixed so, position of observer change due rotation of earth about it’s own axis. If position of observer change then Z and Z’ will change. Then Horizon and observer’s meridian will also change.

Hour angle RJ will change because it’s move along the celestial equator. When Star is moving. then Position of Hour Circle will change Because hour circle(N.C.P. To Star to S.C.P.) will be changing. So, here Hour circle has change move ,so point R will be moving so that RJ will move.
Or if observer 2 located at different place then for observer 2 has its own observer’s meridian and In that case hour circle has fixed for a finding star. So, here observer’s meridian has change ,so point R will be moving so that RJ will move.

Also visit:

for first method: visit here Horizon system

for basic: visit here

for celestial sphere: Celestial sphere