How can I point stars and planet by knowing declination&right assention? ?

Question

what is the diference between declination&right assention,azimuth&altiude?

Answer 1

I have a telescope that sits on a lazy Susan - it can rotate, and it has a tilt for the telescope to point up and down.  That's azimuth (the lazy Susan) and altitude.  If you put marks on the lazy Susan, and align it with North, then 0 is North, 90 degrees is East, and so on.  And you can measure the angle from the ground that it points up.  You can figure out where a star should be if you know the place on Earth, and the date/time.  It's quite a bit of math, but even really cheap computers can do this easily now.  So there are telescopes like this with a computer, that once it knows where the sky is, it can tell you where stars and other objects are.  My scope's computer only needs to know which way is up, and where two known stars are.

A tracking scope, perhaps with a German Equatorial mount, is aligned to the rotation of the Earth.  The easy way to do this is to align the axis of rotation to the North Star.  Then, a motor moves the scope at the same rate that the Earth rotates - once around per day, but in the opposite direction, so that the scope moves exactly the same rate across the sky that the stars do.  You then align the rotation by looking at any star whose right ascension you know.  Then right ascension is a coordinate of rotation - measured using a 24 hour "clock" instead of degrees, and declination is the number of degrees north or south of the celestial equator.  Positive for North, negative for South.  So Polaris, near true North is nearly 90 degrees.  The cool thing about RA and Dec is that the coordinates of stars and other objects doesn't change much.  So you can look them up, point your scope that way, and there it is.

Answer 2

These are two different coordinate systems used to describe the position of astronomical objects. Azimuth and altitude are relative to the observer's position on the Earth; right ascension and declination are relative to a celestial coordinate system based on the Earth's position in space.

Azimuth and altitude are rarely used, because they are constantly changing and must be recalculated all the time. Right ascension and declination are primarily used by professional astronomers, whose telescopes have accurate scales to measure these. Many amateur telescopes have right ascension and declination scales, but these are usually too inaccurate to point at objects. Amateurs generally find objects be starhopping: moving to an object by starting from a known bright star and following star patterns from a star atlas.

Answer 3

In order to fix a point in 2 dimensional map (any surface, including that of a globe or sphere) we need two co-ordinates. As the Earth spins, Equator & poles get fixed and are projected into the sky (which to all appearance is like another sphere out there). So we have celestial Equator that is right above (at zenith) in the sky over terrestrial equator. So also a pole each on top of its terrestrial counterpart. As objects in the sky are fixed as if on (apparently) a sheet of paper, Earth's diurnal motion makes it rotate (again apparently) in opposite direction. As Sun is the day-maker (='Dina kara' in Sanskrit), his position on the Celestial Sphere marks the mid-day (the principal meridian) and so the longitude is marked as '12 hour' Right Ascension. Along with Sun and like him but in a time-shifted pattern, every part of sky rises and sets over the 24 hour period completing the cycle in that period. So longitudes are divided into 24 hourly intervals and designated accordingly.

But Sun also shifts everyday to a new(apparent) position corresponding to Earth's position in its orbit. That is, Sun appears to occupy a shifted position every next day. When Sun crosses the Equator northward on (Spring Equinox) 21 March, that position on Celestial Sphere is marked as a reference '0 hr' RA (=longitude).

Now, the pole ward Declination (of position) from Equator is parallel to latitudes and Celestial pole is similarly marked 90d (+ for North Pole or - for South Pole is prefixed). So, that is Declination.

Azimuth and Altitude (altazimuth) is applied to your place and not the same as RA & Decl. A (tiltable) table is set and made 'horizontal' with the help of a water or spirit 'bubble' and then fixed. Extension in all directions of this table marks 'horizon'. Actually the table should rest on ground to be perfect. A spot in sky above this can be designated as  /_A (angle) from this 'horizontal' table or 'horizon, as altitude. Zenith is at an altitude (angle) of /_A=90d. Now, this altitude marks a whole circle in the sky and not just a spot. So we need 'Azimuth' (navigators call it 'Bearing') to take a fix of 'one' spot. On 'horizon' circle, there is a direction clearly marked 'North'. North is starting point or direction, Azimuth is reckoned from. It goes right or clock-wise (if you look down onto the 'horizontal' table from above), at 90d is East direction, 180d at the opposite direction is 'South', 90d furthermore at 270d is West and coming back to North after 360 degrees that is same as the starting point 0d.

How do we translate 'RA/Decl' system to Altazimuth?

At your place North can accurately be marked as the direction of Pole star (Polaris=Alpha Ursa Minoris). But this star is on the Horizon only at (on) the Equator, with /_A=0d. Altitude angle of Polaris is hence, the

latitude of your place. The point directly below it on the horizon is the point of Azimuth=0 d. Join East & West on a vertical (imagined) glass wall (plane), that passes through zenith. It is easy to show that from Polaris to these East & West spots (pivoted to Celestial Equator) is 90d. Mark spots (in the sky) 90d away from Polaris and you'll find it as another plane tilted away from the vertical plane by an angle of latitude of the place. This is the Celstial Equator translated to your place. Principal meridian perpendicular to this plane, passing through zenith, Polaris and extended beyond in both directions is the one for your timing of RA(Right Ascension). On March 21, the '0 hrs' stands on this meridian at midnight (00:00 hrs) and you can reckon others accordingly at that instant. From Celestial Equator you can measure off Decl. angles towards Polaris on that particular meridian (Great Circle joining the point with Polaris) if it is + and away from it (from Celestial Equator, of course) if it is -Decl. After March 21 (Spring Equinox), shift this RA on star chart by adding 4minutes (example: 00:04 hrs RA to coincide with the principal meridian on March 22; 02:00 hrs RA on April 21).