What are R.A and Dec? (Right Ascension and Declination)
How do I know where to look to find an object from the given values?
There are two coordinate systems that astronomers use. One is Altitude/Azimuth
which is referenced to your view from earth and is just compass direction and
altitude above the horizon. The compass direction is given in degrees starting
with north=0° and going through east=90°, south=180°, west=270°
and back to north=360°=0°. The elevation is also given in degrees and
is 0° at the horizon increasing to 90° which is overhead. A negative
altitude indicates an object below the horizon. This system is useful for
observation by naked eye or binoculars and for satellite passes.
The other system, the one asked about, is referenced to the
fixed stars. The disadvantage of Alt/Az for the stars is that, due to the
rotation of the earth, the coordinates change with time. It looks to us as
though the stars are going round the Earth, taking approximately one day
to do so. It would be more convenient if we had a system in which the
coordinates of a star or other object were fixed and the same for everybody,
irrespective of where thay are on the Earth.
This is what Right Ascension and Declination do. They are equivalent to
Longitude and Latitude but on the celestial sphere (imagine a hollow sphere
with us at the centre and the stars painted on the inside). The North Pole
of this sphere is (approximately) Polaris (the pole star). The sphere is
divided up into lines of latitude and longitude just like the earth, with a
celestial equator (roughly, but not quite the same as the path of the Sun
through the sky). To fix this in your mind imagine the earth stopped for a
moment so that the stars stand still. Then imagine the earth is transparent
with the lines of latitude and longitude drawn on its surface and a bright
light bulb at the centre of the earth. The shadows of the latitude and
longitude lines on the inside surface of the celestial sphere are the
coordinate system we are looking for, with the shadow of the equator being
the celestial equator. In mathematical terms they are the projection of the
lines of latitude and longitude on the (imaginary) celestial sphere.
The latitude lines are called the declination and are measured in degrees north
or south from the equator, north being positive and south negative. So the
declination can be from +90° to -90°. The lines of longitude are called
Right Ascension and are also measured in degrees, from 0° to 360°.
There is no Greenwich out among the stars, and we certainly do not want to use
the moving shadow of it as a zero reference point. A convention was needed for where
to start numbering from and it was decided to use the point where the Sun crosses
the celestial equator from south to north each year, this being known as the spring
(or vernal) equinox.
If your telescope is mounted on an equatorial mount then you can (usually) set
declination directly on it. To set the RA though you need to take account of the
date and time. There is a direct relationship between Alt/Az and RA/Dec but the
conversion isn't very convenient as it includes the date and time. The easiest
way to do this is through a planetarium program. These normally let you display
either or both sets of coordinates, and having access to the computer's date and
time fields can show you how the heavens are in real time, or at any other time
you set.
If you are not worried about total accuracy and just want a rough conversion from
RA/Dec to Alt/Az then the best way to do this is with a Planisphere. You set the date
and time on a scale round the outside edge by lining up a pointer. Then through
an elliptical window you can see what portion of the sky will be visible at that
time. The map used has the pole star in the middle and the lines of RA and Dec
drawn on the map. The elliptical hole representing the visible sky has north, south,
east and west indicated around the edge, so you can estimate the azimuth.
Similarly you have to estimate the elevation, from 0 on the horizon line, to 90 in the
middle of the ellipse, (note not the middle of the disk where Polaris is).
You can, when you get used to the planisphere, work it backwards so that you can
find out when a particular set of coordinates will be visible.
Until you have a planetary program of your own, you can get star charts for your
own location from www.heavens-above.com.
This is mainly about satellites but has some basic astronomy too.
They also have
a FAQ page which might be useful.
You can get a planetarium program from either of the following:-
Provisos (for this FAQ)
-
The stars are not fixed, they have proper motions, but these are small and can be
ignored by the beginner. Some planetarium programs will take care of proper motion
for you.
-
The celestial north pole is not fixed, it wanders about. It is currently very close
to Polaris. Again this is nothing the beginner need worry about. A decent planetarium
program should take care of this for you if you wander back a few thousand years or so.
-
The point where the Sun crosses the celestial equator is not fixed but moves about.
The precession of the equinoxes. Again the beginner need not worry too much about this,
except perhaps notice that star positions are given with an epoch or date that they are
valid. The current epoch is 2000 and the previous was 1950 (I think). Again your
planetarium program should look after this for you.
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