Last updated: 26 June 2002

Where -is- RA=00:00:00, anyway?

Sent:	Monday, June 24, 2002 21:36:08
From: (Richard Seymour)
Something that pops up from time to time, as people try to
make sense of reading/interpreting the Stellar coordinate
system that history has "blessed" us with, is:

Where -is- RA=0?

The sky is "mapped" by two sets of numbers, roughly corresponding
to the latitude (north/south) and longitude (east/west) system
we use here on Earth (and overlay on other bodies such as the
Moon and planets).
The sky's system uses Declination (distance north/south of the
"celestial equator", which is the line in space directly over
the Earth's equator) and "Right Ascension" (RA).  
Declination (Dec) is measured like Latitude, in degrees from
zero (at the equator) to + or - 90 (directly over the poles).
The "east/west" metric, Right Ascension, is harder to grasp.
First of all, it's measured in the units of time (hours, minutes,
seconds), not degrees.  That is, in part, since things happen
along that direction as time passes... objects rise and set,
and move across the sky from east to west as the night progresses.
The sky advances an "RA hour" for each hour of clock-time.
The RA coordinate rising in the east are "later"... their 
"hour value" gets larger throughout the night.
Since the Earth turns once per day, there are 24 "RA Hours"
snainng the full 360 degree circle of sky... Each "hour"
is a 15 degree piece of the arc.  
Each "RA minute" is 15 arcminutes, or a quarter degree, or
about half the width of the full Moon.
All of the "lines of RA" converge at the Poles, just as all
of the Longitude lines converge at the Earth's poles.
If you are measuring things near the pole, and -cross- the pole,
the RA value will suddenly shift by 12 hours (similar to 
Longitude jumping from "east" to "west" as you walk across the
north pole here on Earth.)

But, unlike Dec, and latitude and longitude, the RA system
is not based upon a ground-reference here on the Earth.
"0 RA" is not fixed, say, over the zero Longitude line
at Greenwich.  It moves with the sky.

WHAT is RA=00? 

By "convention", it's where the Ecliptic (the sun's path against 
the starry background) crosses the celestial equator, which is
the spot where the sun passes from the southern to the northern
hemisphere during its yearly pass around the sky.
When the sun does that, the day length opn the Earth is 
exactly 12 hours long... an "equinox".
It is the date and time when the northern hemisphere enters
"Spring", hence the Vernal Equinox.  The southern hemisphere
starts their Fall.
Unfortunately for us, there's no bright marker star or other
easily seen marker for that spot. So,

WHERE is RA=00?

In the Northern Hemisphere:

If you can see Cassiopeia, and think/see the "W" as being
right-side up (i.e.  W)
The RA=00 line passes just a bit counterclockwise from the
right-top of the W.   (Caph, Beta Andr)
. *   *   *  |
.  \ / \ /   |
.   *   *    |  <--- that line

The RA=6:00 line passes near Betelgeuse, the upper left shoulder
of Orion.

The RA=12:00 line passes (through M109) near Phecda, the lower
left bowl star of Ursa Major

.*----- *|        *  <--pointers to pole
,        \       /
.        |*-----*
.        |

The RA=18:00 line passes near the head of Draco (gamma Draco, 
Eltanin).. i'm not going to try to draw that.

In the Southern Hemisphere: 
The RA=00 line passes a little to the clockwise of Alpha
Pheonix... between Pheonix and Grus.

The RA=6 line passes a little clockwise of Canopus,
the brightest star in Carina.  (and you can probably see
Betegeuse, too)

The RA=12 line passes just to the right (clockwise) of 
.     *          ^  equator  
.     !          |
.*---------*     |
.     !          |
.     !          12
.     !          |
.     *          v   Pole 
     Alpha Crux

The RA=18 line passes a little to the left (counterclockwise)
of the curving tail of Scorpius.

WHEN is RA=00?

The RA=00 line is "above" the poles during the night 
from September 21 to March 21 (give or take a day).  
During the northern summer (southern winter), 
it runs "under" the planet during the majority of the night.

If you are trying to use (or understand) a telescope's
RA setting circles, you want to have reference marks to 
"set" them (just like setting a clock).  You use stars.

The way Setting Circles are used is: Point at a Star you know.
Spin the RA circle so that -that- star's RA is marked by the
When you wish to move to another star, hold the circle steady,
and move the telescope (which carries the pointer) until the
pointer points at the new star's coordinate.

On -some- models of telescope, the numbered circle moves with
the scope, and the pointer remains steady.  But the principle
is the same.

But it's really like that.. you only "set the RA circle" when
you're about to change targets.  While you're watching the target,
the circle setting may become inaccurate (it varies between
telescope models)

have fun

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