Expert: Tom Whiting - 4/12/2006

Question
I am struggling to grasp where right ascension starts from a longitudinal view. I understand that it is at the vernal equinox.  I don't understand where the vernal equinox is from a geographical standpoint.  Is it over Denver? Is it over Cleveland? I understand the hours but I don't know what earthly reference point the "0" hour is located at.  Please help me.

Thanks in advance.

Sincerely,
Tom Moe
Ashland, WI

Answer
Hi Tom,
(another Tom)!

Ok, it's not a "geographical" point on the Earth.
So there is, and cannot be, no Earthly reference....It's in the sky, over everything, every spot, on the equator, at some time during the 24 hour day.  It is located and can only be "seen" by SKY references, which I'll describe below.

It's in the sky, just like the sun or moon on any given date, it's a point in the sky that "rotates" along with the night sky, (due to the fact that the earth rotates, so its an apparent rotation of the night sky.)
And like our own zero point, Greenwich England and the
0 Longitude point on the Earth's surface, the Vernal Equinox is the starting point for Right Ascension, and it's "man made" or imaginary.
Ok, where is it?  It's that spot in the sky where the sun is
on the first day of spring,  March 21.  This is where
the sun crosses the equatorial equator moving slowly
northward, so even that is an arbitrary point.

Since our midnight night sky is opposite the sun, the Vernal Equinox in located (and can be seen visually IF you know your constellations and stars) in our evening
Autumn September sky, when the sun WILL BE opposite of the Vernal Equinox, on Sept 23 or 6 months later.  And this spot, the Vernal Equinox,  is pretty much right below the Great Square of Pegasus, it's eastern side.
 (About 15 degrees, or one full hand span at arm's length, below the southern star of  the Great Square on it's eastern side, so it's due south of the naked eye star Gamma Pegasi).
So that actually puts the Vernal Equinox point, down in
the area we call Pisces,  7 degrees due south of the star Omega Piscis.  (Or about 5 degrees SE of the famous Circlet of Pisces.)

Currently, during our short lifetimes, there is no naked-eye
star that marks that spot in the sky, similar to the south
Celestial Pole...no star there either.  So you have to know
your night sky (just like knowing and memorizing the streets
and buildings of Ashland Wisconsin) to know where the
Vernal Equinox is....visually in the night sky.  (or have a decent star atlas).  Because that's the only reference points in the night sky, that we have available to us.  Actually, you can't see it now anyway, because the Sun is in that location.  But in a few months, by June or July, the Great Square will be rising a few hours ahead of the sun in the east, and then we'll be able to see that part of the sky once again.  If you want to see it in the evening sky right after darkness falls, you'll have to wait until Autumn, as that is our Autumn night sky.

Ok, once you've located that point, you mentally picture
a north-south line running from Polaris and thru that
Vernal Equinox point (basically right down the eastern side
of the Great Square) down to the southern horizon, and that
is the imaginary 0 RA line.   RA's are measured in hour, or
15 degree increments EASTWARD from that line, (because
the Earth rotates 15 degrees every hour) so that
puts the Pleiades, for instance, at about 4 hours or 60 degrees
east of the Vernal Equinox, Orion at 6 hours, Leo at 10 hours,
Scorpius at 17 hours, and so on.

Actually, us amateurs don't use Right Ascension (RA)
and Declination (Decl)  to locate objects...that is reserved
for the professionals who typically don't know their naked
eye night sky anyway...they don't have to.  They are sitting
at a computer console and just punch in the numbers
to a computer which in turn slews a major telescope to that
spot in the sky.  We, as amateurs that move our own scopes
to the correct spot in the sky,  we know our naked eye night
sky, so we just manually move our scopes to that spot by....
star hopping.  We already have memorized, say M-51
the Whirlpool Galaxy  3 degrees below the last handle star
of the Big Dipper, put your scope there, scan around a little, and  BINGO, there's the Whirlpool Galaxy.  So that's really
how we do it...we don't even care about the RA and Decl.

The only time we get involved with the  numbers is if, say
you discover a new object, say like a new comet, then you
have to locate that position in the night sky, transfer that
spot to your star atlas, and read off the RA and Declination
off your star atlas.....only because the people you report
it's position to, want and need the Right Ascension and
Declination numbers to confirm your sighting.

Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA