"Mike Dworetsky" wrote...
in message ...

According to the Simbad data base, the mean radial velocity of M33 is -186
km/sec. This is relative to the centre of our own Galaxy. The radial
velocity relative to the Sun is +48 km/sec.

--
Mike Dworetsky

(Remove "pants" spamblock to send e-mail)

Thanks, Mike! --

So due to the Sun's motion through the Milky Way Galaxy, the
Triangulum Galaxy appears to be going away from us. But if the
radial velocity is plotted with reference to our galaxy's center,
then the Triangulum Galaxy is actually coming *toward* our
galaxy. And apparently its velocity is not as high as Andromeda's.

Now it makes sense... thanks again, Mike! very much!

Now i'm led to ask another question...

We know some things about velocity. We know for example that
a celestial object such as our Moon, a planet, a star or even a
galaxy has a velocity with respect to Earth (or to Sun, or to the
center of our galaxy) that has two main components: radial, which
is the movement toward or away, and transverse, the movement
that is *across* the sky (to the left, right, up, down, etc.).

Our Moon orbits the Earth with a velocity that is very easy to
compute because it's so close to us. The Moon's velocity has a
radial component because it is "falling toward" the Earth (yes, i
know that it appears to be falling away from Earth to the tune of
about 4 cm per year; however, i see this as falling *toward* the
Earth with less and less velocity each year).

Planets, too, are easy when it comes to computing their transverse
velocities and for the same reason: because they're so close.

With stars, the measurements become a little trickier because they
are so far away from us. It seems that the radial velocity is still
easy to compute, while the transverse component of the velocity
gets more and more difficult to measure the farther away the
celestial object is from us.

The farther away an object is, the more *time* we must wait to
be able to sense how far to the left or right the object has moved.

We now know that both of the other large galaxies in our Local
Group have a radial velocity that is in our general direction, that
is, they both show a blue shift. This could mean many things, but
i think it means that all three galaxies, Andromeda, Triangulum
and our own Milky Way are gravitationally bound to a common
center of gravity. All three are "falling toward" each other as they
revolve around... something. (What this "something" is can be
anybody's guess. Since scientists are finding huge masses of dark
matter in the center of other galaxy clusters, it may follow that our
Local Group revolves around a huge clump of this dark matter?)

So my question is this... Is there a formula to compute how long
we must wait before we can gather fairly accurate measurements
of transverse velocity? In other words...

How long would we have to wait to notice a sideways movement
of the Andromeda and/or Triangulum Galaxies? a movement of,
say, 1 mm? Would we already have to know the transverse
velocity to compute this? or is there another way to measure the
amount of time needed? (so we can use it to compute the
transverse velocity?)

Such a formula and measurement would shed an amazing amount
of light on the dynamics of our Local Group!

happy days and...
starry starry nights!

--
"Oh give me please the Universe keys
That unlock all those mysteries!"
You pay your fees, you find some keys
That keeps you always groping.

"Oh give me please the Happiness keys
That ease the pain of biting fleas!"
Today you seize you need no keys,
That door is always open.

Paine Ellsworth