Andromeda is 2 million light years away but?

The parallax method only works for nearby stars in the milky way. It is measured by cepheid variable stars. Cepheid variable stars have a distinct period. Their luminosity changes in a repeated pattern where their peak luminosity I believe happens every couple months. The longer it takes between each peak brightness, the brighter the peak brightness is. To know this, astronomers look at cepheid variables nearby and measure their distance via parallax method. They then measure their apparent magnitude and determine their luminosity with the equation L=A/4πr². Astronomers noticed that given the period of a cepheid variable, it will have a related luminosity. So when astronomers look at cepheid variable stars in andromeda, they measure the period and can measure the luminosity. Once they know the luminosity, they measure the apparent magnitude and solve for the distance by once again the equation L=A/4πr². And yes the andromeda galaxy is moving towards us and is now closer than what we see it as.

It could have moved further away or stayed in the same position. Your reasoning is uncomprehensive. 

Of course it wasn't measured by parallax.  That doesn't work for distances of more than about 50 light years.  And as that distance was measured less than a century ago, it hasn't moved any significant distance since.  Bear in mind it'll take another 4 billion years or so before it collides with the Milky Way.

Andromeda is moving towards us at 110 km per second.   If we assume Andromeda has been moving at the same speed without accelerating, then in the last 2.5 million years, Andromeda has moved only 900 light-years closer to us.

From Wikipedia:

Edwin Hubble settled the debate in 1925 when he identified extragalactic Cepheid variable stars for the first time on astronomical photos of Andromeda. These were made using the 2.5-metre (8 ft 2 in) Hooker telescope, and they enabled the distance of Great Andromeda Nebula to be determined.

By the existing evidence of spiral arms' our galaxy in terms of a galactic time line has encountered another passing galaxy. 

Andromeda and our Milky Way galaxy will have another pass threw.. but that will be in the distant future long after our sun burns out of fuel.

Sleep soundly tonight!   

I wouldn't argue with that

Those guys know their Mathematics

A light year is about six trillion miles.

Even assuming that andromeda was approaching us at one percent of the speed of light means it moves towards us at 1,860 miles per second which is about sixty billion miles per year. That means in a year it becomes a hundredth of a light year closer. In two million years it will be 20,000 light years closer.

That's still only a small change in cosmological terms. And btw, the distance is estimated by measuring the relative brightness of well understood variable stars, not by parallax.

The distance is actually closer to 2.5 million light-years, but the idea remains the same: there are certain types of stars where you can know the proper luminosity (the amount of light emitted) from certain properties. One example is the variability period of a Cepheid star (a type of variable star). By looking at a few Cepheids in the other galaxy and observing their period, you have their luminosity. Compare that with the apparent brightness of the star and you can find the distance.

In astronomy, the distance is given based on the time light takes to go from the object to us (look-back distance). Therefore it is true, in this particular case, that the Andromeda galaxy "now" is slightly closer to us than what we can see. We know the radial speed (how fast it is approaching us) so that we can calculate the real distance where it would be IF we could see it where it is now (i.e., if light was instantaneous). This distance is called co-moving distance.

The radial relative speed of that galaxy is approximately 300 km/s. This sounds fast but space is big. Given the uncertainty in the total distance (2.5 million light-years), if we subtract the 2,500 light-years it would have approached us over that time of 2.5 million years, we'd still call it 2.5 million light-years in co-moving distance.

2,540,000 light-years (look-back distance)

- 2,500 light-years (approach over 2.5 million years)

2,537,500 light-years (co-moving distance)

no, parallax does not work at those distances, not even close. 

reference has more detail, At least four distinct techniques have been used to estimate distances from Earth to the Andromeda Galaxy.

yes, the measurements show the distance 2 million years ago. 

The Andromeda Galaxy is approaching the Milky Way at about 110 km/s, it is expected to collide directly with the Milky Way in about 4 billion years.

given the above, 4 billion year to collide, 2 million LY away, the distance number is not that far from the actual number. 

Parallax:

To measure large distances, such as the distance of a planet or a star from Earth, astronomers use the principle of parallax. Here, the term parallax is the semi-angle of inclination between two sight-lines to the star, as observed when Earth is on opposite sides of the Sun in its orbit.  It is only able to measure parallax angles for stars up to about 1,600 light-years away,