### moving-cluster method

(method of measuring distance to a nearby star cluster)

The **moving-cluster method** is a method of determining the distance
to grouped stars that are moving at the same velocity and are
sufficiently close to us that their proper motion can be
determined. Open clusters are so-grouped and generally moving
together, so this applies to them, if they are not too far.

The method consists of determining a **convergent point**, a point
in the celestial sphere to or from which the stars appear to
be moving, which is presumed to a direction in the sphere (in the
same sense that the apparent meeting point of the rails of a straight
railroad track is the direction to which the track leads). Given
this determination of their direction of motion, the ratio of the
transverse velocity and radial velocity can be worked out, and
with radial velocity measurements by Doppler shift, the transverse
velocity can be deduced, and by comparing that to the proper motion,
the distance can be determined.

I've seen the phrase **convergent point method** described as synonymous,
but one source calls them "closely related". I've also seen reference
to *convergent point method* as a means to determine which stars
are in a cluster; basically, their velocity appears similar.

The moving cluster method depends upon mathematics and probability
to make best use of the observations in light of random measurement
errors, and averaging methods have been developed aiming to get the
best use of observations. The method became less commonly used as
other distance-determination methods improved, and observation
improved, e.g., the accurate astrometry of Hipparcos.
However, more recently the moving cluster method has been retried
because the improvement in astrometry also improves this method's
results.

(*measurement,star clusters,stars*)
http://en.wikipedia.org/wiki/Moving-cluster_method

https://arxiv.org/abs/1112.3542

http://adsabs.harvard.edu/full/1950ApJ...112..225B

https://arxiv.org/abs/astro-ph/9903239

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