We have used new parallax measurements , obtained by the Hipparcos satellite , of fifteen nearby , metal-poor stars to re-define the subdwarf main-sequence .
All of these stars have parallaxes determined to an accuracy of at least 12 % .
Comparing these measurements against previous ground-based data for nine stars reveals a systematic offset of 5 % , in the sense that the Hipparcos parallaxes are smaller ( i.e .
the inferred distances are larger ) .
The availability of the Hipparcos observations expands the local subdwarf sample to the extent that we can separate the stars by abundance into intermediate ( [ Fe/H ] \sim -1.4 ) and extreme ( [ Fe/H ] \sim - 2 ) subsets .
Main-sequence fitting techniques are then used to match stars of the appropriate abundance range to the colour-magnitude diagrams of the seven globular clusters M5 , NGC 6752 , M13 , M15 , M92 , M30 and M68 .
We derive respective distance moduli of 14.45 , 13.17 , 14.48 , 15.38 , 14.93 , 14.95 and 15.29 magnitudes , with formal uncertainties of \pm 0.1 magnitude .
The metal-poor systems M68 , M15 and M30 have moderate foreground reddening , and varying E _ { B - V } by \pm 0.02 magnitudes can change the derived distances by up to \pm 7 \% .
With the exception of NGC 6752 , however , our derived distances exceed previous estimates , particularly in the case of the four [ Fe/H ] \sim - 2.1 globulars , where our distance moduli are \sim 0.3 magnitudes higher than the current standard values .
We discuss briefly how these findings affect the RR Lyrae distance scale , isochrone-based estimates of the age of globular clusters and our picture of the early stages of star formation in the Galaxy .
We note that our results go some way towards reconciling the apparent contradiction between the cluster ages and recent determinations of the Hubble constant .