The absolute distance of the Virgo cluster is still a matter of debate. Distances quoted range from 15 to 22 Mpc. In general, the most reliable extragalactic distance indicators are of course the Cepheids. Cepheids at (and slightly beyond) the Virgo cluster distance are now within the reach of HST. This achievement was so long awaited that the first Cepheid-based distance determination of a Virgo cluster spiral (M100) by Freedman et al. (1994) had an enormous impact. The resulting distance of ca. 17 Mpc was simply taken as the distance of the Virgo cluster. But we know (cf. above) that Virgo spirals avoid the cluster core and may be in the field far off the cluster. M100 is probably lying at the near side of the cluster. Indeed, as more spiral distances are nailed down by HST-observed Cepheids, the average distance of the spirals is growing (Tammann and Federspiel 1997). Recently, Böhringer et al. (1997) have made the clever suggestion to use spirals as HST-Cepheid targets that show clear signs of ram pressure stripping, thereby ensuring proximity to the cluster core.
The safest would be to use only elliptical and S0 cluster members (or, ideally, M87 alone!) for a distance determination. Unfortunately, the primary RR Lyrae stars are much too faint at the distance of Virgo even for HST. The secondary distance indicators which can be applied to Virgo ellipticals give controversial results: globular clusters, Dn - , and novae tend to give large distances (D 20 Mpc), surface brightness fluctuations (SBF) and planetary nebulae (PN) lead to a small D 16 Mpc (an overview of the methods can be found in Jacoby et al. 1992). Great efforts are spent in the application of the SBF method (Tonry et al. 1997) because its claimed distance uncertainty for an individual galaxy is almost as small as with Cepheids ( 0.2 mag). However, Tammann (e.g. Tammann 1996, Tammann & Federspiel 1997) argues that the method is not yet mature for use, as long as the variations of the stellar populations among ellipticals are not really understood. A different problem might also undermine the PN method (Tammann 1996).
Fortunately, the absolute distance of the Virgo cluster is not very relevant for our discussion, and for definitiveness I continue to use D = 20 Mpc. More interesting is the question of the depth of the cluster. Can we resolve this depth with any of the distance indicators in use? What accuracy in the distance modulus would be required? According to Fig. 2, the angular width of the Virgo cluster in the sky is 8°. Assuming the cluster is as deep as it appears wide (i.e. approximate spherical symmetry), and with a mean distance of 20 Mpc, the front-to-back depth is 2.8 Mpc, or 0.3 mag in distance modulus. So the cluster could just barely be resolved with the most accurate distance indicators.
But again: this assumes spherical symmetry. Should the cluster be deeper than wide, in the form of a cigar or finger pointing towards us, we might resolve (or claim to resolve) the cluster depth with even the worst distance indicator. This happened in the early days of the SBF method (Tonry et al. 1988) when the cluster literaly exploded. Since then, people have become more cautious, and an apparent dispersion of the SBF distances among Virgo ellipticals beyond of what can be expected from spherical symmetry is usually ascribed to unaccounted-for variations in the stellar populations (Pahre & Mould 1994, Jensen et al. 1996). A recent claim by Young & Currie (1995) that dwarf ellipticals are distributed in a prolate structure pointing towards us, based on the shape of the luminosity profile of these galaxies, has been shown to be flawed (Binggeli & Jerjen 1998). There is presently no indication that early-type galaxies in the Virgo cluster are not as strongly clustered in space as they are observed to be in sky projection.
Remains the Tully-Fisher method for spiral galaxies (and of course also the Cepheids, which, however, are too costly for a gross application) to map the outskirts and the large-scale environment of the Virgo cluster. There is consistent evidence that Virgo spirals are distributed in a prolate cloud, or filament, stretching essentially from the cluster backwards to the so-called ``W cloud'' - again roughly along our line of sight (Fukugita et al. 1993, Yasuda et al. 1997, Federspiel et al. 1998). There is no doubt about the reality of the feature: spiral and irregular galaxies in the Local Supercluster are known to be gathered in filamentary ``clouds'' of galaxies (cf. Tully & Fisher 1987). The Virgo spiral filament is probably part of a very long filament that runs from Virgo way back to the ``Great Wall'' at the distance of the Coma cluster (Hoffman et al. 1995), and it might even be connected, on the near side, with the ``Coma-Sculptor cloud'' that runs through, i.e. includes the Local Group (cf. Tully & Fisher 1987). If so, we should not be surprised to observe a ``finger of God'' - because we live in a finger of God.