  We study the visibility of sunspots and its influence on observed values of
sunspot region parameters. We use Virtual Observatory tools provided by
AstroGrid to analyse a sample of 6862 sunspot regions. By studying the
distributions of locations where sunspots were first and last observed on the
solar disk, we derive the visibility function of sunspots, the rate of magnetic
flux emergence and the ratio between the durations of growth and decay phases
of solar active regions. We demonstrate that the visibility of small sunspots
has a strong center-to-limb variation, far larger than would be expected from
geometrical (projection) effects. This results in a large number of young spots
being invisible: 44% of new regions emerging in the West of the Sun go
undetected. For sunspot regions that are detected, large differences exist
between actual locations and times of flux emergence, and the apparent ones
derived from sunspot data. The duration of the growth phase of solar regions
has been up to now underestimated.
