The multi-frequency multi-temporal sky
  Contemporary astronomy benefits of very large and rapidly growing amounts of
data in all bands of the electromagnetic spectrum, from long-wavelength radio
waves to high energy gamma-rays. Astronomers normally specialize in data taken
in one particular energy window, however the advent of data centers world-wide
and of the Virtual Observatory, which provide simple and open access to quality
data in all energy bands taken at different epochs, is making multi-frequency
and multi-epoch astronomy much more affordable than in the past. New tools
designed to combine and analyze these data sets are being developed with the
aim of visualizing observational results and extracting information about the
physical processes powering cosmic sources in ways that were not possible
before. In this contribution blazars, a type of cosmic sources that emit highly
variable radiation at all frequencies, are used as an example to describe the
possibilities of this type of astronomy today, and the discovery potential for
the near future.
