The CBA organizes professional and amateur astronomers
around the world in campaigns to monitor a few
interesting stars intensively.
Many of their targets are cataclysmic variables
which, in outburst, can undergo strong variations
(0.05 to 0.25 mag)
over short periods (several hours).
The AAVSO collects measurements of variable stars
from visual, photoelectric, and CCD observers.
Like the CBA, they organize
campaigns on very interesting targets.
This group concentrates on one particular type
of variable star: eclipsing binaries.
Some projects require several full nights of
work, others (Times of Minima) less than an hour of
(very carefully selected) measurements.
These objects are usually faint and boring, but every now
and then produce a bright outburst. By comparing
new images to charts, you can see immediately if an outburst
has started, and inform others to watch it intensively.
The MPC serves as the central repository of information
on the orbits of minor planets (and comets).
It is difficult to discover a new asteroid,
but very easy to make important contributions
to the orbital elements of certain asteroids.
NEOs are a subset of asteroids which happen to fly
past the Earth at relatively small distances.
They are often visible for only a few days before they
move so far away that they fade into invisibility ....
and while they are visible, they move through the
sky very quickly.
CALL members collect photometric measurements of
asteroids, rather than astrometric ones.
The light curve of an asteroid can tell us a bit
about its physical properties.
Occultations of relatively bright stars
by asteroids happen more frequently than you might think.
We can use them to measure the sizes and
orbits of asteroids precisely, if we have
careful measurements
by several observers.