Sky surveys are quite responsive to the seasons and to the phase of the moon. As even the least observant person can attest, there are less stars visible at full moon than on a very dark night; and precisely the same is true of any optical observing. So the
phase of the moon means that faint objects can only be observed during part of the month, and that only bright objects (eg Jupiter) can be observed at full moon. In Skyalert, all of the CRTS telescopes shut down for several days around full moon. If you downloaded the CRTS iPhone app during full moon, you might give up on it as useless because nothing comes through for several days. But press on, keep watching!

Another active survey in Skyalert is the MOA (Microlensing Observations in Astrophysics) stream, which looks for brightening of stars due to gravitational microlensing. This method can be used to detect extra-solar planets, probe stellar atmospheres, and other science. The survey needs to observe the densest concentrations of stars, and thus monitors the center of the galaxy, the area of greatest stellar density, which is also known as the galactic bulge. This area is in Sagittarius, and so the survey cannot operate unless Sagittarius is well above the horizon at night: in the northern hemisphere this is May through August, and in the southern hemisphere May through November. So these surveys work only during the “bulge season” for a few months each year.

Another seasonal effect is weather. The CRTS1 and CRTS2 streams are observed from the mountains above Tucson, Arizona. During the summer, violent thunderstorms can happen suddenly, black clouds rolling in fast, then torrential downpour with a lightning storm. A delicate astronomical telescope could be ruined by such a storm: if not by water damage, then the exquisitely light-sensitive camera can be blown out by the brilliance of a lightning flash. Because of these risks, these two streams do not observe from the end of June to early September. Of course another reason may be that it is so hot during the summer that everyone involved is on vacation elsewhere! However, the third of the group, CRTS3, is taken from Australia, and is not affected by the Arizona monsoon season, so that will continue through the next months.

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