When to organize an observation

New observers should choose a period around the maximum of one of the major showers when more than 15-20 meteors are visible per hour. Not only it is much more fun to see lots of meteors, it is also important to see plenty in order to get sufficient practice of recording data, and to become familiar with the appearance of meteors generally. If one practises on an average night, only a few meteors will appear and there are not enough opportunities to make brightness estimates, etc. Major shower activity is not the sole determinant of how many meteors can be expected. There are at least two other circumstances to be taken into account.

Lower radiant elevations mean fewer meteors are observable. The ZHR (zenithal hourly rate) of a shower gives a measure of that shower's activity, and is the number of shower meteors per hour an observer might count under perfect sky conditions with the radiant in the zenith; major showers have maximum ZHRs of more than 20). Table 1 gives the number of meteors an observer might expect to see at different radiant elevations if the ZHR amounted to the impressive value of 100. Now you can imagine what you will see with a radiant elevation of 10° if the ZHR is not as impressive as 100. Therefore, when planning your watch you should take care that the radiant will have a reasonable elevation for the whole watch. For several reasons the lower limit is about 20°.

Table 1: Numbers of meteors visible per hour for a ZHR=100 shower and different radiant elevations h (limiting magnitude of 6.5m)

The ZHR refers to perfect sky conditions. If there is haze, clouds or any kind of sky illumination (the Moon, artificial lights, twilight) the observable number of meteors decreases since a considerable fraction of the fainter meteors is missed. Haze and clouds cannot be foreseen in planning an observation but twilight and the Moon can.

• The position of the Sun should be at least about 12° below the horizon which is the begin or end of the nautical twilight.
• Moonlight problems strongly depend on the Moon's phase. While the effect can be neglected from around 5 days before to roughly 5 days after new Moon, the full Moon reduces the observed number of meteors by a factor of about 10! The disturbance is less if the Moon is only a few degrees above the horizon.
• Except the period of about ten days centered on new Moon, you should observe only when the Moon is below or just above the horizon.

Now you can plan when you can usefully go outside to carry out your meteor watches.