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(meteorobs) What's the best time to watch a shower?
People (not the sort who are going to observe for hours on a freezing cold
night) ask me when the "best" time to watch for the Leonids is. I'd like to
know, myself. The short answer for the UK in November is "anytime you're
lucky enough to have a clear sky, and as dark a location as you can get
to". But it got me thinking, particularly about the balance between the
time of the expected peak and the "you see more meteors after midnight"
principle. I've outlined my thinking below, and I'd appreciate comments on
factors I might have missed or got wrong or on faults in my logic. More to
the point, perhaps, does this accord with observations?
I've seen various times quoted for the predicted maximum, but a consensus
range would be between 1800 and 2300 UT on November 17. Since UT is our
local time in winter in the UK, this puts the expected maximum well before
midnight.
It seems to me that the following factors affect the number of observed meteors
1 The expected time of maximum ZHR (the published peak).
2 The height of the radiant above the horizon - the higher the better.
3 The local time. More meteors will be seen after local midnight, since the
observer will be on the side of the earth going "forwards" in its orbit.
4 The phase of the moon and the time of moonrise and moonset.
5 The limiting magnitude.
The moon: Since the moon is about new for this year's Leonids, it will set
around sunset, so I'll ignore it.
The limiting magnitude: This is unlikely to be predictably time-dependent,
so I'll ignore that.
Height of the radiant: I'm not clear how far meteors can go from the
radiant, but if they can go up to 90 degrees away, then you would see all
meteors from a radiant at the zenith, but only half the meteors from a
radiant at the horizon (because half would go down below the horizon, where
you wouldn't see them, and half would go above the horizon). So this makes
a difference of a factor of two. If the meteors can be seen more than 90
degrees from the radiant, then the number seen becomes *less* sensitive to
the height of the radiant.
Local time: I don't know exactly how the number of meteorites varies over
the 24 hours but the maximum will be at 6 am when the observer is on the
"frontmost" part of the planet in its orbit, and the minimum will be at 6
pm. Based on radiodetection of sporadics Philip Gebhardt (Sky & Telescope,
Dec 97) quotes a ratio of maximum to minimum of 4 to 1 (for mid-latitudes).
So the difference between after midnight and before can't be more than
fourfold.
Taking the last two factors together, assuming the radiant is above the
horizon, then the most difference that the time of night can make is an
eightfold difference. Provided that the ZHR around the peak is more than
eight times the ZHR outside the peak, then you are better off watching at
the expected peak than waiting until after midnight. Does this all make
sense?
Clear skies
David
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