Event driven astronomy
through the dust scattered around the orbit of a comet, can appear more
organised and produce what’s known as a meteor shower.
Annual meteor showers of note are the Perseids which peak around
the 13th of August and the Geminids which peak around the 13th of
December. A typical meteor shower lasts for several days if not weeks
but peak activity only tends to happen around a narrow window often just
hours in width. A perspective effect makes it seem that shower meteors
come from the same general point in the sky, known as the shower
radiant. Over the period the shower is active, it’s not unusual for the
radiant position to drift slightly in the sky. The constellation or star
that the radiant is closest to when the shower’s activity is at its peak tends
to be the driving force behind naming the shower. So, for example, the
peak output of the Perseids occurs when the radiant is in the constellation
of Perseus.
The number of meteors a shower can produce is known as its zenithal
hourly rate or ZHR and this is a figure which can lead to confusion. The
ZHR of a shower is the number of meteors you’d see under perfect
conditions with the radiant directly overhead at the zenith and assumes
you could see the whole sky in one go. These conditions are rarely met
and the visual rate is often significantly lower than the quoted peak ZHR
which should be used as a relative indicator of how active the shower is,
rather than what you’re likely to see.
The peak ZHR for the Perseids is between 80-100 meteors per hour
while that of the Geminids is 120. Some showers exhibit variable peak
ZHRs. One notable example is the Leonid shower which shows storm
level displays roughly at 33-year intervals. The last spectacular displays
were at the start of the current century when thousands of meteors per
hour were seen around the night of 17/18 November.
The streak a meteor makes as it passes across the sky is known as a
meteor trail. After a bright meteor event, it’s sometimes possible to see
a glow along the trail path, a bit like a short section of a plane’s vapour
trail. This is caused by a decaying column of ionized gas and is known as a
1718[17] Depiction of a cometary nucleus warming as it gets closer to the Sun. Material is
vented off the surface and as the nucleus rotates, this material forms a glowing shroud of
gas and dust that forms the comet’s head or coma. Radiation pressure exerts force on the
dust particles pushing them away from the Sun where they spread out to form a curving
dust tail. Electrically charged particles are carried back by the solar wind to form a long
straight gas or ion tail.
[18] Meteor Crater averages 1.2 kilometres in diameter. Its scale can be appreciated by
looking at the size of the visitor centre on the right part of the rim (Meteor Crater Arizona).