Meteor observing: the basics
Familiarise yourself with how meteors appear in the night sky
What is a meteor?
The name meteor describes the phenomenon that occurs when a small particle – a meteoroid – enters Earth’s atmosphere and vaporises. From the ground, the swift moving path of light that results is what’s known as a meteor trail. An average trail is produced by a meteoroid similar in size to a grain of sand.
Larger meteoroids will produce bright trails, and a trail brighter than mag. –4 (similar to Venus) is known as a fireball. Bright trails are often followed by a glowing column of ionised gas called a meteor train, which fades over time. Persistent trains may last for many seconds, becoming distorted by high altitude atmospheric winds.
What is a meteor shower?
Meteor showers are typically associated with comets, although a handful are linked with asteroids. As a comet orbits the Sun, it releases dust. Over many returns, dust spreads around the orbit. Earth passes through numerous dust streams annually and, when this happens, the number of trails seen increases. Peak activity occurs when we pass through the densest part of the stream. Perspective causes the incoming trails to emanate from a small area called the shower radiant, which slowly moves over the duration of the shower. The constellation in which peak activity occurs gives its name to the shower. For example, the Perseids show peak activity when the radiant is in Perseus.
Identifying shower meteors
Not every trail will belong to a currently active shower. Multiple showers may have overlapping activity and random or sporadic meteors may occur at any time.
Various checks can be applied, the most important of which is whether a trail appears to come from the shower’s radiant. If this isn’t the case, it’s definitely
not a shower meteor. Trail lengths also vary with distance from the radiant: those starting close appear short due to perspective. The trails’ apparent ▶
▶ length grows up to 90° from the radiant, after which it shortens again. Trails further than 90° away start
to converge to the shower’s anti-radiant. Long trails
starting near the radiant are statistically unlikely to belong to a shower.
Trickier checks concern colour and speed. Colour, visible in brighter events or in photographs, will often
be characteristic for a specific shower. Similarly, trail
speed varies between showers: a fast trail among a slow shower is unlikely to belong.
What is a sporadic meteor?
Random meteors not associated with a particular shower may be seen at any time without warning. Known collectively as sporadic meteors (pictured, right), they can appear to come from any direction. Although sporadic meteors don’t belong to a cometary stream, many are related in terms of their source area in the sky.
Sporadic meteor sources
Sporadic meteors tend to originate from one of six sources: helion, antihelion, north apex, south apex, north toroidal and south toroidal (see illustration, below). The helion source is close to the Sun, producing meteors that aren’t likely to be seen. The north and south toroidal sources arise from debris in highly inclined ecliptic orbits and aren’t well understood. The anthelion source is from particles on low inclination
solar orbits. This radiant is 195˚ of ecliptic longitude east of the Sun, shifted from the expected 180° by Earth’s own orbital motion. Like all sporadic
sources, it’s large at around 20° across. Although up
all night, it is best positioned at 02:00 BST (01:00 UT) for the UK.
The two apex sources arise from retrograde particles hitting Earth
head-on. The radiants are
15° above and below the ecliptic, 90° west of the
Sun. This produces activity
in the morning sky. Typically, sporadic sources produce
around five meteors
per hour.
Random meteors may be seen at any time without warning. Known collectively as sporadic meteors they can appear to come from any direction