Spotting meteors during a shower is a pretty easy task to carry out, requiring nothing more than a pair of eyes and something to record your results with. And with the 2020 Perseid meteor shower set to peak around 12 August, now is a good time to plan a meteor observing session from your back garden.
You could even get family or friends to help you complete a scientific record of what you see.
As with all scientific recordings, there are guidelines to follow. Getting this right will elevate your observations and help advance our common knowledge of how specific meteor showers work.
Best of all, it doesn’t take a lot of effort to get to this level and the resulting observations may help predict future meteor activity.
Read more about meteor showers:
- A beginner’s guide to meteor showers
- Interview: the science of space rocks
- The Perseid meteor shower in pictures
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.
1-3 Shower trails
4 Non-shower trail; wrong colour
5 Non-shower trail; doesn’t originate from the radiant
6 Non-shower trail; travelling in the wrong direction
7 Non-shower trail; too long for its proximity to the radiant
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 trail’s 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, below), 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.
Sporadic meteors originate from one of six sources: helion, antihelion, north and south apex and north and south toroidal.
What equipment do you need to observe a meteor shower?
Typically, equipment for observing a meteor shower doesn’t need to be elaborate or expensive. A clipboard, dim red torch, accurate watch and a ruler or piece of string will suffice.
Meteor reporting forms (see below) can be downloaded from organisations such as the British Astronomical Association, while a dim red torch allows you to look at the form and (unilluminated) watch without ruining your dark adaptation.
If you want to plot trails, widefield charts can be pre-printed prior to the session.
These can be generated from many planetarium programs such as the freeware Cartes du Ciel. A ruler or piece of string is useful to hold up to the sky to check on the path of the trail, a good way to confirm to yourself that the trail does project back to a shower radiant.
Meteor observing involves statistical recording, but the very act of looking down at a form will affect results because trails may occur while you are looking away from the sky.
This can be addressed if you observe in a group and nominate one person to be a central recorder.
The rest of the group then arrange themselves like petals of a flower, around the central recorder looking out from the centre.
It’s the central recorder’s job to take down information shouted out by the meteor watchers around them.
Alternatively, for solo sessions consider using a voice recorder.
What’s the best way of staying comfortable?
Comfort is important when observing meteors and using a garden chair, preferably a recliner or sunbed, is recommended. Neck support for long watches is particularly important.
Aim to look up at 60°, where the atmospheric thickness isn’t great enough to reduce the brightness of trails, but remains sufficiently thick for the number of meteors to remain optimal.
It’s also important to wrap up warm, even if temperatures are fairly mild at the session’s start.
Ideally you would want to find a dark location, free from stray light. Give your eyes at least 20 minutes to adapt to the darkness before you start a watch.
Ideally, aim to observe for periods that are multiples of 30 minutes long, say 30, 60 or 90 minutes and pick a direction giving a clear unobstructed view.
Aim to learn at least 10 stars with different visible brightnesses on the night for magnitude estimates (see below).
The quiet lulls in activity that typically occur during meteor-viewing sessions are useful times to learn the constellations.
However, avoid looking down at charts for prolonged periods because this may cause you to miss trails.
Zenithal Hourly Rate (ZHR)
Zenithal Hourly Rate (ZHR) is used to normalise meteor shower rates for comparative purposes and isn’t intended to represent expected visual rates.
The term assumes a shower’s radiant is directly overhead (at the zenith), a perfectly clear, dark sky with a limiting magnitude of +6.5 and no visual obstructions.
Few, if any of these conditions will be met in reality, so the actual number of meteors seen is often significantly lower than the quoted ZHR.
Visual hourly rate (Nv)
An observer’s visual hourly rate is the number of shower meteors recorded per hour. If the observing period is less than an hour, the count should be multiplied by 1 over the hour fraction (T).
For example, seven shower meteors over 15 minutes (T = 0.25 hours) gives a visual hourly rate of 7 ÷ 0.25 = 28 meteors per hour.
Field of view correction (F)
In an ideal world, you’d be looking at a totally clear sky. In reality, foreground objects or passing clouds get in the way.
F corrects for this, calculated as 1 ÷ 1–k, k representing the fraction of sky lost (0 = clear, 1 = obscured). If a one-third loss of sky is noted, F works out as 1 ÷ 1–0.33 = 1.5.
Population index (r)
A shower’s population index (r) indicates the average dimness or brightness of its trails.
A value below 2.5 indicates more bright meteors than average, while values below 3.0 indicate the shower has a larger proportion of fainter trails.
Values for r can be obtained from various sources such as the International Meteor Organisation.
Limiting magnitude (nelm)
Naked-eye limiting magnitude (nelm) is a way to assess how clear your sky is by monitoring the faintest stars visible using nothing more than your eyes.
For typical ZHR calculations, a limiting magnitude of +6.5 is assumed. Moonlight can greatly affect the nelm value.
Several magnitudes are lost when a bright Moon is present.
Radiant altitude (Hr)
The meteor radiant’s height makes a big difference to the number of meteors seen.
A low radiant altitude will produce trails that will either occur below the horizon or be seriously dimmed by the thicker layer of atmosphere near to the horizon.
The value is measured in degrees.
Rate variation over time
Earth entering the outer edge of a comet’s debris stream marks the start of shower activity. The shower’s ZHR will be low at this point, increasing to maximum as Earth moves ever deeper into the stream.
The peak is related to Earth’s orbital position defined by the Sun’s ecliptic longitude. The rise to maximum may result in a narrow, sharp peak lasting only a few hours.
Alternatively, if the densest part of the stream is wide, the peak may last for days.
In addition to this natural rate of variation, on any given night the number of meteors seen fluctuates according to radiant altitude and whether you’re observing before or after midnight local standard time (UT for the UK).
Before midnight, meteoroids play catch up to enter the atmosphere. After midnight, Earth turns so trails are the result of meteoroids colliding head on with the atmosphere.
This raises the collision energy, resulting in brighter and, consequently, visually more numerous trails.
Use a BAA form to record your sightings
Get involved in real scientific observing by downloading a meteor reporting form from the British Astronomical Association (britastro.org) you can help with its national survey
1 Date is normally stated as a double-date (say, 13/14 December 2020). This removes any ambiguity as it covers the evening to morning observed.
2 Observer(s) records the name(s) of anyone who contributes to the form.
3 Observing site identifies where you observed from, latitude and longitude being ideal. This can be obtained from a map or an online resource such as Google Earth.
4 Sheet of is used to ensure no loose sheets are missed, for example Sheet 2 of 4.
5 Correspondence address is where you can be contacted if the need should arise.
6 Observing conditions is intended as a general statement of the conditions throughout the watch. Augment with times if conditions vary greatly during the watch period.
7 Stellar limiting magnitude is a record of the faintest star which can be seen overhead. Again, augment with timed values if conditions vary greatly during the watch session.
8 The Start, End and, for convenience, Duration of the watch should also be recorded. Use UT throughout.
How to log each of your sightings
A Code No. is a sequential number used to identify a trail. If you’re in a group and performing central recording functions, it’s useful to leave a gap of say 5 or 10 between numbers so any trails reported out of sequence can be given the correct number even though they are in the wrong row.
The row sequence can later be corrected when the form is formally written up.
B Time UT indicates when the trail was seen. To the nearest minute is acceptable, although recording in higher time resolution is always useful.
C Magnitude can either be a magnitude estimate of the meteor trail or the name of the nearest star of equivalent brightness. The latter tends to remove observer bias.
It’s useful to have located a number of comparison stars of varying magnitudes before a session for this purpose.
D Name of shower or if sporadic is for the shower name the trail belongs to or whether it’s a sporadic meteor.
E Constellation(s) in which seen is used to describe the starting and ending constellation of the trail.
F Train details and time to fade is to record any special qualities about meteor trains. Time to fade records the number of seconds the train takes to become invisible to the naked eye.
It’s good practice to start counting in seconds in your head after any bright trail, just in case there’s a train visible afterwards.
G Notes is used to indicate any peculiarities about the trail. Examples would include ‘it exhibited a strong green colour’ or ‘meteor trail showed a terminal burst at the end’.
Pete Lawrence is an experienced astronomer and a co-presenter on The Sky at Night. This article originally appeared in the April 2020 issue of BBC Sky at Night Magazine.