Meteors

meteor1Meteors, more known as “shooting stars” or “falling stars”, are one of the most enjoyable astronomical phenomena. An extra bonus is that they are easy to observe. All you need is clear skies and patience. Every evening one can see a few shooting stars per hour. And if you pick the right night this number could rise to tens or sometimes hundreds, or very rarely even thousands! Such phenomena are called (meteor) showers, or meteor storms when the hourly rate is in the thousands.

Unlike those few falling stars per hour that come from random directions (sporadic meteors), showers have a radiant. This is the “point” on the sky from where all meteors, related to the shower, come from. In other words, if you draw all meteors on the sky or a map, and continue these lines towards the place of origin, all lines will intersect in one small area – almost a point. This is the radiant.

The visual phenomenon we call shooting stars, is the track that small particles leave, while burning in the atmosphere. Such particles are called meteoroids. They vary in size, but usually are not larger than a grain of sand. They enter the Earth’s upper atmosphere, approximately 70-120 km above the ground, where they burn due to friction. Their velocities are in the range of 20-100 km/s.

On average, you could observe more falling stars after midnight than during the first part of the night. This is the so-called “windshield” effect. As the Earth is spinning around its axis, it’s also going around the Sun. Up until midnight, you and the sky above you are looking at the direction from where our planet is coming from. From midnight till dawn, you are facing the direction of motion. Since we are revolving around the Sun at ~30 km/s, you can imaging how Earth is swiping particles (potential meteors) on its way. Have you noticed what is happening when driving a car in a rainy day? Even a mild rainfall may feel like a torrent on your windshield when driving fast. Hence, the “windshield” effect.

Fireballs

Every now and then you may see a “fireball”. These are meteors brighter than Venus. However, the odds are not on your side. For every 1000 (or even more) shooting stars you may see, only one will be as bright as Venus. Bolide (often used as synonym of fireball) we call extremely bright meteors with magnitude of -14 brighter.

Stationary meteors

Another interesting phenomena are stationary meteors. They do not make tracks and appear like new stars. They show suddenly and are gone in seconds. This makes them much harder to find. They appear like point sources, because they are dashing right towards you. Don’t worry, though! They burn entirely in the sky… most of the time at least ;).

Meteor trails

When a fireball or a bright meteor rushes through the sky, look for trails. In general they have almost the same size as the meteor track you’ve just seen. After the falling star is gone they don’t stick around for too long. The trails usually last seconds, or sometimes minutes if you’re luck. Initially they look like thick, but faint lines, though their shape changes quickly, so be aware of that.

Take notes and describe the trail in as much details as possible – lifetime, change in shape, color, etc. If you are photographing, terminate the current photo and start taking 10-20 sec exposures of the trail. If you plan a photo session during active meteor shower, bring a spare camera (if you have) dedicated just for trails.

How to observe meteors?

The observing doesn’t require much. Grab a pencil, something to write on (or some sort of recording device), blanket and/or sleeping bag, proper clothing and you are ready to go. Let your eyes accommodate to the low light conditions, and start one-hour session (you’re only human after all and a five minute brake is needed very once in a while). Before and after each session check the limiting magnitude (needless to say that this is for naked eye). How to do that? There are many “standard areas”, triangles or rectangles, created by stars. You just have to look in some book/almanac, and pick at least two that would be visible in the night of your meteor hunt. Then just count how many stars you see in each field, and check to what limiting magnitude that corresponds to for the chosen field. As simple as that! If you decide to make sessions longer than an hour just repeat this every hour.

Try to record as much details about the meteors as possible:

  • Brightness – initially try to compare them to planets or bright stars – something more familiar to you. Latter, you may try to assign magnitudes.
  • Radiant origin – Leonids, Perseids, etc.
  • Speed – very slow, slow, fast, etc.
  • Color.
  • Size – the length of the path they traveled in degrees.
  • Whether they left trails or not.
  • Anything else that I’ve missed here.

Can you use a telescope to observe meteors?

Yes. You can use a telescope (or a binocular) with low magnification and try to catch one. Some meteors are not bright enough for your eyes to detect them, and these can be seen in a telescope. These are called “telescopic meteors”. Don’t expect to see one, though. The sky is vast and the scope’s filed of view is tiny. That’s why I don’t recommend this technique for beginners.

When to observe?

You can observe every night if you like, but with ~5 meteors per hour your patience might fade away sooner rather than later. Every so often there are meteor showers throughout the year (see table below), when the zenith hourly rate (ZHR) is higher. Usually ZHR changes between 10 and 100. Keep in mind that these numbers are for the ZHR, meaning that they are calculated when/if the radiant is in zenith, when the limiting magnitude is 6.0, zero cloud coverage, etc. In other words the perfect conditions. So when you don’t see as many meteors as advertised do not wonder what’s wrong. The number you count can and will be affected by numerous things.

Name of
the Meteor Shower
Period
of Activity
Date of Maximum ZHR Coordinates of the
Radiant
Velocity

km/s

RA

h m

DEC

deg

Quadrantids Dec
27 – Jan 07
Jan
03
100 15 17 +50 41
Lyrids Apr
16 – Apr 25
Apr
22
20 18
06
+34 49
η Aquarids Apr
16 – May 28
May
06
40 22 26 -02 66
δ Aquarids S Jul
08 – Aug 19
Jul
28
20 22
36
-16 41
Perseids Jul
17 – Aug 25
Aug
12
95 03
04
+58 61
Orionids Oct
02 – Nov 07
Oct
21
25 06
04
+16 66
Leonids Nov
14 – Nov 21
Nov
18
20 10
08
+22 71
Geminids Dec
07 – Dec 17
Dec
14
110 07
28
+33 35
Ursids Dec
17 – Dec 26
Dec
22
50 14
28
+75 40

You might have noticed that ZHR in the table above rarely exceeds 50. So, why was I talking about thousands? Even though these numbers don’t vary much between the years, there are some exceptions. Take the Leonids shower for example. The shooting stars that we observe are debris from a comet and every November the Earth passes through their orbit. However, when the Earth swipes a denser region of debris something interesting happens. We don’t have a meteor shower anymore, but an entire storm! With Leonids this happens every 33 years. In 1933 and 1966 the peak ZHR was in order of 100,000! 1999 was not as glorious as the previous two cycles. Nevertheless, Leonids were generous enough during 1998-2001, and the ZHR was always above 1000.

Where to look

Up? Well… yes and no. Do not stare directly at the radiant. Why? The closer to the radiant you look the shorter the meteors would be (which makes them harder to detect), and their numbers will be lower. Try to center your view 40-45 degrees away from the radiant.

Photographing Meteors

There are two general types of meteor photos – camera on a tripod or camera trailing the stars (when mounted on a telescope). The difference is that in the first case the stars make nice curved tracks, and in the later the stars are points. The second case requires additional equipment – telescope, mount, guiding, etc. Thus, your first task is to decide whether your camera will be mounted on a tripod or a telescope.

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To capture meteor in your camera it must be of at least of 2nd magnitude. Everything fainter than that may not show on your photographs. So how to maximize your chances?

  • If possible, find location away from cities and light pollutions. You want the sky to be as dark as possible.
  • Use the shortest focal length that you have, so that you can get the widest angle, thus the largest covered area.
  • Set your lens to the lowest f-number to get as much light as possible. Note that for exposures larger than 10 min you may want to keep the f-number at ~2 and not lower, even though your lens might have faster settings. It is true that the lowest setting will collect the most amount of light, but it will also get the most of the airglow. The sky brightness depends on your location, and will be higher near light polluted areas like cities. Since you want the sky on your photos to be as dark as possible, limit your f-number when doing long exposures.
  • Point the camera in the optimum direction. See “Where to look?” section.
  • If you use digital camera you may not care much about how many photos you’ll take. In this case set your exposure times to a few minutes. Of course, you might not have a laptop nearby to dump your photos. In this case experiment with the exposure to achieve best results – long enough not to fill your camera memory too fast If you are using film camera you may want to make 20-30 min exposures.
  • Film speed and camera sensitivity. If you are using film camera, I recommend 200 or 400 ISO negative. 100 ISO, could work, but lower than that is just not sensitive enough. If you decide to go with 800 ISO or higher make sure to do shorter exposures, or the airglow will be quite bright. Another reason I don’t like very fast negatives (800 ISO and above) is that they are too grainy. The same could be said about digital cameras as well with a word of caution. In general, the sensitivity and quality of digital detectors vary quite a bit. There are cameras such that at 800 ISO the image could be quite pleasing, and some that even at 400 ISO will produce noisy picture. However, most DSLR (Digital Single-Lens Reflex) cameras will give good results in the 200-400 (or even 800) ISO region. In conclusion, stick with 200-800 ISO range for digital cameras, but do some test first to see what is the optimal solution for your particular model.
  • Use multiple cameras. That way you’ll get better coverage. Even 6×6 cm film cameras won’t encompass the entire sky. You can get the whole sky in one shot if you use “fish eye” lens. In general I don’t recommend such lenses, because the image will suffer a significant distortion. They are suitable for getting more artistic effects and if that’s what you are aiming for, go for it.

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If you want to read more about meteors, visit the International Meteor Organization website.

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