Observational astronomy/Meteor observing

Leonid Meteor Storm of November 12-13, 1833

A Meteor, or Shooting Star, is a streak of light seen in the sky. It is caused by a small particle of interplanetary material (a Meteoroid) hitting the Earth's atmosphere and burning up.

Occasionally, an exceptionally large meteoroid will not burn up completely, and part will hit the ground. This is called a Meteorite.

Meteors are of two types:

• Sporadic meteors: These can occur at any time of year and in any part of the sky. They tend to be more frequent just before dawn rather than earlier in the night, because at that time the observer is facing towards the Earth's direction of motion around the Sun and so meteoroids strike head on.

• Meteor showers: These are due to clouds of meteoroids in orbit round the Sun. Such clouds are believed to be expelled from comets, and most of them can be identified with a particular comet. The Earth will pass through a cloud once a year, and observers will see an enhanced rate of meteors. Due to perspective, the paths of meteors from a shower will appear to diverge from a point in the sky known as the shower radiant. Many showers are named after the constellation in which the radiant is located. For example, the Leonids (pictured) have their radiant in Leo. (Their parent comet is Tempel-Tuttle.)

Guidelines on Measuring Meteor Activity

A very bright fireball meteor captured by an automated sky camera, January 2012.

It is very valuable to have a record of the level of meteor activity, hour by hour and day by day, particularly at the time of major showers so that their activity can be monitored. For example, showers are often more active when their associated comet is near perihelion (the point in the orbit of a planet or comet where it is nearest to the sun). Monitoring meteors can be done easily by amateurs with minimal equipment, but it does require commitment, as observing sessions need to last for several hours.

Observe the sky from a place as far as possible from lights, as these can drown out faint meteors. Wrap yourself with warm clothing, especially in winter, though you can feel chilled even apparently warm summer nights and it may get colder after midnight. Sit in a deck chair or lie flat on the ground so that you can see a large part of the sky easily. It is essential to be comfortable as you will be observing for a long time. It is a good plan to have two or three observers in the same spot, so that the whole sky is covered between them. With enough people, you can have observers to work on rotation basis to share the effort.

Before observing, try to decide how bright are the faintest visible stars. This will help those analyzing the observations to assess whether an apparently high or low meteor rate is distorted by particularly good or bad observing conditions. Ideally, re-assess this visibility limit at intervals through-out the night.

Make a note of each meteor you see, and the approximate time so you can deduce hourly rates. If there is a shower, try to see if the meteor is headed away from the position of the radiant; if not, it is a sporadic meteor and should be recorded separately so as not to inflate estimates of shower activity. Note if it is particularly bright or otherwise unusual. For example, some meteors have a flare due to the meteoroid exploding rather than burning up steadily. Notes may be taken on paper or a tape recorder. It would be ideal to have a person do this separately from the observers.

To make these observations of value, they must be reported regularly to some group that can collate and publish them. Some of the leading groups have their web sites listed below.

Zenithal Hourly Rate

If the radiant of a shower is at a low altitude, the meteors will tend to be further away and seen through a greater thickness of atmosphere, hence the rate of meteors will appear to be lower. The less the altitude, the fewer meteors will be seen. To complicate matters further, the altitude of the radiant will vary during the night. Shower rates are often expressed as the Zenithal Hourly Rate or ZHR, the rate to be expected if the radiant were at the zenith. If you are inexperienced, there is no need to worry; the collating group will be able to do the adjustment. To get the ZHR, multiply the observed rate by 1.25 if the altitude of the radiant is 52º, 1.67 (altitude 35º), 2 (altitude 27º), 5 (altitude 8.6º) and 10 (altitude 2.6º).

Observing activities

To find out when the next meteor shower will occur see the Weekly Meteor Activity Outlook.

See also

• SkyCam - Sky Camera testing on nights of meteor showers

External links

• Meteor Showers
• American Meteor Society
• International Meteor Organization
  • International Meteor Organization visual observation
• British Astronomical Association Meteor Section
• NASA forward scatter meteor radar