Meteors
Meteoroids, meteors and meteorites
While our planet moves around the Sun, it is hit by a number of small celestial bodies called meteoroids. The phenomena caused by those bodies when they hit our atmosphere is called a meteor, and when the meteor intensity is particularly strong, it is called a fireball. Sometimes, solid remains of the meteoroid can be found on the Earth's surface, and these are called meteorites. Often, popular expressions like shooting stars or falling stars are used to refer to meteors.
Meteoroid swarms
Most meteoroids are small remnants of comets, and usually travel in space forming groups called swarms. Some of these swarms intercept Earth's orbit at some point, producing what is known as a meteor shower when the Earth reaches it. The Solar System body that produced the meteoroids of the swarm is called the "parent body".
Due to the effect of perspective, when a swarm of meteoroids hits the atmosphere, they appear to come from a single point on the celestial sphere, called the radiant. The different meteor showers receive names related to the constellation where the radiant is located, or any bright star next to it. For example, the eta Aquarids receive that name because their radiant is near the star eta Aquarii, belonging to the constellation of Aquarius.
Meteoroid swarms allow us to study, indirectly, the population of minor bodies in the solar system and thus its formation process, and their evolution mechanisms. Thanks to meteor observing we can obtain data about comets that sometimes no longer exist or are inactive, and even pick up fragments of Solar System bodies without the need to send expensive space probes for that. In a more practical approach, information on meteoroid swarms is essential to plan human activities in space, like "space walks".
Recently, interest in the study of meteoroids has been revived in both theoretical and observational grounds. Professional and amateur astronomers are assembling networks of video stations (using CCD-type sensors) to derive the orbits of observed meteoroids (when the same meteor is observed from two or more stations). And increasingly detailed theoretical models of these swarms are being developed, improving our knowledge of the swarms and the reliability of shower activity estimates. As a result, we can carry out predictions like those shown in the meteor shower calendar.
Meteor shower calendar
While meteors can be seen anytime of the year (there are sporadic meteors that are not related to any specific shower), this calendar helps to choose the right time for observing meteor showers. If you have little experience, it is preferable to focus exclusively on the main showers, which are those that have a large Zenith Hourly Rate (ZHR). If you have an advanced web browser, the calendar will automatically pick up the best showers for your location. To prepare the observation, it is advisable to follow these tips to observe meteor showers.
Open the calendar for the desired year: [2010] [2011] [2012].
For the main annual meteor showers, the radiant name, activity interval and the date of maximum are given, together with other data:
- R.A. and Decl.: Right ascension and declination of radiant at maximum, referred to ICRS. ICRS is the reference system used by modern star catalogs.
- Vg: Geocentric velocity in km/s.
- ZHR: Zenith Hourly Rate. It is the maximum number of meteors that an observer could see under ideal conditions, with clear skies and the radiant at zenith.
- % Moon: Percentage of Moon's illumination at 0h TT of maximum's day.
Due to their nature, meteor shower predictions are only approximate, so the appropriate caution must be taken when using the ephemerides. ZHR prediction is notably difficult, and is consequently omitted sometimes.
Predictions are based on data from:
The first two organizations are the main sources of data for the calendar and other meteor ephemerides on this site, while the IAU telegrams are useful for last minute updates in the case of any activity beyond expectations. The showers listed in the calendar are intended to be visually observable, with a ZHR at maximum that is greater or equal to 2, or showers with a very irregular activity, which may have annual rates of less than 2 but occasionally have important outbursts.
The home page includes a daily ephemeris for the annual showers and for the main sporadic pseudo-radiant (the Anthelion Source). Besides the calendar showers and the Anthelion, it is possible that the daily ephemeris -and the monthly listing- include some other radiant with weaker activity. On the other hand it is normal if, due to rounding and boundary definition issues, in some cases the calendar indicates that a shower is starting or ending a particular day, while the daily ephemeris at zero hours UT for that same day shows no activity.
The shower calendar is usually published two years in advance from the corresponding year; for example, the 2010 issue went online in 2008. However, the calendar is updated when there is any novelty with the predictions. Those updates are not very common, but it is appropriate to review the calendar before planning an observation, since there could have been some updates. Also, upgrades are planned on the online software tools that customize the calendar for you.