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angular speeds of meteor showers

Posted: Sat Mar 14, 2015 3:57 am
by Perseus
I have heard the angular speed of meteors variously described as "slow", "medium", "fast", etc., and I know that the angular speed of meteors is sometimes expressed in terms of degrees per second.

I have also heard that different meteor showers have characteristic angular speeds associated with them, but when I try to look them up online (e.g., at the IMO's website), all I get are tables which assume the reader knows the values of several variables that I don't know.

Does anyone know of some place online where I can find the average or typical angular speeds for given meteor showers (at least the major ones), preferably in degrees per second, or at least in descriptive terms, such as "fast" or "slow"?

Thanks for any help that anyone can provide.

Re: angular speeds of meteor showers

Posted: Sat Mar 14, 2015 7:57 pm
by David Frydman
. Hi Perseus and welcome,

. Norton's star atlas, at least the 19th edition, gives meteor speeds as does the BAA Handbook.

It lists them as slow, very slow, fast etc.

Regards, David

Re: angular speeds of meteor showers

Posted: Sat Mar 14, 2015 8:35 pm
by Perseus
Hi, David -

Thanks for answering, but since you didn't explicitly say so, I have to ask: Norton's Star Atlas and the BAA Handbook give ANGULAR meteor speeds, correct?

Also, I don't suppose you know of any free or inexpensive ONLINE source for this information, do you?

Thanks again.

Re: angular speeds of meteor showers

Posted: Sun Mar 15, 2015 10:06 am
by mike a feist
Surely the angular speed (in degrees rather than a description) is difficult to used because the lengths of each meteor depend on the distance from the radiant, hence the general description is used. Nortons 20th Edition does include the terms fast, medium and slow too. regards maf

Re: angular speeds of meteor showers

Posted: Sun Mar 15, 2015 12:18 pm
by Alastair McBeath
There are actually two elements to this question.

The meteor shower speed categories listed in various publications using terms like "fast", "very slow", etc., are in general based on the atmospheric entry velocities of the meteors in question. This velocity measurement runs from approximately 11 to 72 km/sec (very slow to very fast), and is dependent on the direction from which the meteors approach the Earth. The Taurid meteors, which we see each autumn from Britain, are slow-moving, so are passing through space in a fairly similar direction to the Earth, having an atmospheric entry velocity of about 25-30 km/sec. The Perseids, which peak in mid-August annually, are swift-moving by contrast, with a velocity of almost 60 km/sec on average, as they approach the Earth from more nearly head-on.

However, the apparent shower meteor speeds you can see under the sky do not always match such broad expectations, because the apparent angular velocity of a given meteor, usually measured in degrees per second, effectively depends on its distance from the meteor's radiant and the elevation of the meteor above your horizon. This is because the meteor is an object moving in three-dimensional space relative to you as an observer on the Earth's surface, so you will be only rarely viewing a meteor which passes at right-angles to where you are (when it would show its greatest apparent angular motion). For example, a meteor moving directly towards or away from you will have both a very short apparent path in the sky, and be very much slower moving than you'd expect from the simple "Perseid meteors are fast" concept.

The IMO's tables are so seemingly complex, because they're trying to provide a workable guide for visual observers which take into account these variables - different atmospheric entry velocities, different meteor heights above the horizon, and different distances between the meteors and their radiants.

So, we can't estimate "typical" angular meteor velocities for shower meteors, but we can estimate the likely average apparent velocity meteors from a given shower should show as we view them from Earth, by using the atmospheric entry velocities listed in places like the IMO's annual Meteor Shower Calendar (direct link to free download of the 2015 PDF version). Table 5 on page 25 of this 2015 Calendar has a convenient listing of the average atmospheric entry velocities for each shower under the "V-subscript-infinity-symbol" column, in km/sec (sorry; can't do the right formatting to show this symbol combination here, but see p.24 of the Calendar for what I mean - it's the fourth item on the "Abbreviations" list).

If you're searching on the Web more generally, you may also run across other meteor velocity measurements which aren't so helpful for surface-based observers, commonly "V-subscript-H" and "V-subscript-G". The first is the heliocentric velocity (effectively the orbital velocity of the particle around the Sun, hence "helio" = Sun), the second the geocentric velocity, which is the particle's velocity compared to the theoretical centre of the Earth. Neither of these are of use to an Earth-surface observer, so are best avoided, unless you wish to explore meteor orbital dynamics.