Astronomy & Celestial Mechanics
the sky, computed
Concept

Meteor shower — where it appears

A number of meteors arriving over a few nights from one point in the sky, as the Earth crosses a stream of grains shed by a comet or asteroid. The date gives the stream's node and the speed its inclination. Thin or old streams are found only statistically, from large camera catalogues.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

How fast a meteor arrives is its orbit's inclination. The speed at which a meteoroid on a near-parabolic orbit with perihelion at 1 AU meets the Earth, against the orbit's inclination to the ecliptic: relative to the Earth far away (dashed) and at the top of the atmosphere after the Earth's gravity has added its pull (solid). A prograde orbit in the ecliptic overtakes the Earth at 12 km/s and enters at 17; a retrograde one meets it head-on at 72 and enters at 73. The points are four showers computed from their parents' elements at the node nearest 1 AU: Draconids 24 km/s at 31.9°, Geminids 38 km/s at 22.3°, Perseids 61 km/s at 113.5°, Leonids 72 km/s at 162.5°. A meteor's speed is a reading of the angle between its orbit and the Earth's.

A shower that is a reading of an orbit

Every August the Earth runs through the dust of comet Swift–Tuttle, and the meteors arrive at sixty kilometres a second from a point in Perseus. Each of those facts is an orbital element in disguise. The date is the longitude of the stream's node, the speed is its inclination, the radiant is the direction it comes from, and the shower exists at all only because one element happens to put a node within a hair of the Earth's orbit.

orbits · Orbital elements How many strangers an orbit will match by chance. The number of chance partners each sporadic orbit has inside a fixed distance, against the number of orbits in the catalogue, for thresholds of 0.2, 0.1, 0.05, from pair distances measured in a schematic Earth-crossing population. At the original 0.2 a catalogue of 360 gives each orbit 0.5 partners, one of a hundred thousand 136, and one of a million 1362. No orbit in a large catalogue is alone at that threshold.

A distance between orbits that depends on how many there are

To say two meteors belong to one stream, a distance has been defined between their orbits, and below a threshold they count as kin. The threshold chosen in 1963, for a catalogue of 360 orbits, was 0.2. Camera networks now hold hundreds of thousands, and at that threshold an orbit chosen at random has hundreds of chance partners. The criterion has not failed. It was always a statement about a catalogue, and a list of showers is partly a function of how many meteors were counted.

orbits · Orbital elements

Named alongside it

The objects these essays reach for when they reach for this one.

Orbital elementsArgument of perihelionAscending nodeClusteringCometFalse-positiveMeteoroid streamOrbital inclinationPercolationPrecession of the equinoxesRadiant

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