The 1833 Leonid storm proved shooting stars come from space
On a November night in 1833, meteors fell over eastern North America at an estimated hundred thousand or more an hour. Denison Olmsted noticed they all streamed from one spot in Leo and concluded they came from a cloud of particles in space, not from the weather as many scientists then assumed.
Until then, meteors were widely treated as an atmospheric effect. Antoine Lavoisier had speculated in 1789 that lightning fused airborne dust into fiery lumps. When Ernst Chladni argued in 1794 that meteorites arrive from outer space, sceptics including, at first, Alexander von Humboldt objected, since Newton had held that space must be empty for planets to keep moving. Olmsted's analysis of the 1833 storm, which he noted was brief and unseen in Europe, turned the tide. In 1866 Giovanni Schiaparelli showed that the Leonids share an orbit with Comet Tempel, linking meteors to comets. Biela's Comet split in two in 1846, and when Earth crossed its path in 1872 a strong shower followed.
The source is comet dust. Each pass near the Sun warms a comet's ice, and escaping vapour drags out dust, sand and pebbles that spread along its orbit as a meteoroid stream. Fred Whipple pictured comets as dirty snowballs in 1951. Peter Jenniskens has argued that many showers instead come from occasional breakups of largely dormant bodies; the Geminids trace back to 3200 Phaethon, which fragmented about 1,000 years ago. A typical shower meteor is smaller than a sand grain and burns up 70 to 110 kilometres above the ground. Leonids hit the atmosphere at about 71 kilometres a second, Taurids at around 27.
Because the particles travel in parallel, perspective makes them seem to burst from a single point, the radiant, just as railway tracks meet at the horizon. Showers are named for the constellation or star nearest that point, with an -id ending: meteors near Delta Aquarii are the Delta Aquariids. Rates rise as the radiant climbs, peaking in usefulness when it sits halfway up the sky, and most showers improve after midnight as your side of Earth faces forward along its orbit, so the hours just before dawn are often best.
People have watched showers for millennia, with Spartan observations suggested as early as 1200 BCE, and the Lyrids recorded in China since 687 BCE. Forecasting took longer: attempts to predict Leonid storms for 1898 and 1899 found the dust lay inside Earth's orbit, and reliable dust-trail predictions arrived only in the 1990s.
Source: Meteor shower
