The sky — astronomy

Meteor Showers: A Calendar of Shooting Stars

A shooting star is a meteor: the streak of light made by a tiny grain or pebble of space debris heating up and vaporising in Earth's atmosphere. A meteor shower happens when Earth passes through a trail of debris left by a comet or asteroid; the best known are the Quadrantids (January), Lyrids (April), Perseids (August), Orionids (October), Leonids (November) and Geminids (December).

What a shooting star really is

Shooting stars have nothing to do with stars. Stars are distant suns, while the bright streak that flashes across the sky is a meteor, a light phenomenon created when a small piece of space material enters Earth's atmosphere at very high speed.

Three similar terms are often mixed up:

• A meteoroid is the small body itself while it is still in space, anything from a dust grain to a boulder. Most of those behind ordinary shooting stars are about the size of a grain of sand.
• A meteor is the streak of light we see when the meteoroid hits the atmosphere.
• A meteorite is a piece that survives the trip and lands on Earth's surface.

Meteoroids hit the atmosphere at tens of kilometres per second; NASA gives a range of roughly 11 to 72 km/s. At those speeds the glow is not ordinary burning. The object compresses and heats the air in front of it, and its own material vaporises and glows. This happens at altitudes of around a hundred kilometres, far above the clouds.

Comets as the source

Most major showers are linked to comets. As a comet approaches the Sun, its ice vaporises and releases dust and small particles. These stay along the comet's orbit and gradually spread into a stream of debris around its whole path.

When Earth, on its yearly trip around the Sun, crosses such a stream, the particles enter our atmosphere and we see a meteor shower. That is why showers return at about the same dates every year: Earth reaches the same point of its orbit at the same time.

Not every shower comes from a comet. The Geminids, for example, are associated with 3200 Phaethon, which is classified as an asteroid. The Quadrantids are also linked to a small body that is not a typical active comet. The line between comets and asteroids is blurrier than once thought.

Radiant and peak

If you trace the paths of a shower's meteors backward, they appear to spread out from one small area of the sky called the radiant. This is an effect of perspective: the particles travel in nearly parallel paths, just as railway tracks seem to converge in the distance. Showers are named after the constellation containing the radiant: Perseids (Perseus), Geminids (Gemini), Leonids (Leo).

You do not need to look straight at the radiant. Meteors can appear anywhere in the sky, and their trails are often longer some distance away from it.

The peak is the time when the shower is most active. It is often described by the ZHR (zenithal hourly rate), the theoretical number of meteors per hour under a perfectly dark sky with the radiant overhead. In practice you will see fewer, because of light pollution, clouds, a low radiant and moonlight. Exact peak dates shift by a day or so from year to year, so check an up-to-date calendar such as the one published by the International Meteor Organization (IMO).

The main showers through the year

Dates are approximate for a typical year:

• Quadrantids: early January (around 3–4 January). A short but intense peak.
• Lyrids: around 22 April, associated with comet Thatcher.
• Eta Aquariids: around 5–6 May. Debris from Halley's Comet; better seen from southern latitudes.
• Perseids: around 12–13 August. One of the most popular showers in the Northern Hemisphere, with particles from comet Swift-Tuttle. See the Perseids.
• Draconids: around 8–9 October, linked to comet Giacobini-Zinner; activity varies a lot between years.
• Orionids: around 21 October. Also from Halley's Comet; the radiant lies near Orion.
• Taurids: October to November. Slow meteors that often include bright fireballs; linked to comet Encke.
• Leonids: around 17–18 November, from comet Tempel-Tuttle; in some past years they produced true meteor storms.
• Geminids: around 13–14 December. Often the richest shower of the year. See the Geminids.
• Ursids: around 22 December, linked to comet Tuttle.

Besides these there are sporadic meteors, which do not belong to any shower. Even on an ordinary dark night you may spot a few per hour.

Tips for watching

• Find darkness. Get away from street lights. Light pollution is the biggest obstacle.
• Check the Moon. A bright Moon drowns out faint meteors. Look at the Moon phases and choose a night with a thin crescent or new Moon if you can.
• Be patient. Eyes need about 20–30 minutes to adapt to the dark. Avoid your phone screen, or use a dim red mode.
• Lie back and look wide. A reclining chair or blanket beats standing for an hour. You do not need a telescope or binoculars; they narrow the field of view. Your eyes are the best tool.
• Mind the hour. Meteor counts usually rise after midnight, when your side of Earth faces forward along its path around the Sun. The hours before dawn are often the best.
• Dress warmly, even in summer, and bring a hot drink.

More on preparation is in How to watch the sky. On a clear night you may even see the Milky Way as a backdrop to the meteors.

Most meteors are quiet and brief, a flicker lasting a fraction of a second. Occasionally a fireball (bolide) appears, a meteor brighter than Venus. That alone is a lovely reason to keep looking up with patience.

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