Comet Thatcher hasn't visited our neck of the solar system since the American Civil War. And scientists predict that the giant hunk of ice and dust won't come back here until the year 2276. But every April, some of its debris puts on a nighttime light show enjoyed by millions of stargazers: the Lyrid Meteor Shower.
The intensity of that display is prone to vary from one year to the next, with some April showers being more spectacular than others. If conditions are favorable — and you know where to look — you can usually expect to see about 10 to 15 meteors per hour when the Lyrid shower peaks. Yet every few decades or so, they'll arrive in far greater numbers, giving us a springtime storm of celestial activity. Usually, the show lasts from April 16 through April 25, give or take a few days. This year is no different. Stargazers can watch the Lyrid for about 10 days, but it's expected to peak on April 22.
An Infrequent Guest
Meteor showers mostly unfold in the pathways of comets. Made of ice, dust, rock and frozen gases, comets are celestial bodies that revolve around our sun and other stars in the universe. So far, astronomers have discovered more than 6,400 individual comets. The Lyrid showers are a byproduct of one particular specimen known as "C/1861 G1 (Thatcher)." Admittedly, that's a mouthful, so feel free to use the object's less formal name, Comet Thatcher.
Amateur astronomer A.E. Thatcher is credited with its discovery. On April 5, 1861, he was watching the heavens from his New York City home. That evening, he noticed a strange, luminous body in the constellation Draco. It didn't stay put. Just over a month later, the object came relatively close to Earth, flying just 31.1 million miles (50.1 million kilometers) overhead. Then over the next few months, it approached our sun before exiting the inner solar system, the area that encompasses every planet from Mercury to Mars — plus the asteroid belt.
You will never get the chance to see Thatcher's comet up close. Like our beloved planet, it orbits the sun, but we now know that it takes Comet Thatcher approximately 415.5 Earth years to complete one rotation around the life-giving ball of plasma.
Because of its orbital path, Comet Thatcher spends most of its time far beyond the recesses of our inner solar system. A.E. Thatcher was lucky; he got to observe his comet during one of its rare forays into the area. Astronomers think that its next visit to the inner solar system will occur in 2276 or so, ending a centuries-long hiatus.
Although Comet Thatcher seldom comes our way, Earth predictably crosses its orbital path once a year. In the process, our atmosphere gets pelted by rubbish.
Whenever a comet approaches the sun, the heat melts some of its ice, freeing little pieces of dust and rock. So therefore, passing comets leave debris trails behind. And those tiny chunks occupy the same orbit as the comet that shed them. Eventually, the comet itself may disintegrate entirely. However, even after it vanishes, a stream of dust will linger in its place, filling up the orbital route that the icy object used to take around the sun.
That's why Earth gets annual meteor showers. On its own sun-centered journey, the planet reliably crosses paths with the debris-clogged orbits of many different comets — including A.E. Thatcher's.
The detritus starts to burn up once it enters Earth's atmosphere. Fear not; most chunks will never reach the ground. The ones that do hit our planet's surface are called meteorites. They're far less common than meteoroids, those chunks that completely vaporize in our atmosphere. Both meteorites and meteoroids produce flashes of light known as meteors.
Every April, our planet makes its yearly passage through Comet Thatcher's orbit. Stargazers are treated to a meteor shower while Earth intersects the debris trail.
Heroes and Lyres
Now if you're standing on the ground, the meteors in whatever shower you're watching will appear to emanate from the same general point in the sky. (Even though they don't; that's just an illusion.)
This spot is called the shower's radiant. Most of the yearly showers are named after constellations that sit by their radiant points. For example, meteors in the Perseid Shower, which thrills us every August, seem to fly out of the constellation Perseus.
By the same token, the April shower produced by Comet Thatcher is called the Lyrid Shower because its radiant is next to Lyra, a constellation representing the lyre played by the Greek hero Orpheus. Vega — one of the brightest stars in the sky — helps form the instrument's crossbar.
Knowing where the radiant is can enhance your shower-watching experience, but veteran stargazers will tell you not to look at the constellation directly. Meteors will show up all over the sky, and the longest-looking ones are going to appear far away from the radiant.
An Illuminated History
The number of meteors you can expect to witness during the Lyrid shower varies from year to year. At the peak of a Lyrid shower, it's typical to see 10 to 20 per hour. Usually, the Perseid showers are a whole lot denser.
But every so often, an exceptional Lyrid shower occurs. During the peak of the 1982 Lyrids, for instance, viewers in the Eastern United States reported seeing around 100 meteors per hour. Comets don't shed their dust particles at a constant rate, so one year's shower might be more or less intense than the next one.
To try and predict the intensity of a coming shower, astronomers make computer models that take things like comet trajectories and atmospheric changes into account. This year, you can help scientists gather Lyrid data. Simply register with the International Meteor Organization and report your observations after you go Lyrid-gazing.
By the way, it turns out watching Lyrids is an ancient pastime. You see, a Chinese document dating back to 687 B.C.E. refers to "stars [that] fell like rain" one night. Given the timing of this report, scholars think it could be the earliest written reference to a Lyrid meteor shower — or indeed, any meteor shower — in recorded history.