Wednesday, April 22, 2015

The Ancient Lyrid Meteor Shower

Lyrid meteor over Lake McDonald (c) John Ashley
A Lyrid meteor flames across the evening sky above Lake McDonald (click to enlarge)

The Lyrids are seldom spectacular, as meteor showers go. Earth passes through this dust trail between April 16-25 each year, and the number of Lyrid meteors peaks tonight at roughly 10 per hour between midnight and 4 a.m. If you're not the type to stand around in the dark wee hours staring at the sky, then the Lyrid meteor shower is also sprinkled with an intriguing history that you can read about from your comfy chair.

Lyrid meteors light up when the dust left behind by Comet Thatcher rams into our atmosphere at an average speed of 105,000 mph (169,000 km/hr). You and I have never seen Comet Thatcher, and we never will. It's a long-period comet travelling on an oblique, 415.5 year orbit around the sun, and it won't return until 2276. It last passed by here in May of 1861, but it was first spotted on April 5th by A.E. Thatcher, an amateur astronomer living in New York.

By the end of April, Thatcher's "tailess nebulosity" brightened and grew a tail to become naked-eye visible. The comet passed by the sun on June 3rd - inside of Earth's orbit - and was last observed on September 7th, disappearing into the night and not seen since. There are no known sketches or photographs of Comet Thatcher.

Most of our meteor showers are the offspring of short-period comets that make frequent fly-bys to refresh their dusty trails for astronomers and night owls. Halley's Comet refreshes the Aquarids every 77 years, Temple-Tuttle spruces up the Leonids every 33 years, and Tuttle lays down a new track of dust every 13.5 years to re-sparkle our Ursid meteor showers. So this long-period Comet Thatcher must be more like Pigpen, the ever-dusty Peanuts character, for it's debris trails to spark more than 400 years worth of shooting stars.

To keep us interested and watching, there are also thicker clouds of dust hidden within Thatcher's trail that occasionally produce unpredictable and unforgettable outbursts. The Lyrid showers of 1849, 1850, 1884, 1922, 1945, and 1982 were all well above-average, and the Lyrids of 1803 sparkled with 700 meteors per hour on the night of April 19-20. Tonight's show may well be one of the 10 meteor per hour showers, but then we don't really know.

It wasn't until after comet Thatcher's discovery that astronomers first linked meteor showers to comets. From the 1861 observations, professor Edmond Weiss carefully calculated Thatcher's orbit and found that it matched up with the annual Lyrid meteors. By tracing backwards in time, Johann Galle followed the Lyrids back to a Chinese observation on March 16th, 687 BC. In other words, we have been enjoying the annual meteor shower that returns tonight for more than 2,700 years.

If you want to join this centuries old celebration, you have to get up out of your warm chair. From northwestern Montana, tonight's Lyrid meteor shower starts low in the northeast and rises slowly towards the southeast. The radiant where these meteors seem to originate from will be at about 50° east and 9° above the horizon at 10 p.m. By midnight it moves to 70° east and 26° altitude. When nautical twilight arrives at 5:13 a.m., the Lyrid's radiant will be at 151° southeast and 72° altitude.

When you see a meteor, imagine standing shoulder to shoulder with a Chinese sky watcher 2,703 years ago. A dark night sky connects us all.