2016 Perseid Meteor Shower

Date: August 12-13, but activity can last for ± 1 week
Time: midnight to dawn is best, but you can start watching around 8 pm
Place: the northeastern sky

The annual Perseid Meteor Shower is coming soon to a sky near you. The peak of activity is expected on August 12th and 13th, midnight to dawn, but you can start watching for meteors as soon as the sky gets dark. This year, unlike in 2015, we’ll have a waxing gibbous Moon to interfere. But it will set around 1 am, an hour after peak meteor activity begins. And if you’re lucky, you might even witness a bolide or fireball light up the landscape and leave a glowing trail in the sky.

Bolides are fireballs that explode into multiple fragments, each of which can leave a separate trail. I’ve seen dozens of fireballs, and just a few bolides. The meteor that exploded over Chelyabinsk, Russia in February 2013 was a bolide — and it was a large one. Some reputable observers have reported an audible distant rumble following the explosion of a bolide. I’ve listened for that, but never heard any sounds. Because typical bolides explode at high altitudes (40-50 km), the sound could arrive 3-5 minutes after the explosion. So if you do see one, be quiet and be patient.

The meteor shower is named for the constellation Perseus, from which the meteors appear to radiate. Perseus won’t rise until midnight, but you’ll see meteors well before then. If you can see the “W” of Cassiopeia, you’re looking in the right direction. Click on the thumbnail below to see how the sky will appear to the northeast after Perseus rises.

Perseid meteors come from a stream of debris left behind by Comet Swift-Tuttle. This debris orbits the Sun, as does Earth, but it’s orbit is elongated and tilted in a different plane (top graphic). Over time, the debris has dispersed around the entire length of the comet’s orbit. The debris distribution, however, is far from uniform. There are places where that debris is more concentrated. When Earth passes through one of those clumps, you’ll see a lot more meteors.

Unfortunately, we don’t accurately know where all those clumps are located. Predictions for any meteor shower, Perseids included, are seldom more than a “best guess.” But in 2016 Earth will meet up with one of the known clumps. We should see what astronomers call an outburst. On average, the Perseids give you 60-90 meteors per hour. For the 2016 outburst, the prediction is 100-150 meteors per hour.

Of course, those numbers are given for dark and clear skies in a location with a full view to the horizon. And they only apply during the peak period, which is always between midnight and dawn. The Moon is the variable — depending on its phase, it can blot out the dimmer meteors.

As the graphic illustrates, every August Earth passes through this stream of debris. When that happens, the visual effect is much like what you see when driving a car through rain or snow — all the drops or flakes (or meteors) seem to be coming at you from a point directly ahead. For a meteor shower, that point of origin is called the radiant. And for this shower, the radiant is located in the constellation Perseus. Earth’s motion through space is toward that radiant.

You might also see some activity from the Delta Aquarids, which have a very broad peak and often overlap the Perseids. Their radiant is lower in the sky, and to the south of the Perseids. The source of the Aquarid debris stream is unknown.

Most debris streams consist of tiny fragments of rock, metal, and ice. Some can enter Earth’s atmosphere with speeds up to 60 km/s (37 miles/second). At such extreme speeds they burn up from friction, ionize the air to the point where it glows, and create the bright streaks we call meteors or (more commonly) “shooting stars.” Most meteors never make it to the ground — those that do are promoted to meteorites.

So turn off your outside lights, and find a dark location with a clear view of the northeast sky. A reclining chair allows you to look upward comfortably. Give your eyes at least 15 minutes to become dark-adapted. Then settle in for the night and observe one of Nature’s greatest celestial fireworks displays.

Next Week in Sky Lights ⇒ The Doppler Effect

Q&A: The Geometry of Angular Size
The Doppler Effect