The Perseids are rising toward one of the best of their annual displays.
You may see a few of these light-streaks any night from about July 17 to August 24, but they increase gradually to a peak, predicted, this year, for August 13 at 6 Universal Time. That is 7 AM by British clocks, 2 AM in eastern North America. And there are often sub-peaks before the main one, caused by slightly thicker threads in the vast diffuse stream of particles in space through which the Earth is passing.
The exact time is not what matters most, though it happens to be excellent this year for North America and Europe. You can be out watching, in the night between Aug. 12 and 13, preferably as late as possible in that night, but you may also see Perseids in the surrounding nights. (I meant to get this post ready yesterday but my cat prevented it.)
Why as late as possible in the night? (Which could mean 3 or 4 AM if you’re an enthusiast.) The “radiant,” the spot in the constellation Perseus from which these meteors seem to shoot, is in the sky all night, but at first it is low in the northeast; as the night goes on and Earth turns, that part of the sky swings higher, so more of the meteor trails are likely to be above the horizon for you. After midnight is better; best of all is around 5 or 6 AM, when the radiant is almost overhead.
(That’s the way things are for us on the northern side of Earth’s globe. Southerners are unlucky, they don’t get to see the Perseids. The globe is in the way.)
Find a non-cloudy time, find a dark place away from light pollution (sadly difficult in modern times); make yourself comfortable (keep warm). It may take twenty minutes for your eyes to adapt to the dark. Sit back and gaze tranquilly at, say, a direction northeastward and half way between horizon and zenith, but there’s no need to stare fiercely at any particular spot: it will suffice to be dreamily attentive to your sky surroundings. Then you may see nothing for a while; a sudden faint streak over here, a longer and brighter over there; nothing for a while; a flurry of Perseids like a quiet firework; you may miss some behind you, that doesn’t matter. If you count, you could get fifty an hour at the best time, maybe even more, but neither I nor the hero Perseus make promises.
What is especially favorable this year is that there is no light pollution from the Moon, which when in the sky can drown out all but the brightest meteors. It is a day before New: in the direction of the Sun, out of the way.
What are these “children of Perseus”? They are bits of dust and rock that separated, most of them many centuries ago, from a 16-mile-wide flying mountain called Comet 109P Swift-Tuttle. It was discovered in July 1862 by Lewis Swift and three days later by by Horace Tuttle. “109P” means that it was the 109th comet to be recognized as “periodic”: returning in an elliptical orbit. It didn’t come by again till 1992, for its period is long, about 130 years. This means the orbit is vast; its outer end is at about 52 astronomical units (Sun-Earth distances), way beyond Neptune’s 30. So it is amazing that the comet, diving from such a distance, comes so close to Earth’s little orbit.
In fact, it seemed to come dangerously close, and at the next return, in 2126, Earth will be close to that point in its orbit, so – impact? This prompted Gary Kronk, an amateur but a great expert and author on comets and meteors, to make a study of ancient records, which might turn up other appearances of the comet and thus help to refine calculations. And he found that indeed the Chinese had observed it in 69 BC and AD 188. (It is now also identified with a comet of 1737.) This enabled Brian Marsden to calculate a more accurate orbit for the future. On 2126 Aug. 5 the comet will pass 0.15 a.u., not 0.05, from Earth and there will be no danger, but it will be like a naked-eye bright star. There will be a probably closer passage, and slightly higher chance of collision, in September 4479.