Meteor Showers

                                                          Meteor showers

                        
More than a dozen times each year, we experience a strong pulse of "shooting stars" from an annual meteor shower. 
 The two best meteor showers in 2018 will be the Perseids in mid-August and the Geminids in mid-December.



                        

Imagine lying on a soft patch of grass and admiring the starry night sky overhead. Suddenly, you spot a bright streak that comes and goes literally in a flash. You've just seen a "shooting star," and it's time to make a wish — maybe for another one that's even brighter!

There's a chance that your wish will come true, because if you're looking up from a dark location, far from city lights, you'll see brief streaks from meteors a few times every hour. They can catch your eye just at the limit of visibility or be dramatically bright fireballs that appear brighter than Venus and light up the nightscape around you. The rarest of these, called a bolide, shatters into pieces during its rapid descent.

Because they're arriving so fast, it doesn't take a very large particle to make a dramatic flash. Typically they're no bigger than big sand grains, and something the size of a pea can create a meteor that's dramatically bright. Those high velocities give each particle a lot of kinetic energy, which converts to heat due to friction in the upper atmosphere.Derived from the Greek word meteoros (meaning "high in the air"), meteors are bits of interplanetary debris that slam into Earth's upper atmosphere at speeds of 30 to 70 km (20 to 45 miles) per second! And although some meteors look bright enough that it seems you can almost touch them, actually they occur very high up, at altitudes of 80 to 120 km (50 to 75 miles).

Many people think a meteor occurs because the particle is "burning up." But actually friction flash-heats air molecules along the particle's path to thousands of degrees. The air molecules cool down in just a split second, giving off light as they do so.

                              What is a Meteor Shower?

You'll notice the difference if you watch the sky for a half hour or so during one of these events: not only do the number of meteors you'll see go up, but also the meteors seem to fly away from a common point in the sky called the radiant.Meteors can occur at any time on any night and appear in any part of the sky. On a dark, moonless night you might see a half dozen of these sporadic(random) meteors per hour. However, whenever Earth encounters a stream of gritty debris left in space by a passing comet, the result is a meteor shower.

This is a trick of perspective, because all these particles are traveling in parallel — part of a vast but sparse "river of rubble" that's spread all around the comet's orbit. 

                      
                     

Radiant point

Meteor shower on chart

             Because meteor shower particles are all traveling in parallel paths, and at the same velocity, they will all appear to an observer below to radiate away from a single point in the sky.       
             This radiant  point is caused by the effect of perspective, similar to parallel railroad tracks converging at a single vanishing point on the horizon when viewed from the middle of the tracks.
                A shower gets its name not from its parent object but from the constellation where this radiant lies — for example, August's well-known Perseid shower has its radiant in Perseus, December's Geminids appear to radiate from Gemini, and so forth.

 This "fixed point" slowly moves across the sky during the night due to the Earth turning on its axis, the same reason the stars appear to slowly march across the sky. 
           The radiant also moves slightly from night to night against the background stars (radiant drift) due to the Earth moving in its orbit around the sun.

           When the moving radiant is at the highest point it will reach in the observer's sky that night, the sun will be just clearing the eastern horizon. 


For this reason, the best viewing time for a meteor shower is generally slightly before dawn — a compromise between the maximum number of meteors available for viewing, and the lightening sky which makes them harder to see.

Comet Encke's meteoroid trail is the diagonal red glow

Meteoroid trail between fragments of Comet 73P

Dynamical evolution of meteoroid streams

 Milos Plavec was the first to offer the idea of a dust trail, when he calculated how meteoroids, once freed from the comet, would drift mostly in front of or behind the comet after completing one orbit. 

The effect is simple orbital mechanism – the material drifts only a little laterally away from the comet while drifting ahead or behind the comet because some particles make a wider orbit than others.

These dust trails are sometimes observed in comet images taken at mid infrared wavelengths (heat radiation), where dust particles from the previous return to the Sun are spread along the orbit of the comet (see figures).

The gravitational pull of the planets determines where the dust trail would pass by Earth orbit, much like a gardener directing a hose to water a distant plant.
 Most years, those trails would miss the Earth altogether, but in some years the Earth is showered by meteors. This effect was first demonstrated from observations of the 1995 alpha Monocerotids, and from earlier not widely known identifications of past earth storms.

Over longer periods of time, the dust trails can evolve in complicated ways.
      For example, the orbits of some repeating comets, and meteoroids leaving them, are in resonant orbitswith Jupiter or one of the other large planets – so many revolutions of one will equal another number of revolutions of the other. 
This creates a shower component called a filament.

A second effect is a close encounter with a planet. When the meteoroids pass by Earth, some are accelerated (making wider orbits around the Sun), others are decelerated (making shorter orbits), resulting in gaps in the dust trail in the next return (like opening a curtain, with grains piling up at the beginning and end of the gap).

 Also, Jupiter's perturbation can change sections of the dust trail dramatically, especially for short period comets, when the grains approach the big planet at their furthest point along the orbit around the Sun, moving most slowly.

 As a result, the trail has a clumping, a braiding or a tangling of crescents, of each individual release of material.

The third effect is that of radiation pressure which will push less massive particles into orbits further from the sun – while more massive objects (responsible for bolides or fireballs) will tend to be affected less by radiation pressure. This makes some dust trail encounters rich in bright meteors, others rich in faint meteors.

 Over time, these effects disperse the meteoroids and create a broader stream. 

The meteors we see from these streams are part of annual showers, because Earth encounters those streams every year at much the same rate.

When the meteoroids collide with other meteoroids in the zodiacal cloud, they lose their stream association and become part of the "sporadic meteors" background. Long since dispersed from any stream or trail, they form isolated meteors, not a part of any shower. These random meteors will not appear to come from the radiant of the main shower.

Must-See Meteor Showers in 2018

Here are highlights of the major meteor showers for 2018. For the best possible viewing experience, find a dark location, make yourself comfortable in a reclining chair, and wear plenty of warm clothing (as appropriate). And for more information on watching and studying meteors, see our article on meteor basics and the other articles in the Meteor section of our website.

January 4: The Quadrantids

Here's the view looking northeast at about 1 a.m., by which time the Quadrantid shower's radiant is well above the horizon for mid-northern viewers.

The "Quads" can deliver at least 1 meteor per minute under excellent sky conditions. In fact, the idealized zenithal hourly rate (ZHR), for someone with perfectly dark skies and the meteor's radiant overhead, is a very high 120. But in practice few observers ever see anything close to this many, because the shower's maximum activity lasts only several hours and is easy to miss. Seeing 25 per hour is more typical. Unfortunately, this year the peak occurs at 22h Universal Time on January 3rd, which is during daylight for North Americans — and the Moon is just past full.

The parent of this shower is an object designated 2003 EH1, which loops around the Sun every 5½ years between the orbits of Earth and Jupiter. It's not an active comet — more likely it's an "extinct comet" that no longer has any ice. Meteor specialist Peter Jenniskens first realized that this body is responsible for the Quadrantid meteors.

April 22: The Lyrids

As with the Quadrantids, April's Lyrid shower puts on a fairly brief performance. This hasn't been a particularly strong display in recent years, though counts exceeded one per minute during an outburst in 1982. The predicted peak (18h UT) isn't very favorable for North America, and light from a first-quarter Moon will hinder viewing before midnight. Look for a few meteors per hour emanating from a radiant near the Hercules-Lyra border.

May 6: The Eta Aquariids

This annual shower originates from none other than Halley's Comet, and these meteors come in fast — 66 km (41 miles) per second! At its best, under ideal conditions, the Eta Aquariids can deliver a meteor per minute at its peak, which this year should occur near 8h UT on May 6th. The shower's radiant (in the Water Jar asterism of Aquarius) never gets very high above the horizon for observers in the Northern Hemisphere. It rises only a couple of hours before dawn, and this year a fat gibbous Moon will be in the same part of the sky.

July 29: The Delta Aquariids

You might see this long-lasting shower called the Southern Delta Aquariids, because its radiant is below the celestial equator and thus best seen from the Southern Hemisphere. This year light from a full Moon will be a big problem, because the Delta Aquariids tend to be faint — so don't count on seeing more than a few of these meteors per hour.

August 12–13: The Perseids

Even casual skywatchers know about the Perseid meteor shower, because it can deliver at least one meteor per minute under pleasant summer skies. But the shower's peak performance is relatively brief, so timing is important.

A bright Perseid meteor streaked down on August 7, 2010, over buildings at the Stellafane amateur astronomy convention in Springfield, Vermont. 

According to the International Meteor Organization, the shower's 2018 maximum should come between 20hUT on August 12th and 8h UT on the 13th. Fortunately, the midpoint of this range falls during the night of August 12-13 for North America. Moreover, the Moon will be new, so observing conditions are nearly perfect! Start watching on the evening of the 12th as soon as it's gotten dark and the radiant (near the Double Cluster in Perseus) clears the horizon. You might be rewarded with bright firefalls that skim Earth's atmosphere and create long, dramatic streaks in the sky.

These meteors are bits of debris shed by Comet 109P/Swift-Tuttle, which orbits the Sun every 130 years. Careful observers first realized that the Perseids are an annual event in the 1830s.

October 21: The Orionids

Here's another modest shower due to Halley's Comet. This year its peak on October 21st is during daylight in North America. Also, a nearly full Moon will create strong interference. But meteor experts predict that Orionid activity is on an upswing after some poor showings a few years ago. The shower’s radiant is located above Orion’s bright reddish star Betelgeuse.

November 5: The Southern Taurids

Lasting from mid-September to mid-November, this broad, weak display typically produces at most a dozen meteors per hour at its peak. But in 2005 skywatchers were treated to a "Taurid fireball swarm" dominated by bright, slow-moving fireballs from larger-than-average particles. A new Moon will offer no interference when the shower peaks, so be alert for some Taurid fireballs this year. The shower’s radiant is in western Taurus, along its border with Cetus.

November 18: The Leonids

The Leonid shower's parent comet, 55P/Tempel-Tuttle, tends to create narrow concentrated streams of debris that produced prodigious displays in the late 1990s, when it last swung close to the Sun. Since then the shower's activity has varied from year to year, usually offering little more than a trickle of shooting stars radiating from Leo’s Sickle. This year's peak (23h UT on November 17th) suggests looking late that night and especially after midnight on the 18th, by which time a waxing gibbous Moon will have set.

December 14: The Geminids

The Geminid meteors can flash into view anywhere in the late-night sky. But if you follow their paths back far enough, they all appear to diverge from a point in the constellation Gemini.

This end-of-the-calendar shower is usually the year’s best and most reliable, with upward of 100 meteors per hour radiating from a spot near the bright star Castor. The Geminid radiant is well up in the sky by 9 p.m. as seen from at mid-northern latitudes.

This year you'll have some evening interference from the nearly first-quarter Moon, but it will set before midnight. Besides, to see the Geminids at their best you'll want to be out later, in the hours before dawn, because this year the shower's brief peak is predicted for 12:30 UT — which is 7:30 a.m. on the East Coast and 4:30 a.m. on the West Coast.

Geminid meteors come from 3200 Phaethon, an asteroid discovered in 1983 that circles the Sun every 3.3 years. In fact, recent observations suggest that Phaethon might be a "rock comet" that sheds particles when its surface heats up to roughly 1,300°F (700°C) at each perihelion.

December 22: The Ursids

Although the Ursid shower delivers only a modest 10 meteors per hour under even the best conditions, it has the advantage of having a radiant near the bowl of the Little Dipper — so it's in view all night from northern latitudes. Unfortunately, this year its peak coincides with a full Moon, which will likewise be in the sky all night

Viewing Tips

The most common question is “Where can I see the meteor showers?”

The answer is: ANYWHERE in the sky!  Yes, for this celestial wonder, your specific location does not matter.

And the next question is “When can I see the meteor showers?”

The answer is:  Most showers are midnight to dawn.  Yes, it’s for you night owls or early risers. However, the famous Geminids are all night and some showers happen late evening. See the chart below.

METEOR SHOWER VIEWING TIPS

• For everyone, weather conditions and light pollution are the only reasons you can not see the showers. The sky needs to be dark and clear, away from all the city lights. Bright moonlight, within a few days of a full Moon, will also reduce the number of meteors that you will see. 

Where to look? The best place to start is between the radiant and the zenith (straight above you). The radiant is where the meteors appear to start from, such as the constellation Perseus, from which the Perseid meteor showers appear to radiate.

When to look? The time of the year for each shower is determined by when in Earth’s orbit it crosses the stream of meteoroids. On the chart below, see the “date of maximum,” which shows when meteor showers will be the strongest

Note that the “best” viewing times are usually predawn and late evening. In nearly all showers, the radiant is highest just before dawn. (The Geminid meteor showers are visible all night long, since Gemini arises just an hour or two after nightfall; the radiant is highest a little after midnight.) Sporadic meteors (unrelated to a shower and commonly called shooting stars) can be seen on any night, but increase in frequency after midnight and peak just before dawn.

Starting around midnight, your location on the globe spins around to the forward-facing half of Earth (in relation to the direction of orbit). At dawn, your location on the globe directly faces the direction in which Earth is traveling along its orbit. So between midnight and dawn, you’ll be viewing the meteors head-on, for a more frequent display

You don’t need any special equipment. In fact, binoculars do not work for meteor showers. The naked eye is best.

Spread a blanket on the ground and look up in the dark night.

Note that the meteor shower dates do not change much from year to year.  

Principal Meteor Showers
SHOWER	BEST VIEWING	POINTOFORIGIN	DATE OF MAXIMUM*	NO. PER HOUR	ASSOCIATED COMET
Quadrantid	Predawn	N	Jan. 4	25	—
Lyrid	Predawn	S	Apr. 22	10	Thatcher
Eta Aquarid	Predawn	SE	May 4	10	Halley
Delta Aquarid	Predawn	S	July 30	10	—
Perseid	Predawn	NE	Aug. 11–13	50	Swift-Tuttle
Draconid	Late evening	NW	Oct. 9	6	Giacobini-Zinner
Orionid	Predawn	S	Oct. 21–22	15	Halley
Taurid	Late evening	S	Nov. 9	3	Encke
Leonid	Predawn	S	Nov. 17–18	10	Tempel-Tuttle
Andromedid	Late evening	S	Nov. 25–27	5	Biela
Geminid	All night	NE	Dec. 13–14	75	—
Ursid	Predawn	N	Dec. 22	5	Tuttle
*May vary by one or two days    **Moonless, rural sky    Bold = most prominent

• “Predawn” means an hour or so before morning twilight. Best time to view most major showers.
• “Late evening” means approximately between 10 p.m. and midnight (or a little past).

In general, most major meteor showers are best seen after midnight; some do not even appear until after then. Usually, a better time to see them is after 2 a.m., and the best time is about an hour or so just before morning twilight. Geminids, however, can be seen starting earlier, such as around 9 or 10 p.m., until morning twilight.
Sometimes Draconids may be visible at nightfall through early evening.