Meteor showers: what ZHR means, and why the number never happens
A shower's published rate assumes a perfect sky and the radiant overhead. Here is what the figures mean and how to pick a night that is worth staying up for.
A meteor shower is a scheduled event with an unscheduled experience. The schedule is real: Earth crosses the same dust trail at the same point in its orbit every year, to within a day. The experience depends on the Moon, the weather, and whether the radiant happens to be high when you are awake.
The major showers
| Shower | Peak (approx.) | ZHR | Parent body |
|---|---|---|---|
| Quadrantids | January 3–4 | 120 | Asteroid 2003 EH1 |
| Lyrids | April 22 | 18 | Comet Thatcher |
| Eta Aquariids | May 5–6 | 50 | Comet Halley |
| Southern Delta Aquariids | July 30 | 25 | Comet 96P/Machholz |
| Perseids | August 12–13 | 100 | Comet Swift–Tuttle |
| Draconids | October 8–9 | 10 | Comet 21P/Giacobini–Zinner |
| Orionids | October 21–22 | 20 | Comet Halley |
| Leonids | November 17–18 | 15 | Comet Tempel–Tuttle |
| Geminids | December 13–14 | 150 | Asteroid 3200 Phaethon |
| Ursids | December 21–22 | 10 | Comet Tuttle |
Three of those figures deserve a caution. The Quadrantids reach 120 for only a few hours, so a peak that arrives at the wrong longitude is a 20-per-hour night. The Draconids are usually quiet but have produced storms of thousands per hour, most recently in 2011 and 2018, and cannot be forecast reliably. The Geminids are the most consistent high rate of the year, and the only major shower from an asteroid rather than a comet.
ZHR is not a forecast
The Zenithal Hourly Rate assumes two things that are never both true: the radiant directly overhead, and a sky dark enough to show magnitude 6.5 stars. Both corrections matter:
- Radiant altitude. The observed rate scales roughly with the sine of the radiant’s altitude. At 30° above the horizon you see about half the ZHR; at 15°, about a quarter.
- Sky brightness. Under suburban light pollution, limiting magnitude might be 4.5 instead of 6.5, which removes most of the meteors. A useful rule of thumb is that each magnitude of lost sky costs about a factor of two in visible meteors.
Multiply those and a ZHR of 100 under a suburban sky with a 40° radiant becomes roughly 20 per hour — still a good night, but not the “100 an hour” that headlines promise.
The moonlight problem
The Moon is the dominant factor in any given year, and it is fully predictable. A shower peaking within a few days of full moon is effectively cancelled for faint meteors; one peaking near new moon is as good as it gets.
Because the phase cycle is 29.53 days and showers are annual, each shower cycles through the Moon’s phases over about 30 years, spending a few years badly and a few years well. If a shower’s peak falls near full moon this year, the honest advice is to skip it and check again in two or three years — or to look a few nights before the peak, when the Moon sets earlier.
The phase calendar gives the instants, and the moonrise table tells you when the Moon is actually up.
How to actually watch
- Get away from lights. The single biggest factor after the Moon. A dark site is worth more than any equipment.
- Let your eyes adapt. Twenty to thirty minutes with no white light. A red flashlight preserves dark adaptation; a phone screen destroys it.
- Look at a wide area of sky, not at the radiant. Meteors appear to radiate from a point, but the trails are longest and most visible 30–60° away from it.
- Lie down. Neck strain ends most observing sessions before the meteors do.
- Give it an hour. Rates are averages; a 20-minute gap with nothing is normal and does not mean the shower failed.
What you are actually seeing
A grain of comet dust, typically between a millimeter and a centimeter across, hitting the upper atmosphere at 11 to 72 km/s. It vaporizes in a fraction of a second at 80–100 km altitude, and what you see is the air along its path glowing — not the grain itself. The grain is gone; the streak is the atmosphere’s response.
That is also why the same shower produces the same speed for every meteor: they are all fragments of one object on one orbit, hitting at one angle.
Peak dates and ZHR values are from the International Meteor Organization’s calendar. Rates vary year to year, sometimes by a factor of two, and no forecast can promise a specific number.
This page covers the observing, not the meaning. No meteor shower is claimed to predict anything.
Frequently asked questions
- What does ZHR 100 actually mean?
- Zenithal Hourly Rate: the number of meteors a single observer would see per hour if the radiant were exactly overhead and the sky were dark enough to see stars of magnitude 6.5. Neither condition usually holds, so a real observer under a suburban sky typically sees 10–25% of the published figure. The number is a property of the shower, not a forecast for your garden.
- Why is the best time after midnight?
- Because after midnight you are on the leading side of Earth as it moves along its orbit, so you are facing into the stream of dust rather than being overtaken by it. The radiant is also higher, which raises the count. The rule of thumb: the last few hours before dawn are almost always better than the first few after dusk.
- Does a full moon ruin a shower?
- It does. Moonlight raises the sky background and removes the faint meteors, which are most of them — a full moon can cut the visible rate by 80% or more. Checking the phase before planning is the single most valuable thing you can do, and the phase for any date is on this site.
- Where do meteor showers come from?
- Comet dust, mostly. As a comet orbits the Sun it sheds grains along its path, and when Earth crosses that trail the grains hit the atmosphere at tens of kilometers per second and burn up. A few showers, notably the Geminids, come from an asteroid — 3200 Phaethon — which is one of the reasons that shower is unusually rich.
Sources
Each source is a primary or official reference, checked before publication. Links open the original.
- International Meteor Organization, annual meteor shower calendar — peak dates, radiants, ZHR values and parent bodies for the showers in the table on this page (imo.net/resources/calendar/)
- American Meteor Society — the definition of Zenithal Hourly Rate and the corrections for radiant altitude, limiting magnitude and obstruction that turn ZHR into an observed rate (amsmeteors.org)
- NASA Science — Meteors and meteor showers: the cometary dust trails, encounter geometry and the reason rates peak after local midnight (science.nasa.gov)
- U.S. Naval Observatory — lunar phase instants used on this site to state which showers a given year's Moon interferes with (aa.usno.navy.mil)