Sections · Meteors

Vol. Science · Astronomy · The shower

Meteors
under a real sky

A zenithal hourly rate is not the number you count. The IMO rate an ideal observer would see, radiant overhead and limiting magnitude +6.5, is the ZHR. The count this page prints is lower: HR = ZHR × sin(h) / r^{(6.5 − LM)} × (1 − k). h is the radiant’s altitude. r is the population index. LM is the faintest star you can see. k is the fraction of the field that cloud or an obstruction covers. At or below the horizon the rate is 0. The page opens on the IMO table’s row: ZHR 100, magnitude 6.5, radiant at 30°, clear sky, which is 50 an hour. How dark the night is belongs with magnitude and light pollution. When the Sun is still up, the twilight desk says so. Where the radiant sits in your sky is the local sky, not this calculator.

Educational calculator. Not a forecast of tonight’s sky and not a visual-observing report. The radiant altitude is an input. This page does not compute it from a latitude. Moonlight is not its own term: it enters only as the limiting magnitude you set. Sporadic meteors are not in the rate. A personal perception correction is not in the formula. Not a forecast.

Interactive · Observed hourly rate

Sky dome and rate versus radiant altitude · the marker is 30°

Sky dome
Rate
This altitude

The curve is the sine of altitude, scaled by the other factors. It is zero on the horizon and highest at the zenith.

Showers · IMO 2026 working list

Sky · sets the limiting magnitude · approximate

ZHR /h

Altitude deg

LM mag

r r

Cloud %

Shower meteors · not sporadics

50 an hour

Perseids · ZHR 100 · radiant 30°

Sine of altitude0.50
Magnitude factor1
Clear fraction1
Entry speed59 km/s
Energy per gram1,740.5 kJ/g
Parent109P/Swift-Tuttle
PeakAugust 13
Radiant48°, +58°

The sine of 30° is 0.50, so a zenith rate of 100 becomes 50 before anything else. Limiting magnitude 6.5 is the reference, so the population index does not change the count. The field is clear, so the cloud term is 1. The result is 50 shower meteors an hour.

One gram at the Perseids’ 59 km/s carries 1,740.5 kJ. That is ½v² with v in km/s. Faster grains carry more energy per gram. The Geminids are slower and the IMO calendar still calls them bright. Speed is not the only reason a meteor looks bright. 30° is a slider position, not a prediction for your town.

The sliders need JavaScript. The relation is still HR = ZHR × sin(h) / r^(6.5 − LM) × (1 − k), and h at or below 0° gives 0. IMO’s major-shower table, ZHR 100 and limiting magnitude 6.5, prints 50 meteors an hour at 30°. A stated cloud coverage of 50% is a field correction of 2, which cuts that 50 to 25. Working-list peaks for 2026: Quadrantids January 3, ZHR 80, 41 km/s, r 2.1, radiant 230° +49°, λ⊙ 283.15°. Lyrids April 22, ZHR 18, 49 km/s, r 2.1, 271° +34°, λ⊙ 32.32°, parent C/1861 G1 (Thatcher). Eta Aquariids May 6, ZHR 50, 66 km/s, r 2.4, 338° −1°, λ⊙ 45.5°, parent 1P/Halley. Perseids August 13, ZHR 100, 59 km/s, r 2.2, 48° +58°, λ⊙ 140.0°, parent 109P/Swift-Tuttle. Orionids October 21, ZHR 20, 66 km/s, r 2.5, 95° +16°, λ⊙ 208°, parent 1P/Halley. Leonids November 17, ZHR about 15, 71 km/s, r 2.5, 152° +22°, λ⊙ 235.27°, parent 55P/Tempel-Tuttle. Geminids December 14, ZHR 150, 35 km/s, r 2.6, 112° +33°, λ⊙ 262.2°, parent 3200 Phaethon.

What each factor does

The zenith rate is the ceiling. The sky you have is the rest.

Radiant altitude. The count tracks the sine of the altitude. At 30° the sine is one half, which is why the IMO table’s ZHR of 100 becomes 50 when the limiting magnitude is still 6.5 and the field is clear. At the zenith the sine is 1. On the horizon it is 0. Below the horizon this page prints 0 rather than a negative sine.

Limiting magnitude. The reference sky is +6.5. Each magnitude you lose multiplies the miss by the population index, so the formula divides by r raised to (6.5 − LM). A darker sky than 6.5, a larger LM, raises the rate. A city sky lowers it. The three sky buttons are approximate: 6.5 is the IMO reference, 5.6 is the bright end of Bortle’s suburban class 5, and 4.5 is Bortle’s city class 8 “at best.”

Population index. r is how steeply the counts rise toward fainter meteors. A larger r means a bright sky costs more of the shower. At exactly magnitude 6.5 the exponent is zero and r drops out. Cloud. k is the obstructed fraction of the field you are actually watching. The expected count is multiplied by (1 − k). The IMO visual FAQ says a coverage of 50% corresponds to a correction factor of 2.0, which is 1/(1 − k). Perception is not uniform across that field, so this is the simple fraction, not a full model of where the clouds sit.

Working list · seven annual showers

IMO 2026 Table 5, with parents
Shower	Active	Peak	λ⊙	Radiant	V∞	r	ZHR	Parent
Quadrantids · 010 QUA	Dec 28–Jan 12	January 3 · 2026 close to 21h UT	283.15°	230°, +49°	41 km/s	2.1	80	196256 (2003 EH1)
Lyrids · 006 LYR	Apr 14–Apr 30	April 22 · 2026 19h 40m UT	32.32°	271°, +34°	49 km/s	2.1	18	C/1861 G1 (Thatcher)
Eta Aquariids · 031 ETA	Apr 19–May 28	May 6	45.5°	338°, −1°	66 km/s	2.4	50	1P/Halley
Perseids · 007 PER	Jul 17–Aug 24	August 13 · 2026 02h–04h UT	140.0°	48°, +58°	59 km/s	2.2	100	109P/Swift-Tuttle
Orionids · 008 ORI	Oct 2–Nov 7	October 21	208°	95°, +16°	66 km/s	2.5	20	1P/Halley
Leonids · 013 LEO	Nov 6–Nov 30	November 17 · 2026 23h 45m UT	235.27°	152°, +22°	71 km/s	2.5	15	55P/Tempel-Tuttle
Geminids · 004 GEM	Dec 4–Dec 20	December 14 · 2026 14h UT	262.2°	112°, +33°	35 km/s	2.6	150	3200 Phaethon

Literacy · the words on the readout

ZHR: Zenithal hourly rate. The IMO calendar defines it as the calculated maximum an ideal observer would see in perfectly clear skies, reference limiting magnitude +6.5, with the radiant overhead. It is meteors per hour. It is not the number in a bright or cloudy sky, and it is not a promise for one night.
Population index: r, from the shower’s magnitude distribution. The 2026 calendar says r of 2.0–2.5 means a larger fraction of brighter meteors than average, and r above 3.0 is richer in faint meteors. Molau, Barentsen, and Crivello describe r as the factor by which the count rises when the limiting magnitude improves by one.
V∞: Pre-atmospheric velocity, km/s. The calendar’s range is about 11 km/s, very slow, to 72 km/s, very fast, with 40 km/s roughly medium. The energy line is ½v² for one gram at that speed, in kJ/g. It is not a measured fireball energy.
Radiant: Right ascension and declination at maximum, from Table 5. The radiant drifts on other nights. This page does not apply that drift, and it does not turn the coordinates into an altitude for your latitude. Altitude is the slider.
λ⊙: Solar longitude, equinox 2000.0, the Earth’s place on its orbit. Table 5 says the maximum dates are accurate only for 2026. The longitude is the stable mark. A clock time is printed only when that shower’s section gives one.
Speed and brightness: At the same mass, a faster meteor carries more kinetic energy, so it can be brighter, and a fast streak is the kind observers are asked to check for a persistent train. NASA Science calls the Leonids bright and fast, at 71 km/s in the body of that page, and says their fireballs can persist longer than an average streak. The Geminids, at 35 km/s, are the slow end of this list, and the IMO calendar still calls that shower known for bright meteors and fireballs. Particle size matters. The energy number does not replace that.

Sources · the formula and the presets

Cited, not invented

IMO 2026 Meteor Shower Calendar (Rendtel and others; working list correct as of June 2025) — Table 5 supplies the activity window, the 2026 maximum date, λ⊙, radiant α and δ, V∞, r, and ZHR for the Quadrantids (010 QUA), April Lyrids (006 LYR), η-Aquariids (031 ETA), Perseids (007 PER), Orionids (008 ORI), Leonids (013 LEO), and Geminids (004 GEM). The table note says maximum dates are accurate only for 2026. Abbreviations: ZHR is the ideal rate at limiting magnitude +6.5 with the radiant overhead; r is the population index; V∞ runs from about 11 to 72 km/s. Shower sections add the clock times used here, the Lyrid note that ZHR 18 can be variable up to 90, the Orionid header’s 20+ beside the table’s 20, and the parents C/1861 G1 (Thatcher), 109P/Swift-Tuttle, and 55P/Tempel-Tuttle. The Geminid section calls that shower known for bright meteors and fireballs.
IMO — Major shower observations — Table 1, meteors per hour for a ZHR of 100 at limiting magnitude 6.5: 100, 94, 77, 64, 50, 34, and 17 at altitudes 90° through 10°. Those integers are the sine of the altitude, rounded. This page’s clear-sky reference row is that 30° line, 50.
Ivanovic, IMC 2014, “Software for analysis of visual meteor data” — the total correction, citing Rendtel and Arlt 2008, is C = r^{(6.5 − lm)} × F / sin(h). The hourly rate this page prints is the ZHR divided by that C.
IMO — Visual FAQ — a cloud coverage of 50% corresponds to a correction factor of 2.0, so F = 1/(1 − k). The same answer says perception is stronger near the center of the field, and that a 20% cover near the edges is not a 20% loss of meteors. This page still uses the simple fraction and says so.
Molau, Barentsen, and Crivello, IMC 2014 — the population index is the factor by which the meteor count increases when the limiting magnitude improves by one. Their ZHR equation uses r^{(6.5 − LM)} and a field-obstruction factor F. The zenith term there is written with a zenith exponent. This page uses exponent 1, the sine in the IMO table.
NASA JPL Solar System Dynamics — Meteor streams — parent bodies, from the IAU Meteor Data Center and the references there: Quadrantids 196256 (2003 EH1), April Lyrids C/1861 G1 (Thatcher), eta Aquariids 1P/Halley, Perseids 109P/Swift-Tuttle, Orionids 1P/Halley, Leonids 55P/Tempel-Tuttle, Geminids 3200 Phaethon. The three parents the IMO sections already name agree. The Quadrantid, eta Aquariid, Orionid, and Geminid parents on this page are this table.
NASA Science — Meteor showers — the Leonids are associated with comet Tempel-Tuttle; the Aquariids and Orionids are linked to comet Halley; Perseids are pieces of comet Swift-Tuttle.
NASA Science — Leonids — the body text calls Leonids bright and fast and gives 44 miles (71 kilometers) per second, the same 71 km/s as the IMO table. Fireballs can persist longer than an average streak. The fast-facts table on that page prints 69.7 km/s; this desk does not use that second figure.
Bortle, Sky & Telescope, February 2001 — class 4 maximum naked-eye limit 6.1 to 6.5; class 3 rural 6.6 to 7.0; class 5 suburban around 5.6 to 6.0; class 8 city, magnitude 4.5 at best; class 9 inner-city 4.0 or less. The sky buttons use 6.5, 5.6, and 4.5 and are labeled approximate. Bortle’s own essay says a naked-eye limit depends on the observer.

Educational calculator. Not a forecast. Working-list numbers are the 2026 calendar’s Table 5, and the dates are for 2026. Parents the calendar text does not name are the JPL stream table. The hourly rate omits sporadics, perception, and any zenith exponent other than 1. Earthrise preview: NASA / Bill Anders (public domain).

Keep going

The clock

Dome	The dot sits on the meridian at the altitude you set. Zenith is the top.
Curve	Hourly rate from the horizon to the zenith for the other factors as they stand.

Chart	Left and Right nudge the altitude by one degree while the canvas is focused.
Buttons	Enter or Space selects. The pressed button is aria-pressed. Targets are at least 44 pixels on a side.
Phone	Below 900 pixels the long section row becomes a single Sections control. Open it for the same links. The site menu is the Menu button in the top bar.
This dialog	The question mark, or ?, opens it. Escape closes it. Tab stays inside until you close it. Click the dim page behind the card to close it.

Meteors
under a real sky

The zenith rate is the ceiling. The sky you have is the rest.

Cited, not invented

Twilight

Magnitude

Local sky

Light pollution

Moon

Telescope

Meteor desk

Inputs

Chart

Keyboard and touch

Shower	Sets ZHR and r from IMO 2026 Table 5. It does not set your altitude, your limiting magnitude, or the cloud.
Sky	Dark sets 6.5, suburban 5.6, city 4.5. Approximate Bortle anchors. Typing a different magnitude clears the button.
Altitude	Degrees above your horizon. The slider is 0 to 90. A typed value may go to −90. At or below 0 the rate is 0.
Cloud	Percent of the field that is covered. 50% halves the rate. 100% gives 0.

Meteorsunder a real sky

The zenith rate is the ceiling. The sky you have is the rest.

Cited, not invented

Twilight

Magnitude

Local sky

Light pollution

Moon

Telescope

Meteors
under a real sky