Meteor-shower planner

1. Radiants and why showers come back every year

A meteor shower happens when Earth crosses a stream of dust shed by a comet, or occasionally by an asteroid. The grains in a stream travel on almost parallel paths, so as they plunge into the air they all seem to fly out from one small patch of sky, the radiant, in the same way that parallel railway lines seem to meet in the distance. The shower is named after the constellation or star nearest that point.

Earth crosses each stream at the same point of its orbit every year. That point is measured by the solar longitude, the Sun's position along its yearly path through the sky, from 0° at the March equinox to 360°. The Perseids peak at a solar longitude of about 140.0°, which falls on 12 or 13 August. Because the calendar year is not a whole number of days, the date and hour of a peak drift by up to a day from year to year, so the planner works in solar longitude and turns it into your local date and time.

2. How high is the radiant?

The more of a shower's meteors reach your part of the sky, the higher its radiant stands. Its altitude h above the horizon, seen from latitude φ, comes from spherical trigonometry:

sin h = sin φ sin δ + cos φ cos δ cos H

Here δ is the radiant's declination (its celestial latitude) and H its hour angle: how far the sky has turned since the radiant crossed due south (or north). H is the local sidereal time minus the radiant's right ascension. Sidereal time is star time: the sky turns once in 23 hours 56 minutes, about 4 minutes less than a solar day, so every star rises 4 minutes earlier each night.

From latitude φ in the north, a radiant with declination above 90° − φ never sets: the Perseid radiant, at about +58°, never sets from Britain, where 90° − 51.5° = 38.5°. A radiant south of φ − 90° never rises, which is why the Eta Aquariids, at about −1°, are best from the southern hemisphere and the tropics, and Alpha Centaurids are invisible from Europe.

3. From ZHR to what you will actually see

Observers report a shower's strength as its zenithal hourly rate (ZHR): the number one person would see in an hour with the radiant straight overhead under a perfectly dark sky where stars of magnitude 6.5 are just visible. Real conditions are always worse, so the planner turns the ZHR into an expected hourly rate:

rate seen = ZHR × sin h × r^{(m − 6.5)}

The factor sin h allows for the radiant's height. The population index r, usually between 2 and 3.5, describes how many more faint meteors a shower has than bright ones: each step of one magnitude fainter brings about r times as many. m is the faintest star you can see, your limiting magnitude.

Worked example. At the Perseid peak (ZHR 100, r = 2.2) with the radiant 40° up from a suburb where stars of magnitude 5.5 are visible: 100 × sin 40° × 2.2⁻¹ = 100 × 0.643 ÷ 2.2 ≈ 29 meteors an hour. The same night from a dark site, with the radiant at 70°, gives 100 × 0.940 = 94.

4. Moonlight and twilight

The planner finds the Moon's position from a standard series of terms describing its orbit, accurate to about a third of a degree, and its phase from its angle ψ from the Sun as seen from Earth. The lit fraction of the disc is (1 − cos ψ) ÷ 2: 0 at new Moon, ½ at the quarters and 1 at full.

Scattered moonlight hides faint meteors. As a rule of thumb the planner lowers the limiting magnitude by up to 1.8 magnitudes when a full Moon is well above the horizon, in proportion to the lit fraction. It treats the sky as too bright for meteors until the Sun is 12° below the horizon, and as fully dark once it is 18° below (the end of astronomical twilight), shading the chart to match.

5. Why the hours before dawn are best

Earth travels round the Sun at about 30 km/s. After midnight your side of Earth faces forwards along that path, like the windscreen of a car in the rain, so it sweeps up more meteoroids, and meets them head-on at higher speeds. That is why random (sporadic) meteors are about twice as common before dawn as in the evening, and why most radiants are highest in the small hours. The Draconids are a famous exception: their radiant is highest in the evening.

6. How accurate is it?

The Sun's position is calculated to about 0.01°, which puts sunrise and sunset within a minute or two: for London the planner gives sunrise at 04:43 and sunset at 21:21 on 21 June 2024, matching published tables. Full and new Moons come out within about 20 minutes of their true times, and the predicted peaks of the 2024 Perseids, the 2025 Quadrantids and the 2025 Geminids within half an hour of the times forecast by meteor astronomers.

The rates are the least certain part. Showers vary from year to year, a few are capable of sudden outbursts, and every observer's eyes and sky are different. Treat the numbers as a guide to which nights and hours are worth your sleep, not a promise.

7. Making the most of a night

• Give your eyes 20 to 30 minutes to adapt to the dark, and avoid looking at a bright phone screen.
• Lie back and look about halfway up the sky, roughly 45° away from the radiant: meteors near the radiant have short trails, those further out are longer and easier to see.
• Dress for twice as cold as you expect, and take a reclining chair or a mat.
• Watch for at least an hour: meteors arrive in clumps and lulls.
• Children should observe with an adult, and anyone should ask permission before using private land at night.