Orionids

A ZHR of 20 puts them mid-table, but two things work in their favour. The radiant is at declination +15.7°, close enough to the celestial equator that both hemispheres get a usable view — 61° from 45°N and 40° from Sydney. And at 66.1 km/s they are among the fastest showers here, which produces long trails and a high share of persistent trains. It is a genuinely two-hemisphere shower, which the Perseids and Geminids are not.

The Orionids carry ZHR 20 against the Leonids' 12.5, and their radiant at +15.7° declination is friendlier to southern observers than the Leonid radiant at +21.8°. Both are fast showers from famous parent comets. On the tabulated numbers the Orionids are the better bet in an ordinary year; the Leonids only overtake them in a storm year, which this site does not attempt to forecast.

The window is 2 October – 7 November, 37 days. A broad window means the profile around the peak is comparatively flat, so a night either side of the peak is a reasonable substitute if cloud rules out the peak itself. It also means the Moon moves noticeably across the window: a peak under a bright Moon can still be worth watching several nights later once the Moon rises later in the night.

Of 2026 to 2030: 2028 are dark, 2026 and 2029 are badly moonlit. Given the broad activity window, a moonlit Orionid peak is one of the few cases where deliberately watching several nights off-peak is the better decision.

The population index is 2.5, mid-range for this set, so each magnitude of sky brightness divides your count by 2.5. A typical suburban sky costs around two magnitudes against the reference, which is a factor of roughly 6.3. With a ZHR of 20 to start from, that is the difference between a shower and a curiosity. An hour's drive is usually the single most effective thing you can change.

Judged only on how much of the Moon is lit at the peak instant: 2026 — 21 October 2026 (Moon 79% lit); 2027 — 22 October 2027 (Moon 58% lit); 2028 — 21 October 2028 (Moon 13% lit); 2029 — 21 October 2029 (Moon 99% lit); 2030 — 21 October 2030 (Moon 29% lit). The dark-Moon years are 2028; the washed-out ones are 2026 and 2029. Moon illumination is the only one of the four factors that is knowable years ahead — cloud cover is not, and neither is whether you will be somewhere dark.

Because 20 is the zenithal hourly rate: what one observer would count under a magnitude 6.5 sky with the radiant straight overhead. Two corrections pull it down before you ever look up. Rate scales with the sine of the radiant altitude, so a radiant 30° up delivers half of what it delivers at the zenith. And the population index for this shower is r = 2.5, which means each magnitude of sky brightness you lose costs you a factor of 2.5 in the count. A suburban sky two magnitudes shallower than the reference therefore divides the rate by about 6.3. Real counts are typically 30–50% under the modelled figure even after those corrections.