Geminids

They are the strongest shower on this site by tabulated rate: ZHR 150 against the Perseids' 100. The radiant at declination +32.3° reaches 77° from 45°N and 80° from Hong Kong, so it climbs high for most of the populated northern hemisphere — and unlike the Perseids it reaches 24° from Sydney, so the southern hemisphere is not shut out. The honest counterweight is December weather and December temperatures, both of which the rate figure ignores.

On the numbers, the Geminids: 150 against 100 for ZHR, and a radiant that works from both hemispheres. On probability of a clear night and comfort, the Perseids. The population indices are close (2.6 for the Geminids, 2.2 for the Perseids), so neither has a decisive advantage under a compromised sky. Check both peaks against the Moon for the year in question — that usually settles it.

2028 put the peak under a Moon less than a quarter lit; 2027 and 2030 are near full. With a ZHR of 150 the Geminids retain more usable rate under moonlight than any other shower here, so a partly lit Moon is a reason to lower expectations rather than to stay in.

Yes, and better than most northern showers. The radiant reaches 24° from Sydney, 34° from São Paulo and 31° from Johannesburg. Rates are lower than at 40°N because the radiant altitude is lower, but the shower is genuinely observable rather than nominally so.

The geocentric velocity is 33.8 km/s — slower than the Perseids at 58.8 and much slower than the Leonids at 70.3. Slower meteors last longer across the sky, which makes them easier to catch out of the corner of your eye. Combined with a population index of 2.6 and the highest ZHR in the table, that is why the Geminids are usually the most productive night of the year for a patient observer.

Yes, and more than for a weaker shower in absolute terms. A magnitude of extra sky darkness multiplies the count by 2.6; applied to a base of 150 rather than 10, the same multiplier moves far more meteors. The strongest showers are where a dark site pays the largest absolute dividend, even though the percentage improvement is the same.

Judged only on how much of the Moon is lit at the peak instant: 2026 — 14 December 2026 (Moon 25% lit); 2027 — 14 December 2027 (Moon 98% lit); 2028 — 14 December 2028 (Moon 5% lit); 2029 — 14 December 2029 (Moon 65% lit); 2030 — 14 December 2030 (Moon 82% lit). The dark-Moon years are 2028; the washed-out ones are 2027 and 2030. 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 150 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 = 2.6, which means each magnitude of sky brightness you lose costs you a factor of 2.6 in the count. A suburban sky two magnitudes shallower than the reference therefore divides the rate by about 6.8. Real counts are typically 30–50% under the modelled figure even after those corrections.