eta-Aquariids

The radiant is at declination −0.5°, essentially on the celestial equator, so it rises for everyone — but only just clears the horizon in the pre-dawn hours from mid-northern latitudes. From 45°N it can reach 45°; from 20°S it reaches 70°. With a ZHR of 50 the southern view is one of the best of the year and the northern view is a thin one. This is the clearest hemisphere split among the twelve showers here.

The eta-Aquariids, by default. The Perseid radiant is at declination +57.9°, so it never rises at all south of 32.1°S — from Sydney or Cape Town the Perseids are not a shower, they are a rumour. The eta-Aquariid radiant is on the equator and its ZHR of 50 is half the Perseid figure, but half of something beats all of nothing.

Between 2026 and 2030 the dark-Moon peaks fall in 2027 and 2030, and 2026 and 2028 are compromised by a Moon at least three-quarters lit. Moonlight costs more here than the calendar alone suggests. The radiant sits at −0.5° declination, so from the northern mid-latitudes it only clears the horizon in the last couple of hours before dawn — a Moon inside that short window takes most of what there is. With a population index of 2.4, every magnitude of extra sky brightness divides the count by 2.4, so a tabulated ZHR of 50 does not survive much of it.

The window runs 19 April – 27 May, 39 days, which is unusually generous. It does not mean the rate is flat across it: the tabulated ZHR of 50 applies at the peak solar longitude, and activity falls away on either side. What the long window does buy you is resilience against cloud. If the peak night is overcast, a night two or three days either side is still worth going out for, which is not true of the Quadrantids or the Ursids.

At 66.3 km/s the eta-Aquariids are among the fastest of the twelve — fast meteors leave longer, thinner trails and a higher share leave persistent trains. The population index is 2.4, mid-range for this set, so the shower does lose more to a bright sky than the Quadrantids do: each magnitude of sky glow divides the count by 2.4.

Judged only on how much of the Moon is lit at the peak instant: 2026 — 6 May 2026 (Moon 81% lit); 2027 — 6 May 2027 (Moon 0% lit); 2028 — 5 May 2028 (Moon 89% lit); 2029 — 6 May 2029 (Moon 43% lit); 2030 — 6 May 2030 (Moon 13% lit). The dark-Moon years are 2027 and 2030; the washed-out ones are 2026 and 2028. 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 50 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 = 2.4, which means each magnitude of sky brightness you lose costs you a factor of 2.4 in the count. A suburban sky two magnitudes shallower than the reference therefore divides the rate by about 5.8. Real counts are typically 30–50% under the modelled figure even after those corrections.