Southern delta-Aquariids

A ZHR of 25 against the Perseids' 100 makes this look like an easy decision, and from the northern mid-latitudes it usually is. Two things argue the other way. The radiant is at declination −16.1°, so from 30°S it climbs to 76° against the Perseid radiant never rising there at all. And the two activity periods overlap through late July and August, so on a given night you may be counting both — the site's hourly model treats them separately and reports whichever is stronger for your latitude.

Because r = 3.2 is the highest population index of the twelve showers here. A high r means the stream is rich in faint meteors and poor in bright ones, so every magnitude of sky brightness you lose divides your count by 3.2 rather than by the 2.1 of the Quadrantids. Two magnitudes of suburban sky glow cost you a factor of about 10.2. If you only make one drive out to a dark site all year, this is the shower to spend it on.

Yes, but low. The radiant declination of −16.1° means it culminates at 22° from London and 29° from the Canada–US border latitude. The sine correction alone takes roughly two-thirds off the rate at 51.5°N before any sky-brightness penalty. From the southern hemisphere the same shower is a completely different event.

Of 2026 to 2030, 2027 and 2030 put the peak under a Moon less than a quarter lit; 2026 are badly moonlit. Given r = 3.2, a moonlit delta-Aquariid peak is close to a write-off — this shower has almost no bright-meteor reserve to push through the glare.

For most showers that is a reasonable gamble. For this one it is the worst combination on the list, because the faint-rich population is exactly what moonlight removes first. The population index here is 3.2, the highest of the twelve showers on this site — against 2.2 for the Perseids and 2.6 for the Geminids. The verdict on this site accounts for that automatically: the Moon penalty is applied to the limiting magnitude, and the limiting magnitude is then raised to the power of the population index. At the model’s 2.0-magnitude ceiling for moonlight that is a factor of about 10.2, where the same Moon costs a Perseid observer a factor of 4.8. A bright Moon therefore drops the delta-Aquariid verdict faster than it drops the Geminid one.

Judged only on how much of the Moon is lit at the peak instant: 2026 — 31 July 2026 (Moon 96% lit); 2027 — 31 July 2027 (Moon 4% lit); 2028 — 31 July 2028 (Moon 75% lit); 2029 — 31 July 2029 (Moon 70% lit); 2030 — 31 July 2030 (Moon 2% lit). The dark-Moon years are 2027 and 2030; the washed-out ones are 2026. 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 25 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 = 3.2, which means each magnitude of sky brightness you lose costs you a factor of 3.2 in the count. A suburban sky two magnitudes shallower than the reference therefore divides the rate by about 10.2. Real counts are typically 30–50% under the modelled figure even after those corrections.