Ursids

The tabulated ZHR is 10 and the window is only 10 days — 17 December – 26 December — so this is a narrow, quiet shower falling in the busiest week of the year. The one thing it has going for it is the radiant: at declination +75.8° it is close to the north celestial pole, so from mid-northern latitudes it is circumpolar, never sets, and sits at 66° from London. There is no waiting for it to rise.

No. South of 14.2°S the radiant never rises at all — the radiant at declination +75.8° stays below the horizon all night from anywhere south of 14.2°S. The modelled rate there is zero, not merely low.

Only if the Geminid peak was clouded out or badly moonlit that year. The Geminids carry ZHR 150 against 10 here, an order of magnitude apart. The Ursids are best understood as a second chance eight days after the Geminids rather than as a target in their own right.

Of 2026 to 2030: 2027 and 2030. With a base rate of 10 per hour and a 10-day window, a moonlit Ursid peak leaves essentially nothing — this is the shower with the least margin on the site.

The tabulated drift is 2.4654 degrees of right ascension per degree of solar longitude, several times the typical value. That is a coordinate artefact, not a physical one: at declination +75.8° the lines of right ascension converge towards the pole, so a small real motion across the sky corresponds to a large change in right ascension. The engine applies the drift in the tabulated units and the resulting radiant position is correct; the number simply looks alarming.

Judged only on how much of the Moon is lit at the peak instant: 2026 — 22 December 2026 (Moon 98% lit); 2027 — 23 December 2027 (Moon 21% lit); 2028 — 22 December 2028 (Moon 36% lit); 2029 — 22 December 2029 (Moon 97% lit); 2030 — 22 December 2030 (Moon 5% lit). The dark-Moon years are 2027 and 2030; 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 10 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 = 2.8, which means each magnitude of sky brightness you lose costs you a factor of 2.8 in the count. A suburban sky two magnitudes shallower than the reference therefore divides the rate by about 7.8. Real counts are typically 30–50% under the modelled figure even after those corrections.