Everyone talks about 'equinox auroras' — but the Harvest Moon may have other plans

Last winter I made two trips to see the aurora, one to northern Canada and the other to northern Sweden. Both trips had something in common — there were very few tourists around. While many so-called aurora-hunters go north in the depths of winter when the nights are longer (and colder), I rarely do, instead preferring the start and end of the aurora season — September and March.

The reason is magnetism — and, more specifically, the "equinox effect". It sounds more precise than it is. Do auroras peak on the equinox? That's too simplistic, but "equinox auroras" are statistically real. In the past few years, I've made multiple trips around the equinoxes and never been disappointed.

Geomagnetic activity has a well-established semiannual pattern, with enhanced activity statistically more common around the March and September equinox seasons. One major explanation is the Russell-McPherron effect, named for the researchers who described how the orientation of Earth's magnetic field and the interplanetary magnetic field can favor more efficient magnetic coupling at particular times of year.

It can sound incredibly complex, so let's break it down. Remember the school experiment where you sprinkle iron filings around a bar magnet and watch them trace its invisible magnetic field? Earth is essentially a giant tilted magnet rotating as it orbits the sun, and the solar wind carries the sun's magnetic field out across the solar system. That magnetic field (called the interplanetary magnetic field, or IMF) constantly changes direction, so when it reaches Earth, it can have a component pointing north or south — expressed by scientists as positive or negative. A southward, or negative, magnetic field is particularly important because it points opposite to Earth's northward magnetic field, allowing the two magnetic fields to connect and solar-wind energy to enter Earth's magnetic environment. What the R-M effect describes is how Earth's changing orientation as it travels around the sun makes this favorable magnetic arrangement more likely around the equinoxes.

What it does not mean is that Earth suddenly develops a crack in its magnetic shield precisely on equinox night or that aurora is guaranteed on Sept. 22 — you still need the sun to do something. Auroras ultimately require energy from the solar wind, often enhanced by a coronal mass ejection or a fast solar-wind stream. The equinox geometry can help Earth respond efficiently to suitable conditions, but it cannot create a geomagnetic storm from nothing (nor does it guarantee clear skies!). The R-M effect helps load the dice.

That distinction matters this week because September's seasonal geometry is getting favorable just as the observing conditions are becoming decidedly less so.

The moon reaches first quarter on Sept. 18 and then waxes rapidly towards the Harvest Moon on Sept. 26. So by equinox night there will be a bright waxing gibbous moon polluting the sky with its reflected light. Crucially, that will be the case for the entire week after the equinox, which is when the R-M effect can really kick in.

For strong aurora, that's not necessarily a problem. A vivid display can punch through moonlight beautifully, and moonlit landscapes can make aurora photographs more interesting.

For the faint green arc that barely reaches above the northern horizon, however, moonlight can be brutal. So this year's equinox aurora season comes with an annoying contradiction — the statistics improve, but the sky brightens.

What's happening and when to look

The September equinox occurs at 8:05 p.m. EDT on Sept. 22, when the sun crosses the celestial equator heading south, but there's no need to regard that precise minute as an aurora deadline.

In practical terms, aurora hunters should keep watching space-weather forecasts throughout late September and into October. If activity is predicted, find a site with an unobstructed northern horizon and as little artificial light as possible. The waxing moon will increasingly dominate the southern sky, so use buildings, woodland or terrain to keep direct moonlight away from your eyes and camera lens if possible.

Stargazer's corner: September 18-25, 2026

Sept. 18 is busy. The moon reaches first quarter, while Venus reaches greatest brilliancy later that day at around magnitude -4.8. Look very low towards the western sky soon after sunset for Venus, though its evening apparition is now becoming increasingly difficult from northern latitudes. Equinox — a word derived from Latin for "equal night" — on Sept. 22 marks the instant the center of the sun crosses the celestial equator southwards — but there's nothing specific to see in the night sky aside from the rise in the east at sunset of fall constellations such as Perseus, Pegasus and Pisces. Meanwhile, the moon grows brighter each evening as it approaches full on Sept. 26.

Constellation of the week: Ursa Major

Aurora does not belong to a constellation, but photographs can make northern lights appear fixed among the stars when they are actually happening only hundreds of kilometers above Earth's surface. See the northern lights, and you'll almost always have photographs that prominently feature Ursa Major — better known for its central seven stars, the Big Dipper or the Plough.

Use the two stars forming the outer edge of its bowl — Merak at the bottom and Dubhe at the top. Imagine a line running from Merak through Dubhe, then continue it roughly upward. Travel roughly five times the distance between those two stars, and you'll reach Polaris, the North Star. Aurora may form below it, around it, or, during strong displays, spread far beyond it across the entire sky. From lower northern latitudes, the first suspicious glow often appears much closer to the northern horizon. That is where to look first.