We all know that fall is all about incredible reds, yellows, and oranges as deciduous trees shed their leaves, but they're not the only source of color this season. This Saturday sees the beginning of fall in the northern hemisphere. It's the point halfway between the summer solstice and the winter solstice, and when the day and night are of almost identical length. This is the autumn equinox –– equal night –– but how do you celebrate it? By looking for the Northern Lights, that's how.
Known to be stronger around the autumn equinox because of the planet's position and angle in relation to the sun, those in the border areas between the U.S. and Canada, as well as Northern Canada, Alaska, Scandinavia, and Northern Russia, should all look skywards this week and next in hope of a strong display of aurora borealis.
When is the autumn equinox?
Autumn equinox 2018 occurs on Sept. 22 at 9:54 p.m. EDT/6:54 p.m. PDT, which marks the moment that the center of the sun crosses the celestial equator. However, it's not a visual spectacle (indeed, the sun won't even be visible to half of the planet), so the possibility of strong aurora provides something to look out for to mark the change in the seasons.
What is the autumn equinox?
Also known as the southward equinox, this is when the sun crosses the celestial equator to create days and nights of virtually equal length for a short period. It happens every year between Sept. 21 and 24. It occurs because Earth orbits the sun, and rotates on a tilted axis. At the autumn equinox (and also at the spring equinox) that axis is perpendicular to the sun, and that has huge consequences for the likelihood of Northern Lights displays.
Why are Northern Lights stronger at autumn equinox?
"It's well known that the Northern Lights are stronger around the equinoxes, and it makes sense because of the geometry of Earth and the fact that the Earth is tilted," said Dr. Melanie Windridge, author of "Aurora: In Search of the Northern Lights."
She explains that the Northern Lights are caused by the solar wind hitting the Earth and charged particles being accelerated down the field lines of the Earth's magnetic field, but she stresses that there's a reason strong displays of aurora don’t occur constantly.
"This process only happens when the solar wind's magnetic field is facing southward relative to the Earth, and because of the way the Earth is tilted, that’s more likely at equinoxes," she said. "There's basically more chance of getting a good connection between the solar wind and Earth during the equinoxes."
It means more frequent and stronger Northern Lights displays around both the autumn equinox and the vernal (spring) equinox, which will happen on March 20, 2019, when the sun crosses the celestial equator going north.
What is the celestial equator?
It's an abstract projection of the Earth's equator in the night sky, essentially an imaginary circle on the celestial sphere that divides the night sky into two distinctive parts, north and south. You've heard of Polaris, the North Star, right? You can see it by looking north from anywhere in the northern hemisphere because it sits directly above the Earth's North Pole. Polaris sits above the North Celestial Pole, and the celestial equator sits above Earth's actual equator.
How to see the Northern Lights in 2018
It's usually imperative to head to northern latitudes to the Arctic Circle, between about 64° to 70° North latitude, anytime between September and April when there's lot of darkness. That means heading to Alaska, northern Canada, Greenland, Iceland, Norway, Sweden, Finland or northern Russia. It's also best to avoid the full moon and any light pollution to improve their brightness.
However, during a particular powerful display, the Northern Lights can shift south significantly and can even be visible around the U.S.-Canada border. For example, as part of their short-film series "More Than Just Parks," filmmakers Will & Jim Pattiz captured a magnificent display of Northern Lights in the remote Voyageurs National Park, Minnesota.
So look to the northern horizon this weekend, and for the next week or so, and you just might see the Northern Lights make a special seasonal appearance.