Welcome to fall. Almost. In the early hours of Monday the autumnal or southward equinox will occur, a celestial event that signals the official beginning of astronomical fall. Its coming will bring to the northern hemisphere cooler temperatures, as well as vibrant reds, yellows, purples, and oranges to deciduous forests across North America.
However, the equinox is also a great time to go looking for the Milky Way and, if we’re lucky, it could even see an unusually strong display of the Northern Lights.
When is the Fall Equinox?
Fall Equinox 2019 occurs on Monday, Sept. 23 at 7:50 a.m. UT, which is 3:50 a.m. EDT and 12:50 a.m. PDT. At that exact time the sun crosses the celestial equator, moving south to create days and nights of virtually equal length for a short time. It’s a global event, so occurs simultaneously for everyone on Earth, though for those in North America the 2019 fall equinox will actually happen on the other side of the world, so during the hours of darkness in North America.
What is the Fall Equinox?
It's the point halfway between the summer solstice and the winter solstice when the sun crosses the celestial equator heading south, and Earth’s axis is side-on to the sun. Everywhere on the planet gets the same amount sunlight so the day and night are therefore of almost identical length. The word equinox comes from aequus (Latin for equal) and nox (Latin for night).
How to see the Milky Way around Fall Equinox
Our galaxy’s bright core is only visible to those in the northern hemisphere in summer, with September offering virtually the last chance to observe it. However, it’s also one of the best times to see it because it’s visible soon after dark above the southwestern horizon. It’s important to go looking for it only on nights when the moon is down. By lucky chance, equinox in 2019 occurs when the moon is only 35% lit, and in any case won’t be rising until after midnight. So 2019’s final "Milky Way window" is between about Sept. 22 and Oct. 4.
Why the Northern Lights are stronger around Fall Equinox
Earth’s position and angle in relation to the sun make stronger aurora more likely around the equinoxes. That applies to the fall equinox, but also to the spring or vernal equinox — next on Friday, March 20, 2020 — when the sun crosses the celestial equator going north.
It’s not an exact science because auroral activity depends on the strength of the solar wind — charged particles from the sun interacting with the field lines of the Earth's magnetic field — but during equinox Earth’s 23.5° tilted axis is perpendicular to the sun. That means Earth is nicely lined up with the solar wind's magnetic field at equinox, which is facing southward relative to the Earth.
Where will the Northern Lights be stronger?
They may not be, but if they are, expect stronger displays around the Arctic Circle up at around 64°-70° N in Alaska, northern Canada, southern Greenland, Iceland, northern Scandinavia, and northern Russia. However, if they are particularly intense the Northern Lights may be observable as low as the U.S.-Canada border.