Farewell, flip-flops. And hello pumpkin spice.
Our long, hot summer is finally coming to an end on Thursday with the autumnal equinox, which marks the beginning of fall in the Northern Hemisphere.
When does the autumnal equinox occur?
At a precise moment each September, usually on the 22nd or 23rd, the sun is directly above the equator, marking the autumnal equinox.
This year, on Sept. 22, the center of the sun will be directly over the equator at 9:03 p.m. EDT. In the Northern Hemisphere, the sun will get lower in the sky and days will become shorter until the winter solstice on Dec. 21.
What does 'equinox' mean?
The word "equinox" comes from the Latin words "aequalis" and "nox," meaning "equal night." On the autumnal (and the spring) equinox, day and night are roughly 12 hours long each in most of the world.
This will be true Thursday from as far north as Utqiagvik (formerly Barrow), Alaska – north of the Arctic Circle – to as far south as Wellington, New Zealand, the world's southernmost capital city.
Day and night aren't exactly 12 hours long on the day of the equinox because the Earth's atmosphere refracts, or bends, light in an optical illusion that brings more daylight than there really is. Because of this, the date when day and night are of exactly equal length is usually a few days after the autumnal equinox.
The Earth's axis: What happens at the poles?
Earth has seasons because of its tilted axis, which is at 23.5 degrees to its orbit. As the planet rotates the sun, the axis points in the same direction. On Sept. 22, day and night are roughly equal in length.
Another equinox fun fact: On Thursday, the sun rises almost due east and sets nearly due west for most of the world, except at the North and South poles.
Although some people say the autumnal equinox is the "official" start of fall, no administrative or political organization actually designates that.
In fact, though astronomers say summer ends Thursday, meteorologists and climatologists say summer ended Aug. 31, the final day of the three hottest months of the year (June, July and August).
What's the difference between an equinox and a solstice?
Equinoxes – when day and night are roughly equal – occur in September and March and mark the astronomical beginning of autumn and spring in the Northern Hemisphere, respectively.
Solstices occur in December and June, which mark the beginning of astronomical winter and summer, respectively. The winter solstice occurs when the sun is at its lowest point in the sky, while the summer solstice is when the sun is at its highest point in the sky.
SOURCE Sciencefocus.com, NOAA