Today is the autumn equinox, which marks the astronomical start of fall in the Northern Hemisphere, and the beginning of spring in the Southern Hemisphere.
The fall equinox is one of just two days each year when the number of daytime and nighttime hours are nearly equal all over the world. On this day, most of the Earth experiences about 12 hours of daylight and 12 hours of darkness.
“In the Northern hemisphere, the September equinox marks the start of a period bringing us later sunrises and earlier sunsets,” explained NASA. “We will also feel cooler days with chillier winds, and dry, falling leaves.”
At 3:21 p.m. EST on September 22, the sun appeared to shine directly over the equator. Beyond this point, the Northern Hemisphere will have progressively fewer daylight hours (and more hours of darkness) as our planet slowly tilts its South Pole towards the sun.
Earth is tilted on its axis. This means that as our planet orbits the sun, more light reaches the Northern or Southern Hemisphere according to where Earth is positioned.
During the two equinoxes in spring and fall, however, sunlight equally illuminates both hemispheres.
‘The Earth takes one year (or 365-and-a-quarter days) to orbit once around the Sun and it is tilted over slightly on its axis,”explained Anna Ross, an astronomer at the Royal Observatory Greenwich.
“So for half of the year, the Northern Hemisphere is tilted slightly towards the sun, meaning we have longer daylight hours and hotter temperatures.”
“For the other half of the year, we are tilted slightly further away from the Sun so we have longer nights and colder weather. It’s this tilt that gives us our seasons.”
In the Northern Hemisphere, the fall equinox begins the countdown to the shortest day of the year and the start of winter on December 21, which is known as the winter solstice.
The winter and summer solstices occur when the planet’s north or south pole is most greatly inclined with toward or away from the Sun.
‘There are two points in the year where the Earth’s tilt is at its most extreme relative to the Sun,’ said Ross. ‘These days are known as the solstices.”
“On these dates, depending on which hemisphere you live in, you will either experience midsummer with the longest day of the year or midwinter with the longest night.”
“In between both of these extremes, we get our spring and autumn months, with the exact midpoints being the equinoxes.”